Subscriber access provided by University of Sydney Library
Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers
Self-Assembly, Thermotropic and Lyotropic Phase Behavior of Guerbet Branched-Chain Maltosides Nur Asmak Nabila Saari, Azwa Amanina Mislan, Rauzah Hashim, and N. Idayu Zahid Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01899 • Publication Date (Web): 12 Jul 2018 Downloaded from http://pubs.acs.org on July 16, 2018
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
Self-Assembly, Thermotropic and Lyotropic Phase Behavior of Guerbet Branched-Chain Maltosides N. A. Nabila Saari,† Azwa Amanina Mislan,† Rauzah Hashim,† N. Idayu Zahid†* †
Centre for Fundamental and Frontier Sciences in Nanostructure Self-Assembly, Department of Chemistry, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
ABSTRACT: Five synthetic β-D-maltosides derived from Guerbet branched alcohols, whose total hydrocarbon chain length ranged from C8 to C24, were synthesized to a high anomeric purity and their thermal, liquid crystalline phases and structures were characterized using differential scanning calorimetry (DSC), optical polarizing microscopy (OPM), and small-angle X-ray scattering (SAXS). Thermal investigations of all anhydrous Guerbet maltosides showed that they do not form solid crystals but undergo a glass transition upon temperature change in the range of 35 to 53 °C. The glassy crystalline structure turns into the liquid crystalline structure upon heating or addition of water. In thermotropic studies, the lamellar phase formation is prominent in shorter chain length analogues while the longer chain compounds exhibit more frustrated form of self-assembly in the formation of metastable state, polymorphism and inverse bicontinuous cubic structure (Ia3d). The excess water conditions show that the phase formation is dominated by lamellar phase for the longer chain compounds. Normal micellar solution was observed in the shortest chain length maltosides due to the enlargement in hydrated maltose headgroups. The self-assembly of both dry and fully hydrated Guerbet maltosides which exhibited glass forming abilities, possess surface activity and also ability to act as membrane stabilizing compounds makes them ideal candidates for practical use in industry as well as biomedical research. Keywords: liquid crystal, thermotropic cubic phase, polymorphism, glycolipids
INTRODUCTION
alkyl chain, they exhibit surfactant properties which make them widely used as detergents and cosmetics products. 8 Also, a few of these surface active compounds exhibit glass transition behavior (e.g. octyl-β-D-glucosides9 and maltosides10) and such bifunctional property is useful in preservation of foods and proteins during freezing and drying processes.9 Not to mention, glycolipids are suitable for general biotechnological applications such as drug delivery and membrane protein crystallization.11-13 As amphiphilic liquid crystals, glycolipids can selfassemble in both dry (thermotropic) and solvated (lyotropic) states into various lamellar and non-lamellar mesophases. Synthesis and characterization of various branched-chain glycolipids14-17 have been attempted as these are suitable models for further study on the structure and mechanism of cell membrane. Since natural glycolipids compound have limited availability and require extensive syntheses, similar compounds that can mimic the property of natural materials were synthesized for applications in the field of biophysics and biotechnology. The branched-chain glycolipids have been reported to produce the inverse liquid crystal structures inducing various changes in their physicochemical properties. Moreover, these amphiphiles can exhibit glass transition while being partially ordered to form glassy liquid crystals which referring to combined properties of a glass and liquid crystals. Upon heating, a glass transition is a transition from a glassy state to a liquid state and vice versa upon cooling. Glassy and liquid crystal states have the same structure, but in the glass the molecular motion is frozen. Kocherbitov and Sőderman10 reported the glassy crystalline structure of monoalkylated maltosides turns into the lamellar liquid crystalline structure upon heating or addition of water. The dodecyl-β-D-maltotrioside was also reported to exhibit such behavior.18 The effect of the hydrophobic chain length on the
Used in huge amount in the candy industry, maltose sugar is incapable of crystallization unless possessing purity more than 90% unlike glucose that can be crystallized even with high amount of impurities. It belongs to a disaccharide group which is the simplest oligosaccharides and can be easily obtained in large quantities from starch products like maltodextrin and corn syrup by means of polysaccharides hydrolysis.1 Maltose has also been used in intravenous injection to administer sugar for patients, component in frozen desserts, sweetening agents as well as in baking and brewing industry.2 Chemically, a maltose consists of two glucose units linked by an α(1→4) linkage. The two glucose units are in the pyranose form and are joined by an O-glycosidic bond, with the first carbon (C1) of the first glucose linked to the fourth carbon, (C4) of the second glucose. The glycosidic bond to the anomeric carbon (C1) is in axial position implying it is in the opposite plane from the CH2OH substituent in the same ring. Introduction of hydrophobic hydrocarbon chains to the hydrophilic sugar headgroup produces an interesting surface active amphiphilic molecule, a glycolipid which plays an important role in life science as well as material science. For example, dodecyl-β-D-maltopyranoside functions as a surface blocking reagent in protein separation,3 solubilization of integral protein4 and involves in purification and stabilization of RNA polymerase.5 Glycolipid such as alkylmaltosides can be produced synthetically or enzymatically. Additionally in Nature, glycolipids can be found on the exterior of the lipid layer of cell, where they are involved in intercellular recognition process, signal transduction, transmembrane transportation, cell adhesion and other specialized functions.6,7 Due to the dichotomic balance of the polar headgroup and non-polar
1
ACS Paragon Plus Environment
Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
thermotropic and glass transition behaviors of n-alkyl β-Dglucosides was also investigated by Ogawa et al.9 An attempt to study the size effects of the sugar headgroup (monosaccharides vs. disaccharides) on the Guerbet glycolipids has been made,19 but only their liquid crystalline properties in dry condition were investigated without quantitative structural determination. On the other hand, Guerbet xylosides whose sugar head is an aldopentose ring have been characterized in dry and hydrated forms. 20 From these studies, the phase diagram of several Guerbet glycolipids based on monosaccharide headgroup (glucose, galactose, mannose) have been published and the contributions from anomeric, epimeric and chain length effects have been discussed.21,22 Moreover, these monosaccharide (xylose, glucose and mannose) glycolipids readily form curved mesophases in dry and hydrated conditions. As an extension, the present study aims to elucidate the lyotropic behavior of Guerbet disaccharide maltosides whose hydrophilic-lipophilic balance is significantly different to those of the monosaccharide glycolipids. The synthesized maltosides possess two opposing features i.e. from the headgroup, which contains mostly attractive hydroxyl groups (energetic) and the randomization of chain branching region which gives the hydrophobic effect (entropic). In general, when the chain length is shorter, the former effect is dominant and when the chain length is longer, these two effects are equally dominant resulting in the selfassembly to become frustrated and compromised to give a thermotropic polymorphism. The anhydrous monoalkylated alkylmaltosides shown glass forming abilities and we anticipate this observation for the branched-chain counterparts. Furthermore, their water–temperature partial phase diagrams will be established by combining optical polarizing microscopy (OPM), cross-polarized visual inspection and small-angle X-ray scattering (SAXS).
Page 2 of 15
EXPERIMENTAL SECTION Guerbet Maltoside Synthesis. The Guerbet maltosides were synthesized according to the literature procedure.19 The β-D-maltose octaacetate (98%) was purchased from Carbosynth. The five Guerbet alcohols (97%) namely, 2ethyl-1-hexanol, 2-butyl-1-octanol, 2-hexyl-1-decanol, 2octyl-1-dodecanol and 2-decyl-1-tetradecanol were procured from Sigma-Aldrich. Boron trifluoride diethyl etherate, sodium methylate (97%) and ACS grade solvents which include dichloromethane, acetonitrile, ethyl acetate, n-hexane and methanol were supplied by Merck. All chemicals and solvents were used without any further purification. The anomeric purity of Guerbet maltosides was estimated to be ≥ 97% according to 1H NMR, 13C NMR and thin layer chromatography. NMR data for a homologous series of Guerbet maltosides are available in Supporting Information. The hygroscopic maltosides were extensively dried before any characterization by keeping the compounds in a vacuum oven over di-phosphorus pentoxide for at least 48 hours. The term “dry” is used rather loosely because it is a well-known fact that removing the last trace of water is difficult in sugar lipids.25 Thus, we have tested these consistently “dry lipids” using a Fourier Transform Infrared Spectroscopy (FTIR) for the assessment of their water content by observing the stretching and bending modes. FTIR study of the compounds show that water vibrational peak intensity approximately at 1643 cm-1 lessen considerably with drying in relative to the compound left at ambient and in excess water (see Figure S1 of the Supporting Information). The appearance of symmetric stretching vibration peak of methylene groups in the range of 2853 to 2854 cm-1 in all the studied compounds indicates the presence of liquid-crystalline phases.26 Differential Scanning Calorimetry (DSC). DSC (Mettler Toledo DSC 822 ͤ calorimeter equipped with Haake EK90/MT intercooler) was applied to determine the thermal transitions of the samples through a programmed heating and cooling scans at the rate of 5 °C min-1 under nitrogen atmosphere. Indium was used as the standard sample to calibrate the temperature and enthalpy accuracy. All the samples tested were dried in a vacuum oven over diphosphorus pentoxide for at least 48 hours. Then 4–8 mg of dried sample is weighed accurately and quickly added into the 40 µL aluminium pan and sealed immediately. The calorimetric measurements were carried out for three cycles in a temperature range from (–50 °C up to 250 °C). The data were analyzed using STAR ͤ Thermal Analysis System software. The transition temperatures were obtained from the second heating to remove the thermal history. The enthalpies of these phase transitions were determined by integrating the area between the endothermic peak and the baseline. Optical Polarizing Microscopy (OPM). Liquid crystal textures were identified by an Olympus BX51 polarizing microscope equipped with a Mettler Toledo FP82HT Hot Stage and a temperature controller (FP90 Central Processor). The microscope was connected to Olympus camera for image capture and cellSens microscope imaging software was used for image analysis and storage. The captured images of phase texture are in the presence of cross-polarizing lenses. All compounds were consistently dried before the experiments in the vacuum oven over di-phosphorus pentoxide for at least 48 hours. For the thermotropic studies, dry sample was placed onto a clean glass microscope slide and covered with a glass
Figure 1. The chemical structure of the Guerbet branched βD-maltosides.
Five homologous series of Guerbet β-D-maltosides namely, 2-ethyl-hexyl-β-D-maltopyranoside (β-Mal-C6C2), 2butyl-octyl-β-D-maltopyranoside (β-Mal-C8C4), 2-hexyldecyl-β-D-maltopyranoside (β-Mal-C10C6), 2-octyl-dodecylβ-D-maltopyranoside (β-Mal-C12C8) and 2-decyl-tetradecylβ-D-maltopyranoside (β-Mal-C14C10) have been chosen for this study. Their chemical structures are illustrated in Figure 1. The Guerbet branched hydrophobic tail contains two asymmetric chains ranging from C6C2, C8C4, C10C6, C12C8 to C14C10. This branched chain design has a degree of branching23 of 0.75 and 0.58 respectively, while for the monoalkylated chain this factor equals to 1. We used the lyotropic nomenclature including for describing thermotropic phases since it is more convenient for the amphiphilic mesogens.24
2
ACS Paragon Plus Environment
Page 3 of 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
cover slip. The heating stage was then used to heat the sample until it reached its isotropic phase. The liquid crystal image that formed was taken upon slow cooling at a rate of −2 °C min-1 to obtain the best optical textures. For the lyotropic studies, water contact preparation method was used where deionized water droplet is added to the edge of the cover slip which allowed contact with the sample. The water entered the sample via capillary force action creating a concentration gradient of excess water at the cover slip edge to neat surfactant. The OPM images were captured at least after 30 min of equilibration during which precaution was taken to reduce water loss. Binary Phase Behavior. Amphiphile–water mixtures were prepared by weighing out small amounts of the dry lipid into glass ampules, followed by adding the appropriate volume of water by weight. Samples ranging from 5% (w/w) to 90% (w/w) water content at approximately 2 to 3% (w/w) water intervals were prepared for the phase boundary determination. The glass ampules were flame-sealed, and the lipid−water mixture was mixed as much as possible by centrifugation and heating, followed by cooling. The sealed samples were illuminated and examined through crossed polarizers in a heated water bath. Heating of the water bath was increased stepwise by 10 °C and then left to equilibrate for 1 h at each desired temperature. Small-Angle X-Ray Scattering (SAXS). Sample Preparation. The dry sample were packed into the paste cell sample holder and heated to their respective isotropic temperature. The samples were allowed to cool to room temperature and left at least overnight in the vacuum. The SAXS measurement is performed on the next day or after the sample had reached equilibration. Hydrated samples were prepared by adding appropriate amount of water and dry lipid into a 2.0 ml microcentrifuge tube. The samples were then homogenized by repeated heating combined with up and down centrifuging. The homogenized samples were then allowed to equilibrate for at least fortnight prior to SAXS measurement. Approximately 50 mg of sample was transferred to a paste cell holder and loaded into the X-ray machine. We prepared at least two specimens for each maltoside to confirm that the two specimens give identical SAXS profile. Measurement. SAXS was used to confirm the liquid crystal phases observed by OPM and to extract the main parameters of the structures obtained. The scattering patterns of the dry and fully hydrated samples were characterized by using an analytical small angle X-ray scattering from SAXSpace (Anton Paar, Austria). The instrument was equipped with an X-ray tube (DX-Cu 12x0.45, SERFERT) generating Cu-Kα with wavelength 𝜆 = 1.542 Å at 40 kV and 50 mA. The Goebel-mirror focused and Kratky-slit collimated X-ray beam was line shaped (17 mm horizontal dimension at the sample) and scattered radiation from the samples (measured in the transmission mode) was recorded with a onedimensional MYTHEN-1k microstrip solid-state detector (Dectris Switzerland) within a q-range of 0.01 to 0.5 Å−1 at a sample-to-detector distance of 317 mm, where q is the magnitude of the scattering vector applying the conversion q [Å−1] = 4π(sinθ)/λ, with 2θ being the scattering angle with respect to the incident beam and λ the wavelength of the Xrays in Å. Silver behenate which has a periodicity of 58.38 Å (where d [Å] = 2π/q1 is the lamellar spacing obtained from the position of the first order reflection q1) was used as
standard. The measurement was run at various temperature using a temperature controlled sample stage (TCStage 300) within an accuracy of ± 0.1°C. Both dry and hydrated samples were left to equilibrate for 30 min at the desired temperature prior to a 1-hour acquisition time. The data were calibrated by normalizing the primary beam in the SAXStreat software. The liquid crystal phases and the corresponding lattice parameter was determined by using an SGI software (Space Group Indexing, V.03.2012). RESULTS AND DISCUSSION Thermal Properties from DSC. DSC was used to determine the thermal properties of the Guerbet maltosides. All the DSC data are presented in Table 1 and Figure 2. The DSC thermograms in Figure 2(a) showed the typical endothermic clearing transition into the isotropic phase, but no melting was observed instead step-shaped peaks associated with the glass transition were discernible. We shall first discuss the glass transitions detected between 35 to 53 °C for these maltosides. A glass is an amorphous solid which does not possess long-range translational order27 and it can be obtained either from the heating or cooling scans.9,10 For example, on cooling near the glass transition, motions are restricted due to the sugar interaction which is predominantly hydrogen bonded. At the glassy point, molecules are frozen in positions they adopted just before the transition. Consequently, the structure of the newly formed glassy crystals is closer to that of liquid crystals,10 thus we assign this as lamellar glass or LG.28 See OPM textures discussed below in Figure 3(c) and inset for lamellar (L) at 54 °C and glassy lamellar (LG) at ~25 °C respectively. The presence of a glass transition possibly suppressed the melting transition into the liquid crystal phase.29 For this maltoside series, the effect of the Guerbet chain length variation on Tg is small and has no obvious trend (see Figure 2(b)). Similarly, Ogawa et al. reported no obvious effect on Tg upon increasing the chain length (6 n 12) for monoalkylated glucosides.9 Other studies demonstrated that Tg of dodecyl glycosides increases as the number of sugar ring increases,18,28 hence Tg of dodecyl chain of β-glucoside,9 βmaltosides18 and β-maltotrioside18 were reported to be around 12 °C, 65 °C and 100 °C respectively. This implies the glass transition is predominantly a function of the sugar headgroup, which is unsurprising since glassy phase is common in sugars.30 For example, Tg for sugars glucose, maltose and maltotriose are 39 °C,31 73 °C,32 and 99 °C32 respectively. Moreover, a glass transition is a kinetic phenomenon which depends on the condition of the experiment, thermal history and sample preparation method.10 For comparison, the thermal behavior of a selected monoalkylated maltosides homologous series is given in Table 1. The shorter-chained maltosides (8 n 12) form a glass phase upon lyophilization, but no Tg was observed for those with long alkyl chains (n >12). However, Ericsson et al. reported a glassy state is still possible for β-Mal-C14 by cooling of the liquid crystalline phase to room temperature and let it equilibrate.33 For the same total carbon number of the alkyl chain, the Tg for Guebert maltosides is lower than those of the monoalkylated maltosides. Thus, the chain branching effectively lower the Tg. This is reasonable since intuitively chain branching increase the hydrophobic region and makes the sugar head more mobile compared to the
3
ACS Paragon Plus Environment
Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 15
Figure 2. (a) DSC thermograms with baseline correction for dry β-D-maltosides at a heating scan rate of 5 °C min-1, (b) Tg and Tc trends for the Guerbet maltosides. Enthalpy values (kJ mol-1) are indicated in the square brackets.
monoalkylated systems. A computer simulation study of thermotropic monoalkylated and branched maltosides by Achari et al. supports this observation since the average autocorrelation time for the sugar head is larger for the former compared to the latter.34 On further heating above Tg, the DSC thermograms of Guerbet maltosides do not give any melting peak. In contrast, the homologous series of monoalkylated maltosides from various literatures recorded melting transition as shown in Table 1. From these literatures, it is not obvious if these maltosides give a recrystallization exothermic peak (after the glass transition) as reported by Ogawa et al. for undecyl and dodecyl β-glucosides.9 From Table 1 and Figure 2(b), β-Mal-C6C2 and β-MalC8C4 become isotropic at 138 °C (ΔH = 1.3 kJ mol-1) and 194 °C with ΔH = 1.6 kJ mol-1 respectively. While the next three have the clearing temperatures, 186 °C (ΔH = 0.7 kJ mol -1), 197 °C (ΔH = 0.7 kJ mol-1) and 234 °C (ΔH = 0.9 kJ mol-1) respectively. However, their enthalpies are relatively smaller compared to the shorter chain analogues, implying there are more bond breaking in the latter compared to the former. Interestingly this discriminating behavior between the first two members and the three longer chain members, is also observed in the Guerbet xylosides20 and mannosides.22 In addition, β-Mal-C14C10’s thermogram shows an extra small endothermic peak at 141 °C with an enthalpy of H = 0.6 kJ mol-1, corresponding to a phase transition between an lamellar to a hexagonal phase which will be confirmed by OPM. This additional transition has a relatively less enthalpy change than the clearing phase transition at 234 °C with ΔH = 0.9 kJ mol-1. Elongation of the alkyl chain usually increases the TC as observed in the straight chain alkyl maltosides (Table 1). This is anticipated in increasing the chain length as more energy is required to break interactions to form the isotropic phase. However, this increase reaches a maximum point, beyond which Tc decreases. For monosaccharides, the maxima occur between 12 to 14 carbons35 while for disaccharides headgroup i.e. maltose, such trend may be observed at a longer chain length (n >18).36 Except for βMal-C8C4, Tc increases with increasing chain length due to the higher van der Waals interaction between the
hydrophobic tails with an increase in the alkyl chain length. However, such trend was not apparent for monosaccharide Guerbet glycolipids with xylose, glucose and galactose headgroups.19,20,24 These maltosides are expected to have higher Tc’s compared to the monosaccharides since the former have higher number of hydrogen bonds. Upon comparison with the linear chain counterpart, the branching effectively decreases the Tc resulting in a wider range of liquid crystal, since chain branching increases the hydrophobic volume and increases the disorder in molecular packing.37 Similar observation is made when a double bond is present in cis-9-octadecenyl-β-D-maltopyranoside (β-MalC18:1). von Minden et al.36 reported Tc of β-Mal-C18:1 to be reduced by 7°C to 267°C when compared to β-Mal-C18 which undergo isotropization at 274°C. Moreover, increasing the number of headgroup will increase the chance of hydrogen bonding in the molecules resulting in higher Tc. But this is not observed in dodecyl-β-D-maltotrioside (Tc = 228°C38 and 217°C39), which has, when compared with dodecyl-β-D-maltoside (Tc = 244–245°C18,36-38,40), an extra glucose unit in the headgroup region. Although, rather surprising it shows that TC is not solely governed by the number of hydroxy groups. Liquid Crystalline Phase Behavior from OPM. Typical OPM textures for the Guerbet maltosides are shown in Figures 3–5. The transition temperatures determined by OPM for these dry branched chain maltosides are listed in Table 1. The clearing temperatures are in good agreement with those from DSC. However, for β-Mal-C14C10, we observed additional transition temperatures from liquid crystal to liquid crystal from its thermogram (see Figure 2(a)). On heating, dry β-Mal-C6C2 turns to isotropic at 140 °C, which is comparable to that determined by DSC. Upon cooling, the sample gives the fan-shaped texture that start to appear at 139 °C which correspond to that of Lα phase and the texture remained until the room temperature (Figure 3(a)). Addition of water to the neat surface of the dry β-MalC6C2 at the room temperature reveals non-birefringence texture indicating the micellar solution, L1 structure (Figure 3(b)). We anticipate the micellar solution belongs to the normal micelle L1 and not the inverted structure because it
4
ACS Paragon Plus Environment
Page 5 of 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
Table 1. Phase transition temperatures for anhydrous Guerbet and monoalkylated β-D-maltosides series determined by DSC. Clearing temperatures obtained by OPM are also presented. The total number of carbon is denoted by n. “-” signifies undetected value. Tlc→lc indicates a liquid crystal to liquid crystal transition temperature. Error in temperature is ±1°C while error in enthalpy is ±0.1 kJ mol-1. Transition temperature (°C) [ΔH (kJ mol-1)] Guerbet Maltosides
n Tg
Tlc→lc
Tc (DSC)
Tc (OPM)
β-Mal-C6C2
8
36
-
138 [1.3]
140
β-Mal-C8C4
12
53
-
194 [1.6]
195
β-Mal-C10C6
16
53
-
186 [0.7]
190
β-Mal-C12C8
20
43
-
197 [0.7]
196
β-Mal-C14C10
24
48
141 [0.6]
234 [0.9]
238
Monoalkylated Maltosides
Transition temperature (°C) n
Ref. Tg
Tm
Tc
~60
89-103a
125a
37
54
-
123
10
~60
96-100a
207a
37
58
-
206
10
~60
102
245a
37
102a
245a
36
70b
103a
-c
40
65
-
244
18
-
80a
244a
38
-
107a
264a
36
-
86a
264a
38
-
105
263
33
16
-
105
-d
33
18
-
106a
274a
36
8
10
β-Mal-Cn
12
14
n.b. a data from OPM. b data from SAXS. c compounds darkened and decomposed at ~150°C, hence Tc was not detected.40 d Not reported in the original paper.33
occurs at higher water contents. Similar to the first compound, neat β-Mal-C8C4 exhibits the fan-shaped texture upon cooling down to room temperature suggesting the Lα phase formation with Tc = 195 °C (Figure 3(c)). In lyotropic study, the compound forms an isotropic phase at higher water concentration as indicated by the arrow in Figure 3(d) followed by a stronger birefringence of Lα phase structure when the water content gradually decreased. Upon heating the dry β-Mal-C10C6, no melting was observed by OPM and the compound clears at 190 °C. On cooling, we observed oily streak texture with birefringent bands across a pseudoisotropic region. The texture remains at room temperature as shown in Figure 3(e). The presence of maltese cross structure (see Figure 3(e)) denotes the formation of Lα phase in the samples and SAXS investigations in later section confirms the existence of this phase. The same Lα texture persisted in the presence of water (Figure 3(f)). It was reported that
lamellar amphiphilic phases form pseudo-isotropic textures with oily streaks.41 The fourth analogue, β-Mal-C12C8 in anhydrous state turns to isotropic liquid at 196 °C. Upon cooling, it exhibited unusual birefringent texture between the first and second cooling. A mosaic structure was observed during first cooling (Figure 4(a)) whereas a striated texture (Figure 4(b)) was perceived upon second cooling. The OPM textures are rather uncommon of a sugar lipid and is quite distinct from the focal conic and fan-shaped texture. The rare anisotropic textures are not clear to allow for an unambiguous identification of the liquid crystalline phases, as it could be lamellar, hexagonal, intermediate phase or a mixture of these. Although, a proper phase identification could be performed by SAXS, we shall see later (next section) that this is also not the case for this compound. Figure 4(c) shows a higher birefringence texture was obtained at higher water concentration with the occurrence of myelin structure. The
5
ACS Paragon Plus Environment
Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
observation signifies the formation of Lα phase in this compound. The longest chain member β-Mal-C14C10 exhibits polymorphism upon heating the samples as predicted from the DSC results. A few selected OPM textures representing different mesophases were presented in Figures 5(a)–(c). A birefringent lamellar texture is observed at 52 °C (see Figure 5(a)). With further increase of the temperature to 117 °C, the sample started to form a non-birefringence isotropic phase (Figure 5(b)). This viscous isotropic phase is presumed to be the cubic (V2) phase whose identity shall be confirmed by SAXS. The weak birefringence texture along with the transition bars observed in Figure 5(b) signifies the phase transformation from lamellar to cubic phase. Additional heating of the isotropic phase brings about a birefringence phase of fan-shaped texture of inverse hexagonal, H2 phase at 136 °C, which is comparable to the DSC result (see Figure 2(a) and Table 1). However, the DSC thermogram did not give the phase transition from Lα to cubic, since Lα ↔ cubic interconversion is kinetically controlled.26 Upon further increase in temperature led to the sample became clear at Tc = 238 °C. The molecular structures are more rod-like shaped and possess the zero-mean curvature that favors the formation of Lα at the lower temperature. Upon temperature
(a)
Page 6 of 15
increment, the rod-like shaped of the molecules at Lα phase becomes more wedge-shaped due to the increase motions in the hydrocarbon chains resulting in the formation of cubic. Cubic phase can be regarded as a structural compromise due to the destabilization of molecular moiety in lamellar structure. Further destabilization results in H2 phase as observed in β-Mal-C14C10.16,42 Contact preparation scan of βMal-C14C10 reveals a myelin structure indicating the formation of Lα phase. The formation of myelin in high water concentration region has also been reported in dialkyl glycosyl bearing maltose headgroup.16 Upon second cooling, no morphological changes in OPM textures were observed below and after the Tg. When the liquid crystalline undergo a glass transition, the newly formed glassy crystal phase keep almost the same structure as in the liquid crystal which relaxes into the more ordered glassy phase rather slow. Since it takes a long time, there is no distinct phase changes occurred (see Figure 3(c)), hence, the texture remains the same below and after Tg.9
(c)
(e)
(d)
(f)
Figure 3. Textures of β-D-maltosides, viewed under optical polarizing microscope (10x): (a) Lα of dry β-Mal-C6C2; (b) micellar solution, L1 of β-Mal-C6C2 after contact with water; (c) fan-shaped texture of Lα in dry β-Mal-C8C4. The inset shows the glassy lamellar phase at the room temperature; (d) isotropic and Lα birefringence textures observed from β-Mal-C8C4 after contact preparation scan; (e) coexistence of a maltese cross texture (highlight within a yellow circle) indicating Lα phase and a pseudo-isotropic region of dry β-Mal-C10C6; (f) maltese cross structure and pseudo-isotropic region remain in β-Mal-C10C6 upon addition of water.
In general, the branched-chain maltosides have a wide range of Lα due to strong electrostatic interaction via hydrogen bonding in the hydrophilic region henceforth, the transformation into another non-lamellar phase may occurs at much higher chain length or temperature.43 When two opposing factors from the headgroup and the hydrocarbon
chain are equally dominant, the self-organization becomes frustrated and compromised to give a thermotropic polymorphism, where the system displays a series of phases from lamellar to the non-lamellar (V2, H2) by heating as shown Figure 5. The tendency of the hydrophilic head region to form lamellar structures and the hydrophobic region to
6
ACS Paragon Plus Environment
Page 7 of 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
(a)
(c)
Figure 4. OPM micrograph with 10x magnification for dry β-Mal-C12C8 gives different texture upon repeated heating and cooling cycle. (a) The mosaic structure upon first cooling and (b) striated texture upon second cooling. The hydrated β-MalC12C8 gives Lα structure at high water concentration gradient as shown in (c).
water may give a particularly drastic effect.33 From the results obtained, it was suggested that the headgroup and interfacial hydration determine the thermotropic and lyotropic phase properties. The interaction between headgroup interface and water molecules determines the formation of lamellar phase. In lyotropic studies, the number of phases, the rate of formation and the molecular packing of the phases are also determined by these interactions.
curve result in the packing frustration which induces the formation of a cubic phase.43 The bulkiness (v/l in the unit of Å2) of the Guerbet chains has been previously quantified and compared with those of the monoalkylated kinds. 28,43 v represents volume of the alkyl chain while l denotes its length at full extension (all trans conformation in the longest chain). Both values were estimated from Tanfords’s formula.44 Based on this comparison, it is generally noted the v/l for monoalkylated chains hardly increase in value, whereas those of the Guerbet ones increase significantly. For examples, v/l for C8, C12, C16, C20 and C24 monoalkylated chains increases by ~1% i.e. from 20.9 Å2 to 21.1 Å2. On the other hand, those of Guerbet tail i.e. C6C2, C8C4, C10C6, C12C8 and C14C10, the v/l changes by ~32% from 27.5 Å2 to 36.2 Å2. It also shows that the bulkiness of the Guerbet chains is tunable and depending on the chain length supports either the formation of lamellar or/and non-lamellar phases.28,43 From the previous studies of Guerbet glycosides, the orientation of the C4-OH and the glycosidic linkages modify the arrangement of hydrogen bonding in the hydrophilic region, thus influencing the thermal properties of the selfassembly.43 The Tc from OPM and of DSC obtained for this work are similar to within the error. Compared to previously reported phase transition temperatures,19 the results by OPM are lower for most compounds except for β-Mal-C12C8, whose Tc is a few °C different from the present study. The presence of water in the former study may cause the different behavior since a small amount of water is sufficient to lessen the clearing point of glycolipids. This effect was also observed in 4-cyano-4’-pentylbiphenyl (5CB) dispersion system.45 The phase behavior and transition temperature for β-MalC12C8, between previous and current works are significantly different due to dissimilar degrees of purification of the compound since the previous work reported that β-MalC12C8 contained 5% α-anomer.19 It is widely accepted that the anomeric purity in glycosides considerably affect the mesomorphic behavior.21,36 Nature of the headgroup largely governs the thermal behavior of alkylmaltosides due to the strong intermolecular hydrogen bonds between the glycolipid headgroups. Thermal behavior of the hygroscopic alkylmaltosides is extremely sensitive to sample history and presence of trace
(a)
(c)
(d)
Figure 5. Polymorphism of anhydrous β-Mal-C14C10 found upon heating under optical polarizing microscope (10x) is shown in (a)–(c). (a) Lα structure at 52 °C; (b) isotropic cubic phase starts to form at 117 °C and (c) fan-shaped texture of H2 at 136 °C. (d) Myelin texture of β-Mal-C14C10 after contact preparation scan with water.
Structure Determination by SAXS. Thermotropic Study. SAXS studies of the first three anhydrous maltosides i.e. β-Mal-C6C2, β-Mal-C8C4 and β-Mal-C10C6 (Figure 6(a– c)) reveal a typical Lα phase pattern which is characterized by the reflections [100], [200] and [300]. Their lattice parameters are 28.0 Å, 29.0 Å and 31.6 Å respectively. The results confirmed the observation of the Lα phase under an optical polarizing microscope for these compounds. The
7
ACS Paragon Plus Environment
Langmuir
Dry (thermotropic)
Fully hydrated (lyotropic)
Intensity (a.u.)
β-Mal-C10C6
β-Mal-C8C4
β-Mal-C6C2
Lipid
0.15
0.25
0.35 q (Å-1)
0.45
0.55
0.65
β-Mal-C12C8
0.05
β-Mal-C14C10
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 8 of 15
Figure 6. SAXS patterns of β-D-maltosides under dry and fully hydrated conditions at 25 °C with the following phase: (a) Lα; (b) Lα; (c) Lα; (d) unassignable phase; (e) V2 (Ia3d); (f) L1; (g) L1 and Lα; (h) Lα; (i) Lα and (j) Lα. The intensity data are all in an arbitrary unit (a.u.).
8
ACS Paragon Plus Environment
Page 9 of 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
(b)
Figure 7. Characteristic X-ray scattering of dry β-Mal-C12C8: (a) SAXS pattern after heating and left to equilibrate overnight; (b) WAXS pattern under the same condition as (a); (c) before heating to isotropic and (d) after heating to isotropic and left to equilibrate for a week; (e) SAXS pattern at different temperature under the same condition as (a). The intensity data are all in an arbitrary unit (a.u.).
formation of either Lα or H2 phase in anhydrous β-Mal-C12C8. However, the SAXS pattern of this lipid gave unusual characteristic peaks which is different from the reflection order of a Lα or H2 phases (Figures 6(d) and 7(a)). This phase is characterized by a scattering pattern containing about 12 sharp lines below 0.6 Å-1. The WAXS study identifies this as a liquid crystal phase based on a broad diffuse peak close to 4.7 Å (see Figure 7(b)). Hitherto, the identity of β-Mal-C12C8 mesophase was unassignable to any specific lattice and its complex SAXS pattern obtained upon an overnight cooling from heating to its isotropic phase indicates a possible involvement of kinetic retardations. The results may imply that the compound is kinetically unstable since its SAXS
chain length effect for these shorter Guerbet chains is insufficient to counter the strong influence of the bulky maltose headgroups which support the formation of the lamellar layers. This is due to the molecular balance between the hydrophobic chain moiety bound to the ether bond with the maltose headgroup give a more rod-like packing with equivalent balance between these two parts, hence the formation of lamellar phase is more favored.20 Upon increasing the hydrophobic chain length, the properties of the glycolipids are dominated by the hydrocarbon chain melting process as the lipophilic chains are said to be the main drive for the phase transition. The striated birefringent texture from OPM study predicts the
9
ACS Paragon Plus Environment
Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
peaks appeared to be shifting with time after left to equilibrate for a week as shown in Figure 7(d). Hence, the sample is said to be kinetically metastable and may require longer equilibration time for it to return to its original phase. This metastable phase can exists over long time so that no spontaneous conversion to the ground state occurs in sensible time. The lack of reversibility in lipid phase transitions which is indicated by the occurrence of distinct metastable phases may be attributed to the long hydration/dehydration times, slow reformation of hydrogen bond networks, necessity of large spatial rearrangements in lamellar/non-lamellar transition, relative stability of the interfaces between rigid and fluid domains. Such behavior has been observed in cooling scans of phospholipid and glycolipid dispersion.46 The heating scan for this compound is further investigated using SAXS by varying the temperature and the scattering peaks are depicted in Figure 7(e). As observed in Figure 7(e), the compound remains metastable in a wide range of temperature resulting in difficult peaks assignment for phase determination. It was proposed that two cubic mesophases may co-existing in this particular compound, but we could not confirm the assignment as seen in the phase assignment of previous work.19 From previous study,19 β-Mal-C12C8 was assigned with cubic phase of space group Im3m, Pn3m from SAXS measurement. These discrepancies with the present study may be due to the difference in purity and hydration factors. Finally, Figure 6(e) depicts the scattering pattern of βMal-C14C10 at 25 °C which consists of [211], [220], [321] and [400] reflections. These Miller indices unveil an inverse bicontinuous cubic, V2 structure of space group Ia3d with lattice parameter of 89.7 Å. The assignment of inverse phase behavior is based upon this molecule has a large CPP (>1), as evidenced in the formation of non-lamellar V2 phase. The observed results confirm that chain branching supports curved phases as reported for other branching systems.16,47-49 In the case of branched-chain Guerbet β-xylosides20 and β-glucosides,24 the non-lamellar phase can be formed by chain length as short as C8C4 while the maltoside counterpart requires longer chain length i.e. C12C8 to give curved phases which is consistent with the mean curvature theory.50 The βMal-C14C10 showed polymorphism over a temperature scan with phase sequence as follows: Lα ↔ V2 (Ia3d) ↔ H2. Hence, we anticipated the SAXS pattern of an Lα phase to be formed at 25 °C. However, surprisingly, the pattern for a cubic Ia3d structure was observed upon an overnight cooling from heating to its isotropic phase. From the phase sequence, the cubic mesophase is located between the lamellar and inverse hexagonal phases. Thus, the presence of the bicontinuous cubic phase implies a remarkable metastability over an extended temperature range once formed and possibly can be reset into the Lα phase only by further cooling. Another possible explanation for the different observation between the OPM and SAXS measurement for this phase is due to different sample preparations in the two techniques. In the former, a small amount of sample in a thin layer was used, while in the latter, the cell contained much more sample. Large variation of kinetic origin can be induced in the lipid phase behavior by parameters such as thermal history, scan rate and temperature gradients in the measuring cell.46 Binary Phase Behavior. The five phase diagrams with a controlled water content ranging from approximately 5%
Page 10 of 15
(w/w) to 90% (w/w) are given in Figure 8 (a–e). The temperature ranged from 25–60 °C. The phase boundaries as well as excess water region were obtained visually between crossed polarizer sheets and are shown in the shaded areas in Figure 8. Selected hydrated samples are measured by SAXS to elucidate their mesophases and structural parameters following at least two weeks of equilibration at 25°C. The measurement conducted in the heating direction as a function of temperature and composition are superimposed on the partial binary phase diagrams data. These data are denoted by different symbols representing different type of liquid crystal phases. In general, Lα phase dominates the lyotropic self-assembly of all compounds which determined from the scattering pattern in the following ratios: 1, 2, 3 (see Figure 6 (h–j) for example) and its anisotropic behavior between crossed polarizer sheets. Additionally, the normal micellar solution L1 has a significant region of existence at higher water content of the shorter chain length Guerbet β-Dmaltosides. Its SAXS data revealed a single wide peak which indicates the absence of long-range crystalline order. The calculated lattice parameters for each phase are tabulated as a function of temperature and water content in Tables S1−S5 of the Supporting Information. Figure 8 (a) shows the partial phase diagram of β-MalC6C2. At low water contents up to 15% (w/w), β-MalC6C2/water system showed anisotropic behavior between 25–60°C and the SAXS data at 10% (w/w) revealed the scattering pattern of the lamellar phase. A fluid isotropic region starts to form at 17.5% (w/w) and concentration beyond. Hence, we assign this concentration as the excess water point (dashed line) for β-Mal-C6C2. SAXS measurement at 25% (w/w) reveals two broad peaks which can be associated with the lamellar phase. Nevertheless, using crossed polarizer sheets, the sample has a very low birefringence resembling that of an isotropic phase. The different observation between SAXS and microscopy data may reflect small changes in hydration between the samples. Upon increasing the water content at 50, 70 and 90% (w/w), no sharp scattering peak is observed for β-Mal-C6C2 suggesting the system adopts normal micellar solution, L1 in excess water region. At these concentrations, we can detect a two-phase region between water and a liquid-like medium or liquid crystal phase which supports that an excess water point has been achieved. The SAXS pattern of 90% (w/w) gives a broad peak centered at q of ca. 0.24 Å-1, with a lattice parameter or cell−cell distance of 26.7 Å. The wide peak indicates absence of long-range crystalline order which can be associated with micellar solution (see Figure 6 (f)). Due to the short chain length in this compound, the maltose headgroup plays a dominant effect with its high solubility in water properties resulting in large display of normal micellar solution, L1 in the excess water region. Comparing the solubility of the maltose-based glycolipid with those of the corresponding glucose counterpart, the solubility of the former is substantially higher.21,37 The surface activity which reflects the strong intramolecular H-bond-driven cohesive forces between sugar headgroup with water will be studied in a separate work. For β-Mal-C8C4/water system (Figure 8(b)), the anisotropic behavior appeared between 5–67.5% (w/w) at all temperature studied with the excess water point estimated to be around 37.5% (w/w). SAXS data confirm the existence of lamellar phase in this region with the lattice parameter
10
ACS Paragon Plus Environment
Page 11 of 15
65
55 45 35 25
55
fluid isotropic
45 35 25
0
65
10 20 30 40 50 60 70 80 90 100 Water Content (%)
anisotropic
0
65
anisotropic + water
Temperature ( C)
Temperature ( C)
anisotropic
anisotropic + water
Temperature ( C)
fluid isotropic
anisotropic
Temperature ( C)
65
55 45 35
10 20 30 40 50 60 70 80 90 100 Water Content (%)
anisotropic
anisotropic + water
55 45 35 25
25
0
65 Temperature ( C)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
0
10 20 30 40 50 60 70 80 90 100 Water Content (%)
anisotropic
10 20 30 40 50 60 70 80 90 100 Water Content (%)
Figure 8. Partial binary phase diagrams of β-Dmaltosides/water on heating: (a) β-Mal-C6C2; (b) β-Mal-C8C4; (c) β-Mal-C10C6; (d) β-Mal-C12C8 and (e) β-Mal-C14C10. Various phases identified from the SAXS data are marked on
anisotropic + water
55
the phase diagrams i.e. (L1); (Lα) and (L1 with Lα phase). The coexistence of different phases is also represented by overlapping these notations. Polarizing microscopy results are shown in the shaded areas, denoting three distinct shaded regions: anisotropic, fluid isotropic and anisotropic + water. The excess water points are represented by dashed lines.
45 35 25
0
10 20 30 40 50 60 70 80 90 100 Water Content (%)
ranging from 32–41 Å. Samples beyond 67.5% (w/w) exhibit fluid isotropic phase between crossed polarizer sheets. However, reflection of lamellar phase was observed at 70% (w/w) from SAXS measurement. A single and broad spectrum peaked at q of ca. 0.20Å-1 with lattice parameter of 31.1 Å was observed for β-Mal-C8C4 at 90% (w/w). This is consistent with the isotropic appearance of the sample. Interestingly, the SAXS scan gave a trace amount of possibly a Lα phase with a d-spacing (lattice parameter) of 39.8 Å. This could be the results of local drying. The preliminary lyotropic investigation of β-Mal-C8C4 using an optical polarizing microscope shows a birefringent texture of Lα phase (see Figure 3(d)). Given the fully hydrated sample was equilibrated for at least 14 days, the shorter equilibration
times in OPM lyotropic experiment may have disallowed clear visualization of the L1 phase since it may take times to form. As shown in Figure 8(c–e), the Lα phase largely dominates the phase behavior of the middle and longer branched-chain maltosides i.e. β-Mal-C10C6, β-Mal-C12C8 and β-Mal-C14C10 in which this phase formation is usually governed by the hydrated headgroup. The excess water point for β-Mal-C10C6 is approximately 40% (w/w), while for βMal-C12C8 and β-Mal-C14C10, this occurs at slightly lower water content i.e. 37.5% (w/w) and 30% (w/w) respectively. The lattice parameter of the Lα phase ranging from 35–40 Å, 38–43 Å and 42–46 Å respectively. Evidently, an increase in chain length causes an increase in lattice parameter which is
11
ACS Paragon Plus Environment
Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 12 of 15
Table 2. Phase assignments and lattice parameters for the dry and fully hydrated β-D-maltosides at 25°C. Error in lattice parameter measurement is ±0.1Å. The asterisk, * indicates trace amount. Dry
Fully hydrated
Lipid Phase
Lattice parameter (Å)
Phase
Lattice parameter (Å)
β-Mal-C6C2
Lα
28.0
L1
26.7
β-Mal-C8C4
Lα
29.0
L1, Lα*
31.1, 39.8*
β-Mal-C10C6
Lα
31.6
Lα
40.2
β-Mal-C12C8
metastable
-
Lα
42.9
β-Mal-C14C10
Ia3d
89.7
Lα
46.3
accompanied by a shift in the scattering peak position toward lower q values. Such behavior in lyotropic condition for alkyl maltosides have been reported.16,37 Unlike the longer chain Guerbet monosaccharides i.e. βxylosides and β-glucosides, (with three and four OH groups respectively) which exhibits stable non-lamellar phases such as V2 and H2 in excess water, the corresponding Guerbet disaccharide i.e. β-maltosides (with seven OH groups) favors the formation of Lα phase. This is due to all hydroxyls in maltose is surrounded by extensive water shell (hydration ability), hence, increasing the headgroup area triggering the formation of the lamellar structure in their fully hydrated system.43 The formation of hydrogen bonding between water molecules and the seven OH groups in the headgroup results in larger effective headgroup area diminishing the hydrophobic chain effect and hence, decreases the CPP to 1 (zero mean curvature). The results imply that the Guerbet maltosides have the ability to act as membrane forming or membrane stabilizing compounds. The lattice parameters for the self-assembly structures in dry and in excess water are listed in Table 2. The temperature dependence of the lattice parameter at selected concentrations for all phases observed in Guerbet β-Dmaltoside/water phase diagrams is shown in Table S1–S5 of the Supporting Information. The data show that all phases is observed to be invariant with temperature which suggests that there was no significant removal of water molecules from the sugar headgroup as a result of the strong hydrogen bond between the OH groups of the maltoside and water.51 Figure S2 of the Supporting Information illustrates the dependence of the lattice parameter of the Lα phase on the water concentration for β-Mal-C8C4, β-Mal-C10C6, β-MalC12C8 and β-Mal-C14C10. The Lα phase swells upon hydration in the following manners: β-Mal-C8C4 (38%), β-Mal-C10C6 (25%), β-Mal-C12C8 (13%) and β-Mal-C14C10 (9%) until they reach the saturation point or excess water point. Beyond this point, each phase is in equilibrium with excess water, and the lattice parameter remains constant.
In dry condition, the formation of lamellar glass, LαG was observed in the step-shaped Tg transition in DSC thermogram for all compounds. In addition, the OPM texture of Lα and the subsequent LαG formed upon cooling were found to be similar before and after Tg. The observation confirms that the molecules were frozen at Tg and adopt the molecular arrangement before the Tg transitions. The higher Tc values for Guerbet maltosides than the glucoside counterparts are due to the presence of two pyranose sugar ring as its headgroup which increases the hydrogen bond networks. The Lα phase is prominent for the first three compounds i.e. β-Mal-C6C2, β-Mal-C8C4 and β-Mal-C10C6. The fourth compound, β-Mal-C12C8 was found to be kinetically metastable in a wide range temperature for a long period of time in its dry state. On the other hand, the longest chain i.e. β-Mal-C14C10 exhibits polymorphism over a temperature scan with phase sequence as follows: Lα ↔ V2 (Ia3d) ↔ H2 which was obtained from combined observations of DSC, OPM and SAXS results. In hydrated conditions, Lα phase dominates the self-assembly of β-Dmaltosides while the micellar solution was observed in the shorter chain length compounds due to the enlargement in hydrated maltoside headgroups. The prominent Lα phase reflects a stronger hydrogen bond between water molecules with the OH groups in the maltose sugar resulting in larger effective headgroup area and hence diminishing the hydrophobic chain effect. The self-assembly behavior of both dry and fully hydrated Guerbet maltosides which exhibited glass forming abilities and their ability to act as membrane stabilizing compounds makes them ideal candidates for practical use in industry as well as biomedical research. ASSOCIATED CONTENT Supporting Information NMR data and spectra, FTIR spectra (Figure S1), composition dependence of the lattice parameter of the lamellar phase at 25 °C (Figure S2), lattice parameter of Guerbet maltosides as a function of water content and temperature (Table S1–S5). This material is available free of charge via the Internet at http://pubs.acs.org/
CONCLUSIONS In this study, the effect of chain length on thermotropic and lyotropic mesophases of Guerbet maltosides was investigated and their partial temperature−composition phase diagrams were established. These amphiphilic compounds are amphitropic with large hydrophilic headgroup that governs most of its liquid crystalline behavior. The chain length also plays a role in the phase formation as seen in the phase behavior of longer chain length that gives polymorphism in thermotropic phase study.
AUTHOR INFORMATION Corresponding Author * Telephone: (+603) 79677022 ext. 2545. Fax: (+603) 79674193. E-mail:
[email protected] 12
ACS Paragon Plus Environment
Page 13 of 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
Oligosaccharides as Hydrophilic Groups. Chem. Phys. Lipids 1994, 72, 111–118. (16) Milkereit, G.; Brandenburg, K.; Gerber, S.; Koch, M. H. J.; Morr, M.; Andrä, J.; Seydel, U.; Vill, V. Synthesis and Mesomorphic Properties of Glycosyl Dialkyl- and DiacylGlycerols Bearing Saturated, Unsaturated and Methyl Branched Fatty Acid and Fatty Alcohol Chains: Part II. Mesomorphic Properties. Chem. Phys. Lipids 2005, 135, 15–26. (17) Hato, M.; Minamikawa, H.; Salkar, R. A.; Matsutani, S. Alkylglycosides with an Isoprenoid-Type Hydrophobic Chain Can Afford Greater Control of Aqueous Phase Structures at Low Temperatures. Langmuir 2002, 18, 3425–3429. (18) Ericsson, C. A.; Ericsson, L. C.; Ulvenlund, S. SolidState Phase Behaviour of Dodecylglycosides. Carbohydr. Res. 2005, 340, 1529–1537. (19) Hashim, R.; Hashim, H. H. A.; Rodzi, N. Z. M.; Hussen, R. S. D.; Heidelberg, T. Branched Chain Glycosides: Enhanced Diversity for Phase Behavior of Easily Accessible Synthetic Glycolipids. Thin Solid Films 2006, 509, 27–35. (20) Liew, C. Y.; Salim, M.; Zahid, N. I.; Hashim, R. Biomass Derived Xylose Guerbet Surfactants: Thermotropic and Lyotropic Properties from Small-Angle X-Ray Scattering. RSC Adv. 2015, 5, 99125–99132. (21) Zahid, N. I.; Conn, C. E.; Brooks, N. J.; Ahmad, N.; Seddon, J. M.; Hashim, R. Investigation of the Effect of Sugar Stereochemistry on Biologically Relevant Lyotropic Phases from Branched-Chain Synthetic Glycolipids by Small-Angle Xray Scattering. Langmuir 2013, 29, 15794–15804. (22) Patrick, M.; Zahid, N. I.; Kriechbaum, M.; Hashim, R. Guerbet Glycolipids from Mannose: Liquid Crystals Properties. Liq. Cryst. 2018, DOI: 10.1080/02678292.2018.1492036. (23) Claesson, P.; Krustev, C. S., R.; Johansson, I. Thin Film and Foam Properties of Sugar-Based Surfactants. In SugarBased Surfactants: Fundamentals and Applications, Ruiz, C. C., Ed.; CRC Press: Boca Raton, 2008, pp 144. (24) Brooks, N. J.; Hamid, H. A. A.; Hashim, R.; Heidelberg, T.; Seddon, J. M.; Conn, C. E.; Husseini, S. M. M.; Zahid, N. I.; Hussen, R. S. D. Thermotropic and Lyotropic Liquid Crystalline Phases of Guerbet Branched-Chain β-DGlucosides. Liq. Cryst. 2011, 38, 1725–1734. (25) Loewenstein, A.; Igner, D. Deuterium NMR Studies of n-Octyl α and β–Glucopyranoside Liquid-Crystalline Systems. Liq. Cryst. 1991, 10, 457–466. (26) Garidel, P.; Kaconis, Y.; Heinbockel, L.; Wulf, M.; Gerber, S.; Munk, A.; Vill, V.; Brandenburg, K. SelfOrganisation, Thermotropic and Lyotropic Properties of Glycolipids Related to their Biological Implications. Open Biochem. J. 2015, 9, 49–72. (27) Elliott, S. R. Physics of Amorphous Materials; Longman Scientific & Technical: London, 1990, p 481. (28) Hashim, R.; Zahid, N. I.; Velayutham, T. S.; Aripin, N. F. K.; Ogawa, S.; Sugimura, A. Dry Thermotropic Glycolipid Self-Assembly: A Review. J. Oleo Sci. 2018, 67, 651–668. (29) Sagnella, S. M.; Conn, C. E.; Krodkiewska, I.; Mulet, X.; Drummond, C. J. Anandamide and Analogous Endocannabinoids: A Lipid Self-Assembly Study. Soft Matter 2011, 7, 5319–5328. (30) Kauzmann, W. The Nature of the Glassy State and the Behavior of Liquids at Low Temperatures. Chem. Rev. 1948, 43, 219–256. (31) Noel, T. R.; Parker, R.; Ring, S. G. Effect of Molecular Structure and Water Content on the Dielectric Relaxation Behaviour of Amorphous Low Molecular Weight Carbohydrates Above and Below Their Glass Transition. Carbohydr. Res. 2000, 329, 839–845.
Notes The authors declare no competing financial interest. ACKNOWLEDGEMENT The authors would like to thank the University of Malaya PPP PG030-2016A, Ministry of Higher Education Malaysia FRGS FP040-2015A and UM.C/625/1/HIR/MOHE/05 for the financial support. REFERENCES (1) Lindhorst, T. K. Essentials of Carbohydrate Chemistry and Biochemistry; Wiley-VCH: New York, USA, 2000, p 232. (2) Dziedzic, S.; Kearsley, M. Glucose Syrups: Science and Technology; Elsevier Applied Science Publishers: New York, USA, 1984, p 276. (3) Huang, B.; Kim, S.; Wu, H.; Zare, R. N. Use of a Mixture of n-Dodecyl-β-D-Maltoside and Sodium Dodecyl Sulfate in Poly(dimethylsiloxane) Microchips to Suppress Adhesion and Promote Separation of Proteins. Anal. Chem. 2007, 79, 9145–9149. (4) Kragh-Hansen, U.; Le Maire, M.; Noel, J. P.; GulikKrzywicki, T.; Moeller, J. V. Transitional Steps in the Solubilization of Protein-Containing Membranes and Liposomes by Nonionic Detergent. Biochemistry 1993, 32, 1648–1656. (5) Bujarski, J. J.; Hardy, S. F.; Miller, W. A.; Hall, T. C. Use of Dodecyl-β-D-maltoside in the Purification and Stabilization of RNA Polymerase from Brome Mosaic VirusInfected Barley. Virology 1982, 119, 465–473. (6) Ernst, B.; Hart, G. W.; Sinaÿ, P. Carbohydrates in Chemistry and Biology; Wiley-VCH: Weinheim, 2000; Vol. 1., p 535. (7) Kren, V.; Martínková, L. Glycosides in Medicine:“The Role of Glycosidic Residue in Biological Activity”. Curr. Med. Chem. 2001, 8, 1303–1328. (8) Ruiz, C. C. Sugar-Based Surfactants: Fundamentals and Applications; CRC Press: Boca Raton, 2008; Vol. 143, p 664. (9) Ogawa, S.; Asakura, K.; Osanai, S. Thermotropic and Glass Transition Behaviors of n-Alkyl β-D-Glucosides. RSC Adv. 2013, 3, 21439–21446. (10) Kocherbitov, V.; Söderman, O. Glassy Crystalline State and Water Sorption of Alkyl Maltosides. Langmuir 2004, 20, 3056–3061. (11) Shah, J. C.; Sadhale, Y.; Chilukuri, D. M. Cubic Phase Gels as Drug Delivery Systems. Adv. Drug Deliv. Rev. 2001, 47, 229–250. (12) Drummond, C. J.; Fong, C. Surfactant Self-Assembly Objects as Novel Drug Delivery Vehicles. Curr. Opin. Colloid Interface Sci. 2000, 4, 449–456. (13) Borshchevskiy, V.; Moiseeva, E.; Kuklin, A.; Büldt, G.; Hato, M.; Gordeliy, V. Isoprenoid-Chained Lipid βXylOC16+4—A Novel Molecule for In Meso Membrane Protein Crystallization. J. Cryst. Growth 2010, 312, 3326–3330. (14) Mannock, D. A.; Harper, P. E.; Gruner, S. M.; McElhaney, R. N. The Physical Properties of Glycosyl Diacylglycerols. Calorimetric, X-ray diffraction and Fourier Transform Spectroscopic Studies of a Homologous Series of 1,2-di-O-acyl-3-O-(β-D-Galactopyranosyl)-sn-Glycerols. Chem. Phys. Lipids 2001, 111, 139–161. (15) Minamikawa, H.; Murakami, T.; Hato, M. Synthesis of 1,3-di-O-alkyl-2-O-(β-glycosyl)glycerols Bearing
13
ACS Paragon Plus Environment
Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
(32) Imamura, K.; Ogawa, T.; Sakiyama, T.; Nakanishi, K. Effects of Types of Sugar on the Stabilization of Protein in the Dried State. J. Pharm. Sci. 2003, 92, 266–274. (33) Boyd, B. J.; Drummond, C. J.; Krodkiewska, I.; Grieser, F. How Chain Length, Headgroup Polymerization, and Anomeric Configuration Govern the Thermotropic and Lyotropic Liquid Crystalline Phase Behavior and the Air- Water Interfacial Adsorption of Glucose-Based Surfactants. Langmuir 2000, 16, 7359–7367. (34) von Minden, H. M.; Brandenburg, K.; Seydel, U.; Koch, M. H. J.; Garamus, V.; Willumeit, R.; Vill, V. Thermotropic and Lyotropic Properties of Long Chain Alkyl Glycopyranosides. Part II: Disaccharides Head Groups. Chem. Phys. Lipids 2000, 106, 157–179. (35) Auvray, X.; Petipas, C.; Dupuy, C.; Louvet, S.; Anthore, R.; Rico-Lattes, I.; Lattes, A. Small-Angle X-ray Diffraction Study of the Thermotropic and Lyotropic Phases of Five Alkyl Cyclic and Acyclic Disaccharides: Influence of the Linkage between the Hydrophilic and Hydrophobic Moieties. Eur. Phys. J. E 2001, 4, 489–504. (36) Vill, V.; Böcker, T.; Thiem, J.; Fischer, F. Studies on Liquid-Crystalline Glycosides. Liq. Cryst. 1989, 6, 349–356. (37) Ericsson, C. A.; Ericsson, L. C.; Kocherbitov, V.; Söderman, O.; Ulvenlund, S. Thermotropic Phase Behaviour of Long-Chain Alkylmaltosides. Phys. Chem. Chem. Phys. 2005, 7, 2970–2977. (38) Manickam Achari, V.; Bryce, R. A.; Hashim, R. Conformational Dynamics of Dry Lamellar Crystals of Sugar Based Lipids: An Atomistic Simulation Study. PLoS One 2014, 9, e101110. (39) Vill, V.; von Minden, H. M.; Koch, M. H. J.; Seydel, U.; Brandenburg, K. Thermotropic and Lyotropic Properties of Long Chain Alkyl Glycopyranosides: Part I: Monosaccharide Headgroups. Chem. Phys. Lipids 2000, 104, 75–91. (40) Koeltzow, D. E.; Urefer, A. D. Preparation and Properties of Pure Alkyl Glucosides, Maltosides and Maltotriosides. J. Am. Oil Chem. Soc. 1984, 61, 1651–1655. (41) Saupe, A. Textures, Deformations, and Structural Order of Liquid Crystals. J. Colloid Interface Sci. 1977, 58, 549–558. (42) Garidel, P.; Howe, J.; Milkereit, G.; Rössle, M.; Linser, S.; Gerber, S.; Willumeit, R.; Gutsmann, T.; Vill, V.; Brandenburg, K. Structural Polymorphism of Hydrated EtherLinked Dimyristyl Maltoside and Melibioside. Chem. Phys. Lipids 2008, 151, 18–29. (43) Hashim, R.; Sugimura, A.; Minamikawa, H.; Heidelberg, T. Nature-Like Synthetic Alkyl Branched-Chain Glycolipids: A Review on Chemical Structure and SelfAssembly Properties. Liq. Cryst. 2012, 39, 1–17.
Page 14 of 15
(44) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes; Wiley: New York, USA, 1980, p 233. (45) Noh, J.; Reguengo De Sousa, K.; Lagerwall, J. P. F. Influence of Interface Stabilisers and Surrounding Aqueous Phases on Nematic Liquid Crystal Shells. Soft Matter 2016, 12, 367–372. (46) Tenchov, B. On the Reversibility of the Phase Transitions in Lipid-Water Systems. Chem. Phys. Lipids 1991, 57, 165–177. (47) Mannock, D. A.; Collins, M. D.; Kreichbaum, M.; Harper, P. E.; Gruner, S. M.; McElhaney, R. N. The Thermotropic Phase Behaviour and Phase Structure of a Homologous Series of Racemic β-D-Galactosyl Dialkylglycerols Studied by Differential Scanning Calorimetry and X-ray Diffraction. Chem. Phys. Lipids 2007, 148, 26–50. (48) Hato, M.; Yamashita, J.; Shiono, M. Aqueous Phase Behavior of Lipids with Isoprenoid Type Hydrophobic Chains. J. Phys. Chem. B 2009, 113, 10196–10209. (49) Feast, G. C.; Lepitre, T.; Tran, N.; Conn, C. E.; Hutt, O. E.; Mulet, X.; Drummond, C. J.; Savage, G. P. Inverse Hexagonal and Cubic Micellar Lyotropic Liquid Crystalline Phase Behaviour of Novel Double Chain Sugar-Based Amphiphiles. Colloids Surf. B Biointerfaces 2017, 151, 34–38. (50) Shearman, G. C.; Ces, O.; Templer, R. H.; Seddon, J. M. Inverse Lyotropic Phases of Lipids and Membrane Curvature. J. Phys. Condens. Matter 2006, 18, S1105. (51) Stubenrauch, C. Sugar Surfactants–Aggregation, Interfacial, and Adsorption Phenomena. Curr. Opin. Colloid Interface Sci. 2001, 6, 160–170.
14
ACS Paragon Plus Environment
Page 15 of 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
TOC Graphic
Thermotropic
Self-assembly
Lyotropic
Increasing Guerbet chain
15
ACS Paragon Plus Environment
