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Self-Assembly and Chiral Recognition of Chiral Cationic Gemini Surfactants Lili Zhou, Jiling Yue, Yaxun Fan, and Yilin Wang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02599 • Publication Date (Web): 11 Oct 2018 Downloaded from http://pubs.acs.org on October 15, 2018

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Langmuir

Self-Assembly and Chiral Recognition of Chiral Cationic Gemini Surfactants Lili Zhou,†,‡ Jiling Yue,§ Yaxun Fan,*,† Yilin Wang*,†,‡,§ †

CAS

Key

Laboratory

of

Colloid,

Interface

and

Chemical

Thermodynamics,

CAS

Research/Education Center for Excellence in Molecular Sciences, and §Beijing National Laboratory for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China. ‡

University of Chinese Academy of Sciences, Beijing 100049, P. R. China.

ABSTRACT:

Chiral

cationic

gemini

surfactants

1,4-bis(dodecyl-N,N-dimethylammonium

bromide)-2,3-butanediol (12-4(OH)2-12) including racemate, mesomer and two enantiomers were synthesized, and their self-assembly in aqueous solution has been comparatively investigated by tensiometry, conductometry, 1H NMR, SANS, Cryo-TEM and Cryo-SEM. The chirality at spacer induces different self-assembly behaviors due to the hydrogen-bonding interaction between the hydroxyl groups at the chiral centers. The stereochemistry of the spacer has little effects on the release of the counterions from the surfactant headgroups and on the molecular packing at air-water interface. The critical micelle concentration (CMC) decreases in the order of racemate > enantiomer > mesomer. Above the CMC, the aggregates of enantiomers transit from small spherical micelles to rodlike and wormlike micelles with increasing the concentration, while the mesomer and racemate aggregates transform from spherical micelles to rodlike micelles and platelet-like aggregates. The differences may be caused by the reason that the mesomer and racemate molecules mainly form intermolecular hydrogen-bonds between the -OH groups, but the enantiomer molecules dominantly form intramolecular hydrogen-bonds. Furthermore, it was found that the chiral micelles formed by 1

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the enantiomers exhibit enantioselection ability for bilirubin enantiomers (BR). The recognition capability can be adjusted by the micellar structure, i.e., the rodlike micelles are better than either small spherical micelles or wormlike micelles, which might possess different chiral cavities, controlling BR shape and location. These results demonstrate that the aggregates of chiral gemini surfactants can be used to mimic the chiral recognition in biological membrane.

INTRODUCTION Chiral surfactants are one kind of amphiphiles with chiral centers, bearing the properties of conventional surfactants and chiral molecules simultaneously. The two characteristics play a significant role in the formation and properties of their self-assembled structures. As surfactants, they can self-assemble into a rich variety of assemblies ranging from micelles, vesicles to highly organized fibers, ribbons, helices and tubes.1-3 Meanwhile, chirality is intimately associated with the stability of aggregates and promotes the growth of aggregates, because it has a structure-directing power to promote the organization of monomers into aggregates.4-5 Moreover, the essence of chirality endows the aggregates with widespread applications in stereoselective synthesis,6-9 chiral separation10-12 and the preparation of chiral nanostructures.13-17 In addition, their chiral self-assembled structures, such as micelles, liposomes or Langmuir monolayers, can be applied as simple models to mimic biomembranes18-20 and chiral recognition.21-23 Taking the growing applications into account, a great deal of effort has been devoted to develop new chiral surfactants,24-26 and the effects of stereochemistry on the self-assembly of chiral surfactants are being highly sought. So far, the investigation of chiral surfactants has been focused on chiral single-chain surfactants 2


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and the influence of chiral headgroup stereochemistry on their self-assembly.26-32 Fuhrhop et al.2 studied the aggregation behavior of eight diastereomeric N-octylaldonamides, three enantiomers and the corresponding racemates. It was found that the solubility and aggregate structures of the surfactants depend on the stereochemistry of the polyol headgroups. Undisturbed all-anti chains lead to “whisker”-type aggregates, a bend close to the amide group produces extremely thin helical whiskers of high curvature, and a twist at the end of chain results in larger water solubility and prevents fiber formation. Barclay et al.30 prepared chiral surfactants with glutamic acid headgroup, and found that the left-handed twisted ribbons assemble from the L-isomer and the right-handed twist ribbons from D-isomer. When the D- and L-isomers are mixed at a 1:1 ratio, nanotubes are the dominant morphology. Ambrosi et al.31 studied the surfactants taking L-ascorbic and D-isoascorbic acid residues as hydrophilic headgroups, and indicated that the L-ascorbic acid surfactants form intermolecular hydrogen-bonds, while the D-isoascorbic acid surfactants mainly form intramolecular hydrogen-bonds. The inter- and intra-molecular interactions lead to different hydration and ionization capacity of the polar heads. Sorrenti et al.32 constructed chiral cyclobutane β-amino acid surfactants, whose cis configuration forms intramolecular hydrogen-bonds, while trans configuration presents different hydrogen-bonding patterns. The molecules with cis configuration exhibit better stabilization and larger critical micellar concentration (CMC), and form larger spherical micelles and weaken the formation of insoluble long fibers. Moreover, the spherical micellar aggregates formed by either cis or trans configuration show dramatically different enantioselection ability to bilirubin enantiomers (BR). Therefore, the stereochemistry in chiral surfactants not only alters the solubility and aggregation ability, but also induces the obvious differences in aggregate morphologies and enantioselection. 3


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Compared with chiral single-chain surfactants, chiral gemini surfactants33-36 can offer more possible chiral sites in alkyl chains, headgroups, spacer and counterions.37-45 Nevertheless, only a few works have studied the effect of chirality on the properties of chiral gemini surfactants. Mancini and coworkers37-39 constructed three stereoisomeric cationic gemini surfactants with two methoxyl groups locating at chiral centers of spacer. In enantiomers, only one of the chiral groups is exposed to water and the other one is oriented toward the hydrophobic region of alkyl chains, while in mesomers, both chiral groups are exposed to water. The chirality of the spacer greatly impacts the exposure of hydrophilic headgroups to water and thus changes the assembling tendency, interface curvature, aggregate morphology, phase behavior and relaxation kinetics. More specifically, the chirality in the hydrophobic tail and counterion also affect the aggregation behavior of chiral gemini surfactants. Jaeger et al.40-41 studied the vesicular and monolayer properties of the chiral gemini surfactants with two chiral centers in the two alkyl chains, and found distinct differences in phase transition temperature, permeability and monolayer characteristics. The stereochemical difference among the surfactants is more obvious in the monolayer than in the vesicles due to the tighter surfactant packing in the former. Shankar et al.42 revealed that the enantiomeric monolayers exhibit a more condensed isotherm than the racemic one because of a homochiral interaction at air-water interface. Zhang et al.43 observed the chirality inversion of aggregate nanostructures by varying alkyl chain length of chiral gemini surfactants, which is attributed to the twisting of a headgroup. Oda and coworkers44-45 designed cationic gemini surfactants taking chiral tartrates as counterions, and found that they form twisted multilayered ribbons with chiral tartrate counterions in solution. The handedness of the ribbons depends on the configuration of the tartrate anions, and the twist pitch decreases continuously with increasing the enantiomeric excess. Obviously, chiral gemini surfactants 4


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provide more chiral centers and changeable structural factors to adjust their self-assembly and chiral properties. Thus, it is necessary to explore new stereoisomeric gemini surfactants and further understand their chirality effect on their self-assembly. In the present work, we have successfully prepared chiral cationic gemini surfactant 12-4(OH)2-12 with its racemate ((±)-12-4(OH)2-12), mesomer ((2R,3S)-12-4(OH)2-12), and enantiomers ((2R,3R)-12-4(OH)2-12 and (2S,3S)-12-4(OH)2-12) (Figure 1), following a route starting from chiral tartaric acid diethyl ester (Scheme 1).45-46 The two chiral centers locates at spacer, and hydroxyls are introduced to form intra- or inter-molecular hydrogen-bonds. Cationic gemini surfactant (±)-12-4(OH)2-12 with two hydroxyl groups at spacer has been reported before.46-49 It presents lower CMC, higher interfacial elasticity, tighter molecular packing in monolayer and more abundant aggregate structures compared with the corresponding cationic gemini surfactants with one hydroxyl group at spacer or without any hydroxyl group. However, its enantiomers and mesomer have not been reported because of the difficulty of synthesis and purification. The results in the present work show that the racemate, mesomer and enantiomers display different self-assembly behaviors at air-water interface and in bulk. Furthermore, the expression of chirality in enantiomers is investigated by studying the chiral recognition of bilirubin enantiomers (BR). Remarkably, the rodlike micelles exhibit higher recognition capabilities than small spherical and wormlike micelles formed by the same chiral surfactant. The chiral aggregates may act as simple models to study and understand the chiral recognition of biomembrane.

5


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(2R,3R)-12-4(OH)2-12 (2S,3S)-12-4(OH)2-12

(± ±)-12-4(OH)2-12

(2R,3S)-12-4(OH)2-12

enantiomer

racemate

mesomer

Figure 1. Chemical structures of the enantiomers, racemate and mesomer of chiral cationic gemini surfactant 12-4(OH)2-12.

Scheme 1. Synthesis routes for the enantiomers, racemate and mesomer of chiral cationic gemini surfactant 12-4(OH)2-12.

EXPERIMENTAL SECTION Materials. Diethyl D-tartrate, diethyl L-tartrate, tetrabutylammonium bromide (TBAB), BBr3 and bilirubin (BR) were purchased from J&K (Beijing, China). NaH and LiAlH4 were obtained from Alfa.

(±)-1,4-dibromo-2,3-butanediol

were

purchased

from

Sigma-Aldrich.

N,

N-dimethyldodeylamine and meso-1,4-dichloro-2,3-butanediol were purchased from TCI. Methyl iodide and mesyl chloride were from Energy Chemical Co. (Shanghai, China). Ion exchanger III was 6


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from Merk. All the reactants were commercially available and used without further purification. All organic solvents were purchased from Beijing Chemical Works and were dried and distilled before use. Milli-Q water (18 MΩ·cm-1) was used in all experiments. Synthesis. The enantiomers, racemate and mesomer of the chiral cationic ammonium gemini surfactant 12-4(OH)2-12 were synthesized and purified according to Scheme 1. The synthesis of these surfactants and their precursors, and the characterization data of 1H NMR (Figures S1-S9), 13C NMR (Figures S10-S18), COSY (Figures S19-S22), mass spectra, element analysis and infrared spectra are presented in the Supporting Information. 1H NMR,

13

C NMR and COSY spectra were

recorded on Brucker Avance III 400 MHz and Brucker AVIII 500 MHz NMR spectrometers, respectively. Mass spectra were recorded on a SHIMADZU LCMS-2010 spectrometer for ESI and Thermo Exactive spectrometer for APCI. Elemental analyses were performed by a Flash EA 1112 micro-analyzer. Infrared spectra were measured by Nicolet 6700 FT-IR Spectrometer. Additionally, melting points were recorded on YuHua X-5 digital melting point apparatus and were uncorrected. The optical rotation was determined by Rudolph Autopol VI Automatic polarimeter. Surface Tension Measurement. The surface tension measurements of the enantiomers, racemate and mesomer of chiral cationic gemini surfactant 12-4(OH)2-12 were separately carried out with a DCAT11 tensiometer (Dataphysics Co., Germany) equipped with a Pt/Ir plate. The measurement temperature was controlled by a thermostat at 25.0 ± 0.1 °C. The tensiometer was calibrated by pure water before each set of measurements, and the measurements error was typically ~ 0.2 mN/m. Electrical Conductivity Measurement. The conductivity of the aqueous solutions of chiral cationic gemini surfactant 12-4(OH)2-12 was recorded as a function of concentration by JENWAY model 4320 conductivity meter. The measurement was conducted in a double-walled glass container 7


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with a circulation of water at 25.0 ± 0.1 °C. This method was used to determine both the CMC and micelle ionization degree (α) of the surfactants. Small-Angle Neutron Scattering (SANS). SANS experiments were performed to determine the micelle structures of the enantiomers, racemate and mesomer of chiral cationic gemini surfactant 12-4(OH)2-12 on the SANS2D diffractometers at the ISIS pulsed neutron source in UK. The pattern of the scattering intensity with wave vector transfer, Q, where Q = 4π/λsin(θ/2), θ is the scattering angle, and λ is the neutron wavelength, can derive the information of the micelle structures. On the SANS2D diffractometer, the measurements were made using the white beam time of flight method to cover a scattering vector Q range of 0.004 to 0.30 Å-1. The samples were measured in 2 mm path-length quartz cells in D2O at 25 °C. By using standard procedures, the data were normalized to the background scattering, detector response, and spectral distribution of the incident neutron beam to establish scattering intensity I(Q) in cm−1. Cryogenic Transmission Electron Microscopy (Cryo-TEM). The aqueous solutions of the enantiomers, racemate and mesomer of 12-4(OH)2-12 were embedded in a thin liquid film on carbon-coated holey TEM grids by blotting the grid with filter paper and quickly plunging them into liquid ethane cooled by liquid nitrogen. The frozen hydrated specimens were imaged with a JEOL JEM-2010 TEM (120 kV) at about -179°C. The digital images were recorded in the minimal electron dose mode by a charge-coupled device (CCD) camera (Gantega, Olympus Soft Imaging Solutions). Cryogenic Scanning Electron Microscopy (Cryo-SEM). The aggregates of the enantiomers, racemate and mesomer of 12-4(OH)2-12 in aqueous solution were “sandwiched” between two gold planchettes, then plunged into liquid nitrogen slush. After that, the samples were transferred into a Cryo-preparation chamber (LEICA EM ACE600) under vacuum and sublimed by cooling to -100 °C 8


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for about 15 min. The frozen surface of the samples was coated with Pt to make it conductive under an argon environment (15 mA for 200 s). Then the samples were transferred to the cryostage of -137 °C in the microscope (S-4300, HITACHI, Ltd, Japan). Finally, imaging was performed using a 3 kV landing energy and 10 µA current. Circular Dichroism Spectroscopy Measurement (CD). The CD spectra of the two enantiomers of 12-4(OH)2-12 and their mixtures with bilirubin (BR) were recorded on a Jasco J-815 spectropolarimeter at 25 °C. On the basis of the concentrations used, quartz cells of light path 0.2 cm were used to keep the photomultiplier voltage below 800 V in the entire wavelength range. The samples containing BR were prepared by adding a DMSO solution of BR to the aqueous solutions of the surfactants (v/v < 2%).

RESULTS AND DISCUSSION Synthesis of Chiral Cationic Gemini Surfactants. The synthetic routes of the enantiomers, racemate and mesomer of 12-4(OH)2-12 are depicted in Scheme 1. The enantiomers follow a route starting from chiral tartaric acid diethyl ester. Hydroxyl groups were protected by methylation first for the following reactions.50 Afterwards, the precursors 5a and 5b were subsequently obtained by the reduction, bromination and then demethylation reaction.50-51 In contrast, the racemate ((±)-12-4(OH)2-12) and mesomer ((2R,3S)-12-4(OH)2-12) were directly synthesized by the reaction of

the

hydroxyl-containing dibromoalkane or dichloroalkane with

N,N-dimethyldodeylamine,

because

two equivalents of

(±)-1,4-dibromo-2,3-butanediol

5c

and

meso-1,4-dichloro-2,3-butanediol 6 are commercially available. Two more steps, the anion exchange and

neutralization,

finally

afforded

the

target

product

9


meso-1,4-dibromo-2,3-butanediol

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((2R,3S)-12-4(OH)2-12). The melting points of the enantiomers, racemate, and mesomer of 12-4(OH)2-12 show great differences, which are 192-194 °C, 196-198 °C and 224-226 °C, respectively. The melting point increases in the order of enantiomers > racemate > mesomer. This is probably attributed to the fact that in solid state the mesomer mainly forms intermolecular hydrogen-bonds between -OH groups, while enantiomers mainly form intramolecular hydrogen-bonds, and racemate is sort of in between. Thus, the mesomer appears to be more tightly organized with intermolecular hydrogen-bonds so that more heat energy is required to disrupt the ordered structure, while intramolecular hydrogen-bonds of enantiomers barely affect the melting point.31,52 This is supported by the differences of the -OH stretching vibration in the solid-state IR spectra (Figure S23, Supporting Information). The stretching vibration of the -OH is at ~ 3200 cm-1, ~ 3227 cm-1 and ~ 3252 cm-1 for the mesomer, racemate, enantiomers, respectively. The blue shift is consistent with the increasing order of the melting points, may indicating the enhancement of intermolecular hydrogen-bonds from enantiomers to racemate and mesomer.31,53 Notably, the stereochemistry of the hydroxy groups at space provokes the differences in the physicochemical properties of the chiral cationic gemini surfactant in solid state. CMC and Surface Activity of Chiral Cationic Gemini Surfactants. The self-assembling properties for the enantiomers, racemate and mesomer of 12-4(OH)2-12 in aqueous solution have been studied by surface tension (γ), electrical conductivity (κ) and 1H NMR spectroscopy, and the results are shown in Figure 2, where γ is plotted against the surfactant concentration (C) with a logarithmic scale, κ is plotted against C, and the chemical shifts for the protons of N-methyl are plotted against 1/C. The turning points in the curves correspond to the CMC values of the isomers, and the micellar ionization degree (α) values are determined by the slope ratio of the lines below and 10


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above CMC on the electrical conductivity curves. The CMC values determined by the three techniques and the α values are all summarized in Table 1. The CMC values obtained from the three methods are consistent with each other. The CMC values of the two enantiomers are almost the same, but are slightly larger than that of the mesomer and smaller than that of racemate. As reported in literature,43 the chiral cationic gemini surfactants replacing hydroxyl group with methoxyl group also present the similar changing tendency, larger CMC for the enantiomers while smaller CMC for the mesomer. Moreover, the present (±)-12-4(OH)2-12 shows smaller CMC than the corresponding surfactants with one or no hydroxyl group at spacer.46,49 The possible reason is that the hydroxyl groups may form hydrogen-bonds with water, and thus reduce the contacts between the hydrocarbon tail and water, and then promote the micelle formation. Additionally, the α values for all the isomers are around 0.27, and the surface tensions of all the isomers at CMC (γCMC) are almost the same, about 36.0 mN/m. This implies that the stereochemistry of the spacer has little effects on the release of the counterions from the surfactant headgroups and on the molecular packing at air-water interface. Table 1. Critical micelle concentration (CMC) and micellar ionization degree (α) for the isomers of the chiral cationic gemini surfactant at 25 oC. Surfactants

CMC (mM) 1

Conductivity

(±)-12-4(OH)2-12

0.74

0.93

0.75

0.27

(2R,3R)-12-4(OH)2-12

0.65

0.92

0.72

0.28

(2S,3S)-12-4(OH)2-12

0.65

0.91

0.72

0.28

(2R,3S)-12-4(OH)2-12

0.61

0.81

0.65

0.26

11


H NMR

α

Tensiometry

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Figure 2. Variations of (a) surface tension and (b) electrical conductivity against the concentration (C), and (c) chemical shift for the protons of N-methyl versus 1/C in D2O for the enantiomers, racemate and mesomer of 12-4(OH)2-12 at 25 oC. Aggregate Structures of Chiral Cationic Gemini Surfactant. In order to further explore the impact of chirality on the self-assembly of 12-4(OH)2-12 in bulk aqueous solution, the aggregate morphologies for all the isomers have been studied by SANS, Cryo-TEM and Cryo-SEM. Figure 3a shows the SANS scattering intensity curves for all the isomers at different concentrations, Figure 3b displays the Cryo-TEM images for the aggregate transition of the enantiomers with increasing the concentrations, and Figure 3c presents the aggregate structures in Cryo-TEM and Cryo-SEM images 12


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for all the isomers at the same and larger concentration. Generally, the core-shell model matches the SANS data for the globular micelles. When the aggregation number in a micelle is more than being packed into a spherical volume, the micelle is assumed to change into a prolate ellipsoid. The calculated scattering profiles for the acceptable models are smeared by the resolution function and then compared with the measured data. The parameters in the data analysis are optimized with a customized software package developed by Hayter and Penfold.54-55 In the SANS scattering intensity curves for all the isomers at different concentrations (Figure 3a), the solid lines are model calculations for interacting globular micelles. The data associated model fits, i.e., the radius and length of aggregate, the elliptical ratio (ee) and surface charge (z) at different concentrations are summarized in Table 2. Upon increasing the surfactant concentrations, the length, elliptical ratio and surface charge increase gradually, showing that the micellar growth from spherical micelle to rodlike micelle takes place for the enantiomers (2R,3R)-12-4(OH)2-12 and (2S,3S)-12-4(OH)2-12 at C ≤ 10 mM, but for the racemate (±)-12-4(OH)2-12 and mesomer (2R,3S)-12-4(OH)2-12 at C < 5 mM. Taking (2R,3R)-12-4(OH)2-12 as a representative, the Cryo-TEM images prove the aggregate transition from spherical micelle (Figure 3b1) to rodlike micelle (Figure 3b2). Further increasing the concentration, both the enantiomers self-assemble into longer rodlike micelle (Figure 3b3), and then wormlike micelle (Figure 3b4), while the racemate and mesomer form insoluble, even and ordered platelet-like aggregate (Figure 3c3 and 3c4). The comparison of the aggregate structures for the isomers at the same concentration (fixing the concentration at 10 mM) shows the rodlike micelle for (2R,3R)-12-4(OH)2-12 (Figure 3c1) and (2S,3S)-12-4(OH)2-12 (Figure 3c2), and the platelet-like aggregate of (±)-12-4(OH)2-12 (Figure 3c3) 13


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and (2R,3S)-12-4(OH)2-12 (Figure 3c4). The fitting parameters in the SANS data analysis displays the trend of micellar growth, which is in consistent with the Cryo-TEM results. But the limited full Q range of the SANS measurements tends to underestimate larger micelle dimension.55

Figure 3. (a) Scattering intensity for (2R,3R)-12-4(OH)2-12, (±)-12-4(OH)2-12 and (2R,3S)-12-4(OH)2-12 at different concentrations. (b) The Cryo-TEM images of (2R,3R)-12-4(OH)2-12 at 5 mM (b1), 10 mM (b2), 20 mM (b3) and 50 mM (b4). (c) The Cryo-TEM images of (2R,3R)-12-4(OH)2-12 (c1) and (2S,3S)-12-4(OH)2-12 (c2) at 10 mM, and Cryo-SEM images of (±)-12-4(OH)2-12 (c3) and (2R,3S)-12-4(OH)2-12 (c4) at 10 mM.

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Table 2. SANS micelle model parameters for radius (Å), length of aggregate (Å), the elliptical ratio (ee) and surface charge (z) of chiral gemini surfactant 12-4(OH)2-12 in D2O at different concentrations. Surfactant

(2R,3R)-12-4(OH)2-12

(±)-12-4(OH)2-12 (2R,3S)-12-4(OH)2-12

Concentration

Radius, Å

Length, Å

ee

Charge, z

1 mM 3 mM 5 mM 10 mM 15 mM 1 mM 5 mM 1 mM 3 mM

17.5 17.5 17.5 17.5 17.5 17.5 17.5 17.5 17.5

40.6 42.8 44.8 48.3 54.0 42.5 46.8 43.8 45.2

2.3 2.4 2.6 2.8 3.1 2.4 2.7 2.5 2.6

1 2 12 15 16 1 13 1 10

Molecular Packing in Aggregates. On the basis of the results above, the possible models for the molecular packing of the surfactant isomers in aggregates are proposed in Figure 4. At the early stage of the aggregation within lower concentration range, the racemate, mesomer and enantiomers of 12-4(OH)2-12 all show micellar growth from small spherical micelle to rodlike micelle with increasing the surfactant concentration. The mass action model presents the equilibria between monomer and aggregates. Increasing surfactant concentration makes the equilibria in the systems move to aggregates. Although there is no change in the strength of the hydrophobic interactions between the molecules if the interaction strength is calculated to per molecule, the overall hydrophobic interaction between the 12-carbon chains becomes strong with the addition of more surfactant molecules, leading to the formation of spherical micelles. Further increasing the concentration, the hydrogen-bonds occurring less than 5 Å could be strongly enhanced when more hydroxyl groups stay close to each other in the aggregates due to the stronger hydrophobic interaction, contributing to the excessive free energy to the end-caps and inducing the formation of long micelles with lower surface curvature.56 The role of the hydroxyl groups has been verified in previous work,48 in which 12-4-12 only forms spherical micelles, while hydroxyl-substituted 15


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12-4(OH)-12 and 12-4(OH)2-12 can form long micelle. At the following stage of the aggregation at higher concentration range, the isomers of the surfactant exhibit different aggregate structures. The Chem3D simulation indicates that the minimized energy molecular conformations of (2R,3R)-12-4(OH)2-12 and (2R,3S)-12-4(OH)2-12 are different as shown in the right of Figure 4. Although all the hydroxyl groups are exposed to water in the mesomer and enantiomers, the stereochemistry leads to the different distance and orientation. The O-O distance are about 2.5 Å for (2R,3R)-12-4(OH)2-12 and 3.5 Å for (2R,3S)-12-4(OH)2-12. The hydrogen-bonds can form when the O-O distance ranges from about 2.3 to 3.0 Å.57 Thus, the distance

and

orientation

(2S,3S)-12-4(OH)2-12

of

prefer

the to

two form

hydroxyl

groups

intramolecular

in

(2R,3R)-12-4(OH)2-12

hydrogen-bonds,

while

those

and of

(2R,3S)-12-4(OH)2-12 are inclined to form intermolecular hydrogen-bonds, and (±)-12-4(OH)2-12 may have both forms of the hydrogen-bonds. Once the aggregates are formed, the hydroxyl groups at spacer cover the surface of the micelles and expose to the water. As a result, at higher concentration, the enhanced intermolecular hydrogen-bonds in the mesomer and racemate attract rod-like micelles to align side by side, resulting in the lamellar structure. The introduction of hydroxyl groups into the chiral center generates more significant differences in the aggregate structures among the isomers of the chiral gemini surfactant. Moreover, because the hydroxyl groups at spacer exist at the water/aggregate interface, the chirality at spacer may exert more significant chiral characteristics in the aggregates of chiral gemini surfactants.

16


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Figure 4. The possible molecular packing models for the racemate, mesomer and enantiomers of chiral gemini surfactant in aggregates, and ball-and-stick molecular models for the lowest energy conformation of (2R,3R)-12-4(OH)2-12 and (2R,3S)-12-4(OH)2-12. Chiral Recognition by Micellar Aggregates of Chiral Cationic Gemini Surfactant. Chiral homogeneity as one of the main features of nature is indispensable to life governed by chiral interactions in charge of most life processes. For example, the metabolism is closely related to L-amino acids, D-sugars, right-handed α helix of proteins and right-handed helix of nucleic acids. So it is of great importance to study non-covalent interactions responsible for the organization and functions of biomembranes through chiral recognition and diastereomeric interactions. It is generally considered that biological membranes possibly participate in breaking the symmetry or amplifying the process of enantiomeric imbalance.21,23 Herein we utilize the micelles formed by the synthetic chiral gemini surfactants above to mimic and understand the chiral recognition phenomena in biological membrane. So far, the chiral recognition capabilities of different micellar aggregates have not been reported yet. Bilirubin (BR) is a bile pigment produced by the metabolic breakdown of heme. It is a tetrapyrrole composed of two rigid, planar dipyrrole units jointed by a methylene group (Figure 5).58 It adopts a 17


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ridge-tile structure (with interplanar angle, θ ∼ 100°) existing as two isoenergetic M- and P-helical conformers, which is stabilized by six intramolecular hydrogen-bonds. The ridge-tile conformation of BR is dissymmetric, and BR is a racemic mixture of conformational enantiomers that interconvert rapidly with a barrier of 18-20 kcal/mol.58-61 In this work, we studied whether the BR pigments can be

enantioselected

by

the

micelles

of

the

enantiomers

(2R,3R)-12-4(OH)2-12

or

(2S,3S)-12-4(OH)2-12. Figure 6a shows the CD spectra of BR in 10 mM (2R,3R)-12-4(OH)2-12 or (2S,3S)-12-4(OH)2-12 aqueous solution, where the molecules form rod-like micelles. The CD spectra show a bisignate band and completely opposite CD sign, indicating that the enantiomers can offer completely different chiral environment. The interaction of the locally excited twin dipyrrinone chromophores in BR molecule yields exciton splitting, and leads to two transitions with oppositely signed Cotton effects in the CD spectra. Such a bisignate CD profile of BR is interpreted by the exciton coupling between the component dipyrrinone chromophores.58 Figure 6b shows the CD spectra of different concentrations of BR in 10 mM (2S,3S)-12-4(OH)2-12, and indicates that the intensity and sign increase with the increase of the BR concentration from 10 µM to 50 µM and then decrease from 50 µM to 200 µM (Figure S30). So the maximum BR recognition concentration for 10 mM (2S,3S)-12-4(OH)2-12 in rodlike micelles is about 50 µM. More interestingly, Figure 6c shows that the CD signal becomes stronger first and then weaker with increasing the (2S,3S)-12-4(OH)2-12 concentration. It shows that the (2S,3S)-12-4(OH)2-12 monomers still exhibit chiral recognition capabilities and the weak sign is due to the low concentration. The different (2S,3S)-12-4(OH)2-12 micelles have distinct recognition capabilities to BR (50 µM): the rodlike micelle exhibits higher recognition capabilities than either small spherical or wormlike micelles. This suggests that the chiral 18


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recognition strongly depends on the interaction of BR with the micelles. The chiral groups at spacer provides a chiral surface of micelles, interacting with the soluble BR molecules in water. The longer rodlike micelle has larger surface area and exposes more chiral groups to recognize BR. However, the elongated wormlike micelles are inclined to entangle with each other and form viscous solution, which may constrict the contact between the chiral centers and BR. In order to verify the effect of solvent on the chiral recognition, the CD spectra of BR in the presence of the (2S,3S)-12-4(OH)2-12 at high concentration in water and methanol were investigated. As depicted in Figure 6d, the spectrum is almost CD silent in methanol which demonstrates that a change of solvent could affect the interaction between the surfactant and BR and further chiral recognition. Therefore, the micelles of the chiral surfactants are appropriate chiral matrices that preferentially bind one of the BR enantiomers, and the enantioselective recognition ability is dependent on the micelle structures. The proper long micelle is a better candidate for enantioselective recognition.

Figure 5. Ridge-tile shape M- and P-helicity intramolecular hydrogen-bonds interconverting conformers of BR. Dotted lines represent hydrogen-bonds. The double-headed arrows represent the orientations of the long wavelength electric transition dipoles of each dipyrrinone. The relative helicities (M, minus or P, plus) of the dipoles are shown (inset) for each enantiomer.

19


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Figure 6. CD spectra of (a) 50 µM BR in 10 mM (2R,3R)-12-4(OH)2-12 and (2S,3S)-12-4(OH)2-12, (b) different concentrations of BR in 10 mM (2S,3S)-12-4(OH)2-12, (c) 50 µM BR in different concentrations of (2S,3S)-12-4(OH)2-12, and (d) 50 µM BR in aqueous solution and methanol of 10 mM (2S,3S)-12-4(OH)2-12.

CONCLUSION In this work, we have synthesized a new series of chiral cationic ammonium gemini surfactants bearing two chiral hydroxyl groups at spacer, including its racemate ((±)-12-4(OH)2-12), mesomer ((2R,3S)-12-4(OH)2-12) and enantiomers ((2S,3S)-12-4(OH)2-12 and (2R,3R)-12-4(OH)2-12). It is found that the introduction of chirality into the spacer of gemini surfactants induces slightly different aggregation ability, following the trend of mesomer > enantiomers > racemate, and the hydroxyls of forming hydrogen-bonds enlarge the differences. Meanwhile, they all undergo the transitions from the small spherical micelles to rodlike micelles at lower concentration, but at higher concentration, the enantiomers form rodlike micelles and entangled wormlike micelles, while the racemate and mesomer form platelet-like structures. The obvious differences in the aggregate morphologies for the isomers are induced by the stereochemistry of hydroxyls at the chiral centers, which leads to the formation of intermolecular hydrogen-bonds mainly in mesomer and intramolecular hydrogen-bonds 20


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dominantly in enantiomers. The two different forms of hydrogen-bonds might exist in racemate simultaneously, resulting in the properties of racemate between the other two isomers. Moreover, the rodlike micelles of enantiomers with enriched chiral centers on the micellar surface display stronger recognition capability in the deracemization of bilirubin than either the small spherical micelles or the wormlike micelles formed by the same chiral cationic gemini surfactant. Therefore, the chiral characteristics strongly depends on the aggregate structure, and the micelles of the chiral surfactants can be used to understand the chiral recognition in biological membrane.

ASSOCIATED CONTENT Supporting Information. The synthesis routes of chiral cationic gemini surfactants, 1H NMR, 13C NMR, COSY and IR spectra of the surfactants and their precursors, the variations of 1H NMR spectra with the surfactant concentrations in D2O, fitting the SANS results in micellar solutions, and UV-Vis spectra of BR in chiral cationic gemini surfactants. These materials are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] ORCID Yilin Wang: 0000-0002-8455-390X Yaxun Fan: 0000-0003-0057-0444 21


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Notes The authors declare no competing financial interest. ACKNOWLEDGEMENT We are grateful for financial support from Chinese Academy of Sciences and National Natural Science Foundation of China (Grants 21603239, 21633002, and 21761142007), and for the neutron beam time at the ISIS facility in the UK.

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