Swelling of Bicontinuous Cubic Phases in Guerbet Glycolipid: Effects

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Swelling of bicontinuous cubic phases in Guerbet glycolipid: Effects of additives Malinda Salim, Wan Farah Nasuha Wan Iskandar, Melonney Patrick, N. Idayu Zahid, and Rauzah Hashim Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b01007 • Publication Date (Web): 16 May 2016 Downloaded from http://pubs.acs.org on May 23, 2016

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Swelling of bicontinuous cubic phases in Guerbet glycolipid: Effects of additives Malinda Salim,† Wan Farah Nasuha Wan Iskandar,† Melonney Patrick,† N. Idayu Zahid,† Rauzah Hashim†* †

Center of Fundamental Science of Self-Assembly, Department of Chemistry, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia

ABSTRACT: Inverse bicontinuous cubic phases of lyotropic liquid crystal self-assembly have received much attention in biomedical, biosensing, and nanotechnology applications. An Ia3d bicontinuous cubic based on the gyroid G-surface can be formed by Guerbet synthetic glucolipid 2-hexyl-decyl-β-D-glucopyranoside (β-Glc−OC6C10) in excess water. The small water channel diameter of this cubic phase could provide nanoscale constraints in encapsulation of large molecules and crystallization of membrane proteins, hence stresses the importance of water channel tuning ability. This work investigates the swelling behavior of lyotropic self-assembly of β-Glc−OC6C10 which could be controlled and modulated by different surfactants as hydration-modulating agent. Our results demonstrate that addition of non-ionic glycolipid octyl-β-D-glucopyranoside (β-Glc−OC8) at 20 and 25 mol% gives the largest attainable cubic water channel diameter of ca. 62 Å; and formation of coacervates which may be attributed to a sponge phase were seen at 20 mol% octyl-β-D-maltopyranoside (β-Mal−OC8). Swelling of the cubic water channel can also be attained in charged surfactantdoped systems dioctyl sodium sulfosuccinate (AOT) and hexadecyltrimethylammonium bromide (CTAB), of which phase transition occurred from cubic to a lamellar phase. Destabilisation of the cubic phase to an inverse hexagonal phase was observed when high amount of charged lecithin (LEC) and stearylamine (SA) was added to the lipid selfassembly. Keywords: Guerbet glycolipid, cubic phase, liquid crystal, additives, sponge phase, swelling

(MO), this compound does not contain an ester moiety that is susceptible to hydrolysis at basic or acid conditions. Nonionic β-Glc−OC6C10 self-assembled into an inverse hexagonal phase (HII) in its ‘dry’ state, and transformed into V2 phase on increased hydration.10,11 From Figure 1, an excess water point was estimated to be 32 wt% water, and Ia3d/Pn3m cubic bicontinuous phases were observed up to 80 wt% water content.11 In a separate study, a single phase Ia3d was observed in excess water content of 95 wt% at 25°C. 10 Formation of an Ia3d phase in excess water is uncommon as the G-surface is the most compact space filler. As such, Gsurface is normally observed at lower water content in other lipids such as MO and phytantriol (PHYT); and Pn3m phase is formed when the lipids are fully hydrated.12, 13 The extent of swelling was however different in both MO and PHYT as can be observed in the smaller lattice parameter in fully swollen PHYT (68 Å versus 94 Å in MO).14 The lower swelling ability of PHYT in water was attributed to the more hydrophobic character of the PHYT.15 Cubic phases and the dispersion in MO and PHYT systems have been used in several applications such as biosensing, drug delivery and membrane protein crystallization.16,17,18,19 The ability to tune the size of the water channels in V2 phases is important to obtain a better control of the extent of drug and protein loading, as well as the release behavior.20,21,22,23 Moreover, increasing the water channel size of the V2 phase allows loading of larger compounds. It has been reported that an increased hydration of the Pn3m phase in MO and PHYT, with a widening of the water channel diameter can be achieved via addition of hydrating modulating agents such as charged lipids and peptides as well as non-ionic surfactants.24,25,26,27,28

INTRODUCTION The global market for surfactant is expected to increase to USD 42 billion by 2020.1 This has created the demand for niche, biocompatible and environmental friendly base materials, and improved tunable formulation media for numerous applications. Amphoteric sugar-based surfactants are amongst suitable candidates fulfilling the green features, especially those which give rise to the non-lamellar phases, including the bicontinuous cubic phase. A cubic bicontinuous mesophase can be formed from mixtures of lipid or surfactant with water, where the inverse phase (V2) consists of singlesheeted hyperbolic bilayer immersed in intertwining and interconnected water continua.2 Such saddle-shaped surfaces have zero mean curvature at all points (minimal surface), which, when extended to fill space periodically, forms an infinite period minimal surface (IPMS).3,4 Three basic types of cubic IPMS structures are the simplest Schwarz’s primitive P-surface (Im3m), Schwarz’s diamond D-surface (Pn3m), and the Schoen’s G-surface (Ia3d).5,6 The interfacial mean curvature of these inverse phases is negative, a self-assembly feature commonly observed in double-chained amphiphiles with relatively large hydrophobic domain.3,7 Guerbet branched chain glucolipid namely 2-hexyl-decylβ-D-glucopyranoside (β-Glc−OC6C10), contains two asymmetric hydrocarbon chains that differ by two methylene units.8,9 The corresponding partial binary phase diagram of βGlc−OC6C10/water system from 0–80 wt% water is shown in Figure 1. The two chains are branched at the β-carbon position, where an ether linkage connects the hydrophobic tail to the glucose at the first carbon position C1 (see Figure 2 for the chemical structure of β-Glc−OC6C10).9 Unlike monoolein

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Figure 1. Pseudo-binary phase diagram of 2-hexyl-decyl-β-D-glucopyranoside in water as a function of temperature reproduced from Zahid et al.11 The mesophases identified from SAXS data are marked on the phase diagrams (i.e., green ▲) to denote HII, (×) (Ia3d), (◊) (Pn3m). The coexistence of phases is also marked by combining these notations, thus (◊ with ×) (Ia3d + Pn3m). Polarizing microscopy results are shown in the shaded areas, denoting three distinct regions: anisotropic, viscous isotropic and viscous isotropic + water. The excess water points are represented by dashed line. A stable Ia3d cubic phase was observed in the excess water region.

As has been shown in our previous study, the lattice parameter of a fully hydrated β-Glc−OC6C10 after 3−5 day equilibration was ca. 74 Å, having an averaged water channel diameter of 34 Å.10 The small size of the water channel could limit the application of this glucolipid in many applications such as encapsulation of large biomolecules and membrane protein crystallization. Therefore, in the present work, we aim to induce the swelling of the water channels in Ia3d phase by investigating the influence of several additives on the hydration and phase behavior of β-Glc−OC6C10 using a small angle X-ray scattering (SAXS). The additives of study are non-ionic octyl-β-D-glucopyranoside (β-Glc−OC8), octyl-β-D-maltopyranoside (β-Mal−OC8); cationic surfactant hexadecyltrimethylammonium bromide (CTAB) and stearylamine (SA); anionic dioctyl sodium sulfosuccinate (AOT); and zwitterionic soybean lecithin (LEC). Chemical structures of the additives are shown in Figure 2. This study allows us to understand the hydration and swelling properties of β-Glc−OC6C10 for better utilization of the compound in potential future applications.

is commonly used as charge-inducer in the preparation of non-ionic surfactant vesicles.29 β-Glc−OC8 and β-Mal−OC8 are alkyl glycosides that can be used in membrane protein crystallization.30 All the chemicals were used without further purification. Distilled and deionized water (18 MΩ-cm) was used throughout the experiments. Sample Preparation. Blank sample containing 40 mg freeze-dried β-Glc−OC6C10 in 1 ml water (95 wt% water content) was prepared, followed by homogenization through repeated heating (to 70°C) /cooling (to 25°C) cycles and centrifugation. The sample was equilibrated for 2 weeks prior to SAXS analysis. In the mixed samples, appropriate amount of additive ranging from 1–20 mol%, was added to 40 mg β-Glc−OC6C10 and dissolved in methanol (for βGlc−OC8, β-Mal−OC8, CTAB, AOT) or 2:1 v/v chloroform:methanol mixture (for LEC, SA). The solvent was subsequently evaporated and the samples vacuum dried. 95 wt% water (to β-Glc−OC6C10 ratio) was added to the samples, and the resulting water/surfactant mixtures were homogenized by repeated heating/cooling cycles and centrifugation, followed by equilibration at room temperature for ≥ 2 weeks. The synthetic glycolipid was chemically stable after heat/cool cycles, as the transitions (between liquid crystal and isotropic phase) were reversible. The additives were also stable during the temperature range studied, as no degradations were observed from the 1H-NMR spectra (results not shown). Measurement. Small angle X-ray scattering (SAXS) experiments were carried out on SAXSpace (Anton Paar, Austria) equipped with an X-ray tube generating Cu-Kα radiation (λ=1.542 Å) at 40 kV and 50 mA. The sample to detector distance was 317 mm, and the scattering patterns were recorded on a 1-D diode detector in line collimation mode. Measurements were taken at 25°C, where the

EXPERIMENTAL SECTION Materials. β-Glc−OC6C10 (MW: 404 g/mol) was inhouse synthesized by procedures described previously,8 and has an anomeric purity of ca. 98% as determined using 1HNMR. Octyl-β-D-glucopyranoside (β-Glc−OC8, ≥98%), octyl-β-D-maltopyranoside (β-Mal−OC8, ≥99%), hexadecyltrimethylammonium bromide (CTAB, 95%), stearylamine (SA, 97%), soybean lecithin (LEC), and 1,2Distearoyl-sn-glycero-3-phosphocholine (DSPC, ≥99%) were purchased from Sigma-Aldrich Malaysia. Dioctyl sodium sulfosuccinate (AOT) was from R&M Chemical. SA

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Figure 2. Chemical structures of Guerbet branched chain glucolipid 2-hexyl-decyl-β-D-glucopyranoside and the additives of study.

𝑟𝑤 for 𝐼𝑎3𝑑 = 0.248 𝑎 − 𝑙 𝑟𝑤 𝑓or 𝑃𝑛3𝑚 = 0.391𝑎 − 𝑙 𝑟𝑤 for 𝐼𝑚3𝑚 = 0.305𝑎 − 𝑙

temperature of the samples was controlled by a Peltier system (TCStage 150). Each sample was equilibrated for 30 minutes at the desired temperature prior to a 1-hour acquisition time. The data was calibrated by normalizing the primary beam in the SAXStreat software, and desmeared (correction for slit-smeared scattering intensity) in the SAXSquant software. Space group assignment of the liquid crystal phases and the corresponding lattice parameter was determined using an SGI program (Space Group Indexing, V.03.2012). The unit cell or lattice parameter (a) of the liquid crystal mesophase was confirmed through the following calculations, where, in a cubic phase:31 𝑎=

√ℎ2 +𝑘 2 +𝑙 2 𝑠

“𝑙” is the lipid chain length or monolayer thickness, which can be calculated from the volume fraction (𝜙𝑐ℎ ) of the lipid hydrocarbon chain:

𝑞 2𝜋

𝑙

4

𝑙 3

𝑎

3

𝑎

𝜙𝐶𝐻 = 2𝜎 ( ) + 𝜋𝜒 ( ) 𝜙𝐶𝐻 =

𝑛𝑙 𝜐𝐶𝐻 𝑛𝑙 (𝜐𝐶𝐻 +𝜐ℎ𝑒𝑎𝑑 )+𝑛𝑤 𝜐𝑤

(4) (5)

where 𝜒 is the Euler characteristic of the infinite periodic minimal surface geometries (𝜒Ia3d = –8, 𝜒Pn3m= –2, 𝜒Im3m= – 4). 𝜎 is the dimensionless constant that represents the surface area per unit cell with a lattice parameter of unity (𝜎Ia3d= 3.091, 𝜎Pn3m= 1.919, 𝜎Im3m= 2.345). 𝑛𝑙 and 𝑛𝑤 is the number of moles of lipid and water respectively. 𝜐𝑤 is the molar volume of water (29.9 Å). 𝜐𝐶𝐻 and 𝜐ℎ𝑒𝑎𝑑 is the respective molar volume of hydrocarbon chain (479 Å3 ) and lipid headgroup (193.2 Å3 ) for β-Glc−OC6C10. 𝜐𝐶𝐻 values were calculated based on the molecular volumes of CH (20 Å3 ), CH2 (27 Å3 ), CH3 (54 Å3 ).11,33 𝜐ℎ𝑒𝑎𝑑 was estimated from the density of glucose (1.54 g/cm3), and molecular weight of the headgroup (179 g/mol), whereby: 11,33

(1)

“h, k, l” are the Miller indices. “𝑠” is the reciprocal spacing whereby: 𝑠=

(3)

(2)

with 𝑞 = length of the scattering vector (Å -1). Diameter of the water channel in a bicontinuous cubic phase (𝐷𝑤 ) was calculated from the radius of the water channel (𝑟𝑤 ) that was estimated from lattice parameter based on minimal surfaces as derived by Briggs et al.:32

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𝑣ℎ𝑒𝑎𝑑 =

𝑚ℎ𝑒𝑎𝑑 𝜌ℎ𝑒𝑎𝑑

=

𝑀𝑤 ℎ𝑒𝑎𝑑 /𝑁𝐴

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(6)

𝜌ℎ𝑒𝑎𝑑

where 𝑚ℎ𝑒𝑎𝑑 , 𝜌ℎ𝑒𝑎𝑑 and 𝑀𝑤 ℎ𝑒𝑎𝑑 are mass, density and molecular weight of the headgroup respectively. 𝑁𝐴 is the Avogadro’s number. The calculated water channel diameter of the fully hydrated cubic phase was also compared with the Garstecki and Holyst (GH) model,34 of which the intensities in the experimental X-ray pattern was fitted to the model scattering intensities 𝐼ℎ𝑘𝑙 (𝐿) throughout all the hkl indices by varying the 𝐿∗ (dimensionless lipid bilayer thickness) value. The best fitting was obtained by minimizing the sum of the squared intensity differences. 𝑆∗ 𝐼ℎ𝑘𝑙 (𝐿) = 𝑀ℎ𝑘𝑙 [𝐹ℎ𝑘𝑙

𝑠𝑖𝑛(𝛼ℎ𝑘𝑙𝜋(ℎ2 +𝑘 2 +𝑙 2 )

0.5 ∗ 2 𝐿 )

𝛼ℎ𝑘𝑙 2𝜋(ℎ2 +𝑘 2 +𝑙 2 )0.5

]

(7)

𝑆∗ The values for 𝐹ℎ𝑘𝑙 (dimensionless structure factor), 𝑀ℎ𝑘𝑙 (multiplicity factor), and 𝛼ℎ𝑘𝑙 (correction parameters for particular cubic lattices) of the G structure were taken from reference 34. The lipid bilayer thickness 𝐿 of the cubic phase was calculated from Equation 8, followed by the water channel diameter 𝐷𝑤 .34,35

𝐿∗ = 𝐿⁄𝑎 𝐷𝑤 = 0.707𝑎 − 𝐿

(8)

RESULTS AND DISCUSSION Phase behavior of β-Glc−OC6C10. β-Glc−OC6C10 is a non-ionic surfactant with a viscous texture that is insoluble in water. It has two asymmetric hydrocarbon chains (C6 and C10) attached to a hydrophilic glucose headgroup by an ether bond, having an overall hydrophilic–lipophilic balance (HLB) of 8.9 as calculated using Griffin’s method. 36 This amphiphilic molecule has a large critical packing parameter CPP (>1), as evidenced in the formation of non-lamellar V2 lyotropic mesophases. It has been previously shown that a stable Ia3d was formed in an excess water condition at 95 wt% water content, which has an average water channel diameter of 34 Å and lattice parameter of 74 Å on 3–5 days lipid/water equilibration time.10 On an increased equilibration time to a two-week period however, higher lattice parameter of 87 Å was observed, which stayed relatively stable up to 6 weeks. Therefore, all the compounds were analyzed after two weeks equilibration to ensure that the samples have reached a fully swollen state. The lipid bilayer thickness L of this fully swollen cubic phase was estimated to be ca. 22.5 Å (based on Equation 8), with a water channel diameter of about 39 Å (GH model)34 and 42 Å (minimal surfaces).32 It should however be noted that the reported water channel size is an averaged value, as it may vary locally throughout the bicontinuous network. We examine, in this study, the influence of additives on the liquid crystal structural transition, lattice parameter, and the corresponding water channel diameter using a small angle Xray scattering. The results are summarized in Figure 3, and Table S1 in supplementary material. It is observed that the stability of the Ia3d phase in β-Glc−OC6C10 is hugely dependent on the amount of solubilized additives, be it nonionic or ionic, as is described in the following subsections.

Figure 3. Phase behavior and lattice parameters of βGlc−OC6C10 on addition of non-ionic surfactants β-Glc−OC8 and β-Mal−OC8 (A); charged CTAB and AOT (B); and SA and LEC (C). Symbol representation: ‘o’ for Ia3d; ‘Δ’ for Pn3m; ‘☐’ for HII; ‘◊’ for Im3m; ‘-’ for Lα; ‘+’ for L3. Colour representation: black for blank sample; grey for β-Glc−OC8; clear (non-shaded) for β-Mal−OC8; blue for AOT; orange for CTAB; green for SA; and red for LEC. Dotted lines are drawn to guide the eye to the blank reference sample.

Effects of non-ionic surfactants on the phase behavior of β-Glc−OC6C10. β-Glc−OC8 is a straight chain synthetic glycolipid (HLB=12.3) that has been used as hydrating modulating agent in the swelling of bicontinuous cubic phase in monoolein (MO).24,30,37,38,39 Figure 4A shows the SAXS pattern of β-Glc−OC8-loaded β-Glc−OC6C10/water phase. It is evident that an increase in β-Glc−OC8 content causes a change in liquid crystal phase from Ia3d to Pn3m. The increased hydration is also accompanied by a shift in the scattering peak position towards lower 𝑞 values due to larger characteristic distance or swelling in the self-assembled system. Characteristic Bragg peak spacing ratios and the

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corresponding miller indices of Ia3d and Pn3m cubic phase are given in Table S2 in the supplementary material. At low β-Glc−OC8 content of 1 mol%, an Ia3d phase coexists with a Pn3m phase and a slight increment in the lattice parameter was observed. With an increasing molar concentration of β-Glc−OC8 content up to 10 mol%, the cubic lattice parameters changed from 87.7 to 96.0 Å (Ia3d) and 56.1 to 61.6 Å (Pn3m). The water channel diameters of the Ia3d and Pn3m phase at 10 mol% β-Glc−OC8 are ca. 47 Å. As more β-Glc−OC8 molecules are incorporated into the Guerbet glucolipid at 20−25 mol%, the cubic Pn3m phase remains stable, and about 43% increase in the water channel diameter to ca. 61 Å was observed. This swelling could be attributed to the increase in the effective headgroup area of the resultant lipid mixture, thus decreasing the CPP and magnitude of the negative spontaneous curvature.25 The lipid interfacial curvature is controlled by molecular shape of the lipid or surfactant described by CPP (Equation 9), 40 temperature, water content, and presence of guest molecules.26 In this equation, 𝑣 is the molecular volume of the hydrocarbon chain, 𝑎0 is the interfacial area per molecule at the polar-non polar interface, and 𝑙 is the length of the fully extended hydrocarbon chain. Small CPP (1) and CPP~1 prefers the formation of an inverse (type II) liquid crystal and planar bilayers respectively. 𝐶𝑃𝑃 =

𝑣 𝑎0 𝑙

(9)

Additionally, SAXS pattern of 20 mol% content in Figure 4A indicates an overlapping of the sharp Bragg peaks with a broad shoulder between 𝑞= 0.23 and 0.25 Å-1 which become more pronounced in 25 mol% β-Glc−OC8. Incorporation of more β-Glc−OC8 molecules could therefore impart the instability of the cubic phase (Bragg peaks become smaller) towards a short-range order characteristic of a diffuse scattering peak.41 Comparing the effects of β-Glc−OC8 on the swelling behavior in other lipid, it can be noticed that addition of this straight-chain surfactant into MO gives rise to a more considerable swelling compared to our glucolipid. For example, the average diameter of the water channel in 10 mol% β-Glc−OC8/MO is about 70 Å (a 75% increase from 40 Å in pure MO),24 while only ca. 47 Å (an 11−12% increment) was observed in our lipid mixture. In another study, addition of 10–12 mol% β-Glc−OC8 additive was shown to be sufficient to cause a change of structure from Pn3m to Lα phase at 25°C in an MO system.30 The cubic phase in β-Glc−OC6C10 is therefore more stable to the addition of β-Glc−OC8 molecules, where more molecules could be incorporated without causing a phase transformation to an Lα phase. Influence of a non-ionic surfactant with larger (than βGlc−OC8) headgroup, i.e. octyl-β-D-maltopyranoside (βMal−OC8, a di-glucoside surfactant), on the self-assembly and hydration behaviors of β-Glc−OC6C10 was also investigated. As was seen in Figure 4B, an increase in βMal−OC8 concentration from 0−10 mol% caused a phase transition from pure Ia3d to Ia3d/Pn3m. Although similar phase transition sequence as the mono-glucoside β-Glc−OC8 additive was observed from 1–10 mol%, more swelling occurred in β-Glc−OC6C10/β-Mal−OC8 as the lattice

Figure 4. (A) Small angle X-ray scattering pattern of βGlc−OC8 -loaded β-Glc−OC6C10 in excess water from 1−25 mol%. The Ia3d phase in the blank sample (0% β-Glc−OC8) transformed to a complete Pn3m phase at 25 mol%. Reciprocal spacing ratios of the peaks in an Ia3d phase denoted by the symbol‘+’ are √6,√8,√14,√16, √20,√22, √24,√26. In Pn3m phase (symbol ‘o’): √2,√3,√4,√6, √8,√9. (B) The Xray scattering pattern of β-Mal−OC8-added β-Glc−OC6C10 in excess water from 1−20 mol%. The scattering pattern of 20 mol% β-Mal−OC8/β-Glc−OC6C10/water shows a single broad peak that reflects short-range ordered phase. (C) The Log-Log plot of 20 mol% β-Mal−OC8 to check the decay of scattering intensity for 𝑞 > 𝑞 max. The slope of the line is −1.05.

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Figure 5. (A) Coacervation process of 20 mol% β-Mal−OC8 sample: the coacervate droplets (micrograph in Figure 5B(i)) formed after vortexing (cloudy dispersion) phase separate into a bottom coacervate lipid-rich layer and an upper liquid. Figure 5B(i) and (ii) show the isotropic coacervate droplets and (iii) the formation of an Lα phase on sample drying. Figure 5C(i) shows that the dry 20 mol% βMal−OC8/β-Glc−OC6C10 sample is isotropic. A birefringent Lα layer was formed on addition of water through contact penetration scan (Figure 5C(ii) and (iii)), and an isotropic layer (see white arrow) was formed at higher water content.

parameters of the cubic phases were larger (see Figure 3 and Table S1). This could be attributed to the attraction of more water molecules towards the β-Mal−OC8 through hydrogen bonding, which increases the effective headgroup area and decreases the CPP. On addition of 20 mol% β-Mal−OC8, interesting phase behavior was observed whereby the formation of coacervate was seen, of which the viscous cubic phase transformed into liquid-like medium that did not dissolve in the excess water. This was evidenced in Figure 5A. On vigorous mixing, coacervates (see micrograph in Figure 5B(i)) were formed which then coalesced (Figure 5B(ii)) and sedimented into distinct bulk coacervate phase due to the lipid hydrophobic interaction. Coacervate has been previously described to depict and adopt an L3 phase (also known as melted bicontinuous phase or bicontinuous microemulsion);42,43,44 and the term coacervate was derived from the Latin word ‘co’ (together) and ‘acerv’ (a heap) that implies a phase separation between the upper equilibrium fluid and the bottom lipid-rich coacervate layer.42

Preliminary investigation of the 20 mol% β-Mal−OC8 sample using an optical polarizing microscope shows that both the upper fluid and bottom coacervate layers are not birefringent (Figure 5B). However, an Lα phase was observed as the coacervate layer sample starts to dry. To test the hypothesis that an Lα phase is formed at low water content, water contact penetration scan was carried out on dry 20 mol% β-Mal−OC8/β-Glc−OC6C10 using a polarizing microscope. Results shown in Figure 5C confirmed the formation of an isotropic phase at higher water content, and an Lα phase as the water content decreases. In phase diagrams, the optically isotropic sponge (L3) phase is commonly located close to a highly swollen lamellar phase, as an L3 phase was formed upon dilution of an Lα phase whereby the bilayer sheets were multi-connected.45,46,47 SAXS pattern of the coacervate gives a broad peak (no long-range crystalline order) centered at 𝑞 ca. 0.19 Å-1, with a lattice parameter or cell-cell distance of 33 Å. Although SAXS pattern of an L3 phase typically shows a broad peak at low 𝑞 value of < 0.1 Å-1 due to the large pore sizes,15,48,49 several studies have reported an L3 peak at larger 𝑞 values of

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about 0.26 Å-1 and 0.18 Å-1.35,44 Nevertheless, since bilayer structures such as Lα and L3 are known to follow a 𝑞 -2 decay (for point collimation) and a 𝑞 -1 decay (for line collimation) for 𝑞>𝑞 max, a Log-Log plot of the scattering profile is used to check the 𝑞 -1 decaying behavior.49,50 As seen from Figure 4C, the slope deduced from the linear fit of the Log-Log curve was close to –1, which suggests a characteristic bilayer structure.49 As bilayers that are isotropic and have no longrange order have been attributed to either an L3 phase or a dispersion of lamellar vesicles,51 the phase is postulated to be an L3 due to the defined meniscus between the characteristic smooth liquid-like coacervate and equilibrium water which suggests a more connected structure.43 However, additional tests such as high-resolution electron microscopy, conductivity, self-diffusion, and neutron scattering measurements are needed to further confirm the internal structure of the coacervate. Effects of charged surfactants on the phase behavior of β-Glc−OC6C10. Swelling of the water channels in bicontinuous cubic phase can be triggered by electrostatic repulsion between charged headgroups, as has been shown in several studies.15,25 In the present work, anionic surfactant (AOT), cationic surfactants (SA and CTAB), and zwitterionic lecithin (LEC) were added to the Guerbet glucolipid, and the liquid crystal self-assembly behaviour was observed. In presence of lecithin (LEC) which contains mixtures of phosphatidylcholine (PC) and phosphatidylethanolamine (PE), an increased hydration was observed at 1 mol% as evidenced in the increased lattice parameter of the Ia3d phase, and the formation of a Pn3m phase. A further increase in LEC amount (5 mol% and above) however caused a disruption of the bicontinuous cubic phase and a complete structural transition to an inverse hexagonal HII phase. Similarly, phase transformation to an HII phase was seen in 5 mol% zwitterionic DSPC (result not shown). Swelling of the lyotropic cubic phases in MO has been induced via addition of synthetic phospholipid derivatives such as PC (DOPC), phosphatidylserine DOPS, phosphatidic acid DOPA, and PE (DOPE). An Im3m phase with extensive swelling was obtained in 10 mol% DOPA,25 and an Lα phase was seen in DOPC and DOPS at concentration greater than 20 and 5 mol% respectively.52 In DOPE/MO system where the phospholipid has a large negative effective spontaneous curvature (H0 = –0.37 in water)53, an HII phase was formed at >20 mol% DOPE.52 A curvature is negative when the surfactant film bends towards the water region hence the formation of an inverse phase. Although DSPC (H0 = –0.1 in water)54 has a significantly less negative spontaneous curvature compared to DOPE, and has similar H0 value to DOPA (H0= –0.08 in water),53 no swelling towards a lamellar phase was observed in our glucolipid. This could result from the hydrogen bonding between the hydroxyl group in the glucolipid and the charged headgroup of the LEC and DSPC that results in smaller effective headgroup area and less hydration. However, it has also been reported that there exists a complex interplay between the aforementioned glucolipid-phospholipid interaction and the electrostatic attraction of charged phospholipid-water dipole moment, which complicates the lipid swelling behavior.25

Figure 6. Small angle X-ray scattering pattern of (A) 1–10 mol% CTAB- and (B) AOT-loaded β-Glc−OC6C10 in excess water. In 10 mol% AOT, two phases were observed in which a viscous cubic Im3m phase ‘10% bottom’ co-exists with an upper equilibrium fluid ‘10% upper’ that has a characteristic short range order. Reciprocal spacing ratios of the peaks in an Ia3d phase denoted by the symbol ‘+’ are √6,√8,√14,√16, √20,√22, √24,√26. In a Pn3m phase (symbol ‘o’): √2,√3,√4,√6, √8,√9. In an Im3m phase (symbol ‘*’): √2,√4,√6,√8, √10,√12, √14.

On the contrary, addition of negatively charged AOT and positively charged CTAB surfactants to β-Glc−OC6C10 gives rise to the flattening of the bilayer interface caused by greater distance between bilayers due to electrostatic repulsion. Successive formation of Ia3d – Pn3m – Im3m – Lα phase was observed with increasing CTAB and AOT content (Figure 6), which was in line with the constant mean curvature model.55 In a previous study reported by Angelov et al., 56 onion lamellar structures were obtained in the self-assembly of MO with CTAB additives stabilized by DOPE-PEG2000. Our findings also agree well with a recent study by Boyd and co-workers in a PHYT system,57 where an increased concentration of anionic AOT and cationic DDAB (didodecyldimethylammonium bromide) caused phase changes from Pn3m to Im3m to Lα, albeit the initial Pn3m phase in a fully hydrated PHYT. Furthermore, the authors showed interdependency between the self-assembly of

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charged lipid and the variation of ionic strengths in the local environment. An increased in ionic strength of the aqueous solution resulted in charge screening and a reduced electrostatic interaction between the headgroup, thus increasing the negative curvature of the lipid bilayer. In the current study where water was used as the hydration medium, electrostatic interaction between the charged headgroups was therefore maximized.25 It is however worthwhile to note that understanding the effects of external stimuli (such as temperature, pH and ionic strength of the solution) on the lipid self-assembly is important in many applications including drug delivery.

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Due to an increased swelling, lattice parameters of the cubic phase in CTAB-added system were generally higher compared to AOT (see Figure 3). At 10 mol%, a two-phase (Im3m/Lα) region was observed in the AOT system, while a complete transformation to fluid Lα phase (characterized by its haziness, highly fluid, low viscosity, and optically birefringent)58 has taken place in the CTAB-added sample. The Lα phase was confirmed by the oily streaks and Maltese crosses in the polarizing microscope textures shown in Figure 7.6 Small angle X-ray analysis of the samples (10% CTAB and AOT, Figure 6) give a single broad peak at 𝑞~ 0.24 Å-1, which may signify the lack of long-range order of the lipid bilayer.43 Although an increased lipid hydration can be obtained in AOT and CTAB-doped system through electrostatic repulsion of the headgroup that progressively leads to an Lα phase, addition of positively charged SA at ≥10 mol% resulted in the formation of an HII phase. No significant water channel swelling was also observed at 5 mol% SA, as indicated by a slight increment in the lattice parameter of the cubic Ia3d and presence of the small Pn3m peaks. The observations obtained from our results can be ascribed to the significantly higher HLB value of CTAB (21.4) 59 compared to AOT (13.5), and the low HLB values of SA (7.9) 60 and LEC (8.0).61 HLB values were used to relate the relative affinity of surfactant molecules towards oil and aqueous solution, where a more lipophilic surfactant has lower HLB number, and vice versa.36 These numbers can be estimated from the Griffin’s method (for nonionic surfactant)36 or the Davies’ method (for ionic surfactant),62 and can be used to preliminary assess the relative hydrophobicity of the surfactant additives hence the effects on the self-assembly of β-Glc−OC6C10. Although HLB term is usually used in emulsion studies, it shares the same basic concept, and is related to CPP that connects molecular structure to the packing arrangements and film curvature.63 Based on the general HLB-CPP correlation curve for surfactants,63 the CPP of LEC and SA was estimated to be greater than 1. Thus, inclusion of LEC and SA molecules (low HLB and high CPP) into β-Glc−OC6C10 drives the lipid interface to curve more towards water (more negative mean curvature). Direction of the shift in curvature for the different additives was highlighted and graphically presented in Figure 8 for ease of comparison. Charged surfactants CTAB and AOT induced a greater decrease in negative curvature followed by the non-ionic glycosides, while the more hydrophobic LEC and SA increased the negative curvature of the lipid mixtures towards the formation of an HII phase.

Figure 7. Images of birefringent Lα phase in (A) 10% CTAB and (B) 10% AOT upper phase recorded using polarized light microscopy. (C) The samples viewed placed between crossed-polarizer sheets.

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Figure 8. (A) Liquid crystal self-assembly of lamellar Lα and inverse phase Im3m, Pn3m, Ia3d, HII, LII. (B) Effects of additives on lipid curvature and the direction of the phase changes.

X-ray scattering pattern of cholesterol (CHL)-loaded βGlc−OC6C10 (Figure S1), table of effects of additives on the liquid crystal phase, lattice parameter, and the corresponding water channel diameter (Table S1), table of Bragg peak spacing ratios and the corresponding Miller indices for bicontinuous cubic phase Ia3d, Pn3m, and Im3m (Table S2). This material is available free of charge via the Internet at http://pubs.acs.org/

CONCLUSIONS β-Glc−OC6C10 is a Guerbet branched chain synthetic glucolipid that forms a bicontinuous cubic phase of an Ia3d space group in excess water. This cubic phase could potentially be useful in a range of applications including but not limited to active compound delivery and as a template for membrane protein crystallization. In this work, we investigate the effects of different additives, both charged and uncharged, on the phase behavior and hydration properties of the glucolipid. It can be concluded that addition of guest molecules to β-Glc−OC6C10 /water system has a profound effect on the lipid self-assembly structures. However, enlargement of the bicontinuous cubic water channels in β-Glc−OC6C10 was lower compared to commonly used monoolein lipid. The largest attainable water channel size was ca. 61 Å in the Pn3m phase of 25 mol% βGlc−OC8, while an astounding 250 Å was obtained in the Im3m phase of MO/5 mol% DOPS/30 mol% cholesterol mixture.25 Although cholesterol (CHL) has been shown to increase the hydration of MO headgroups, 25 the very high negative spontaneous curvature (H0 = –0.5 in water)54 triggered the formation of an HII phase at low concentration of 5 mol% (result shown in supplementary material Figure S1). The difference in water swelling capacity between βGlc−OC6C10 and MO could be attributed to the hydrocarbon chain branching (in β-Glc−OC6C10) that makes the Guerbet glucolipid less flexible.

AUTHOR INFORMATION Corresponding Author * Telephone: (+603) 79674009. Fax: (+603) 79674193. Email: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENT The authors would like to thank the University of Malaya FRGS FP007-2014B, and the Ministry of Higher Education High Impact Research Grant UM.C/625/1/HIR/MOHE/05 for financial support. ABBREVIATIONS β-Glc−OC6C10, 2-hexyl-decyl-β-D-glucopyranoside; βGlc−OC8, octyl-β-D-glucopyranoside; β-Mal−OC8, octyl-βD-maltopyranoside; CTAB, hexadecyltrimethylammonium bromide; SA, stearylamine; AOT, dioctyl sodium sulfosuccinate; LEC, soybean lecithin; CHL, cholesterol; DSPC, 1,2-Distearoyl-sn-glycero-3-phosphocholine; DOPC,

ASSOCIATED CONTENT Supporting Information

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Swelling of Bicontinuous Cubic Phases in Guerbet Glycolipid: Effects

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