3071
J. Phys. Chem. 1994,98, 3071-3072
An Inversion in the Chirality in an Amphiphilic Cholesteric Liquid Crystal Where the Chiral Dopant Is the Chloride of the Decyl Ester of L-Proline Keith Radley' and Neil McLay Department of Chemical and Biological Sciences, The University of Huddersfield, Queensgate. Huddersfield HDI 3DH., U.K. Received: August 5, 1993: In Final Form: February 3, 1994'
The chloride of the decyl ester of L-proline has been used as a chiral dopant where the achiral host detergent was tetradecyltrimethylammoniumbromide. The twist and the sense of the helix were measured as a function of the chiral dopant concentration, using the fingerprint texture and Grandjean planes observed under a polarizing microscope. The magnitude of the twist as a function of the chiral dopant concentration initially rose and fell, while the sense of helix was positive. At about 10%the twist magnitude passed through zero, and then with increasing amounts of the chiral dopant the sense of the helix became negative. This inversion in the twist sense is thought to be due to a concentration-dependent distribution of the cis and trans molecular rotamers derived from rotation around the >CH-COz- bond. The existence of cis and trans rotamers is confirmed using 13C
NMR. Inversions in the sense of the twisted helix (chirality) in a cholesteric liquid crystal were classically thought to only be possible if two chiral centers acting against one another were present in the chiral mesogen, Le., compensatednematics.' Chiral mesogenic states are created when one or more chiral centers are introduced into a nematic liquid crystal. Dissymmetry in the molecular structure is not a sufficient condition for optical activity. The condition for rotation of the plane of polarization in linearly polarized light must be a dissymmetry in the molecular electronic interactions.2 Recently, in thermotropic liquid crystals (single mesogens) inversions in the sense of the helix have been reported, where only one chiral center is present in the c h o l e ~ t e r o g e n . ~ ~ In one of the most recent studies, where a biphenyl ester was involved, it was proposed that the origin of the inversion in the sense of the helix involved competition between the distribution of different molecular conformers, rather than the averaging out of the relative twisting powers of individual chiral center^.^ In amphiphilic systems inversions in the chirality have been observed which were created through the organic chemistry reversing and changing the molecular configuration with respect to the chiral micelle surface. Constituent isomers can cause an inversionin the amphiphilic cholesteric liquid crystal helix sense,* while acylated amino acids with simple side chains all appear to have the same sense of helix twist in the doped amphiphilic cholesteric liquid crystal samples, although in isotropic solution they can have different signs of optical r ~ t a t i o n . ~ It will be reported here that while attempting to extend these studies of the inversions in the sense of helix twist in amphiphilic cholestericliquid crystals, an inversionin the chirality was created with a single host, when the chlorideof thedecyl ester of L-proline, L-DEPCl, was used as the chiral dopant. The chiral detergent L-DEPC1was synthesized, and the amphiphilic cholesteric liquid crystal samples were prepared as previously described.1° The host samplewas0.32g (33.2%)of tetradecyltrimethylammonium bromide (TDTMABr), 0.045 g (4.66%) of decanol, and 0.6 g (62.2%)of water containing 3% KCl and 2% K2CO3. The samples were prepared by adding 0.005-0.10 g of L-DEPC1 in 0.005-g divisions to the host sample. In all cases the micelle shape was inferred to be disk shaped from the oily streak textures observed under a polarizing microscope from the neighboring lamellar phase in the concentration gradient set up through allowing a thin film of the sample to dry out. The twists were measured at Abstract published in Advance ACS Abstracts, March 15, 1994.
I-
o
12.0
2
P-1,
MOL% DOPANT Figure 1. Laser diffraction twist measurements at 25 O C as a function of the mole percent of the chiral dopant L-DEPCI ( 0 ) .Also plotted are the intensity ratioof the two >CO I3CNMR spectra peaks at 22 O C (0).
25 OC by observing the fingerprint textures under a polarizing microscope. Some of the areas of the sample under observation which gave rise to Grandjean planes could be used to determine the sign of the optical rotation. The signs of the optical rotation were then used to infer (opposite sign) the sense of the helical twist in the cholesteric phase.ll.12 This sense of the twist was checked by mixing the sample with various chiral dopants. The results for L-DEPCl are presented in Figure 1, where the twist (1/P)is plotted against mole percent of the chiral dopant. The twist concentration data for the chiral dopant L-DEPClrises, falls, and then passes through zero. The sense of the helix twist was reversed, where the sense was initially positive at small concentrationsand negativeat high concentrations. It is proposed that the inversion in the twist sense originates from a distribution between two molecular conformations derived from the free rotation around the >CH-C02- bond as shown in the diagram. This inverts the dissymmetry in the molecular electronic states in the detergent. The explanation is similar to that in a previous thermotropic study also involving an ester with X H - C O z - bond r~tation.~ The distribution between cis and trans conformers of the C-N rotation in polypeptide derivatives has been investigated using NMR and X-rays and is well documented.13-16 Similar studies with the cis and trans interconversion in the ester of amino acids are not well documented. As far as the author knows, no
0022-3654/94/2098-307 1%04.50/0 0 1994 American Chemical Society
3072 The Journal of Physical Chemistry, Vol. 98, No. 12, 1994
PPM Figure 2. I3C N M R spectra of L-DEAC1 dissolved in CD3OD at 22 O C produced on a Bruker 270-MHz N M R spectrometer.
experimental data have been published. It was suggested in a recent study that rotation about the amide bond in the chiral headgroup influences the pitch in amphiphilic cholesteric liquid crystals. These previous data only partially confirm any assertion involving cis and trans rotamers. No NMR spectra were presented there showing resolved rotamer peaks. The data are more consistent with changes in the molecular orientation as was suggested." When alanine is part of a polypeptide, the cis and trans ratio is always several orders of magnitude. It might be reasonably presumed that a similar situation would apply to chiral detergents derived from acylated alanine. The l3C NMR spectra of L-DEPCl dissolved in CD30D are shown in Figure 2. There are six pairs of peaks shown in the spectra; three pairs of peaks that are easily resolved and three pairs of peaks are not so easily resolved into separate peaks. In 13CNMR spectra the peak intensitiesare not directly proportional to the number of l3C nuclei present. The cis and trans ratio is not easily calculated. The peaks due to >CO carbons are easily assigned at 171.076 and 169.785 ppm. The doublet peaks are marked with an "a" on the spectra. Some of the samples with the chiral dopant L-DEPCl were remade, where the decanol was left out and 0.2 g of D2O was added. A 13CNMR spectrum was taken from these samples and recorded with long acquisition times of 20 s, so that the spectra peak intensities would be proportional to the cis and trans rotamer populations. The intensity ratio of the 13CNMR > C = O peaks derived from the cis and trans rotamers is plotted out against mole percent of
Letters chiral dopant; see Figure 1. At high chiral dopant concentrations the rotamer population ratio is over 2:l and drops to zero at low chiral dopant concentrations where only one rotamer is present. The assignment of individual peaks to the cis or trans rotamer is not possible without a further investigation, which is beyond the scope of this paper. It is concluded in the present study that the inversion in the chirality created by the chiral dopant L-DEPClin the TDTMABr host sample was due to a concentration-dependent distribution of the cis and the trans molecular conformers created by rotation around the >CH-C02- bond. The presence of the cis and trans rotamers in the chiral detergent L-DEPCl was confirmed using 13CNMR. These amphiphilic chiral systems could become very important in the future, particularly in their relationship to biological systems and molecular recognition at the surface of membranes.
Acknowledgment. The author thanks Professor M. I. Page in the Department of Chemical and Biological Sciences at the University of Huddersfield for facilities a self-supportingvisiting research fellowship beginning January 1991. The author also thanks the Rector of the University for providing a small research grant to buy chemicals. References and Notes (1) Sackmann, E.; Meiboom, Snyder, L. C.; Meischer, A. E.; Dietix, R. E. J . Am. Chem. SOC.1968, 90,3567. (2) Gibson, H. W. Liquid Crysruls rhe Fourrh Srurc of Murrer, Saeva, F. D., Ed.;Marcel Dekker: New York, 1979. (3) Finkelmann, H.; Stegemeyer, H. Z . Nururforsch. A 1973,28,799. (4) Heppke, G.; Lotzsch, D.; Destreider, F. Z . Nururforsch. A 1987,42, 279. (5) Baessler, H.; Malya, P. A. G.;Nes, W. R.; Labes, M. M. Mol. Crysf. Liq. Cryst. 1970, 6, 329. (6) Martinet-Lagarde, P. H.; Duke, R.; Durand, G. Mol. Crysr. Liq. Crvst. 1981. 75. 249. . (7) Styring, p.; Vuijk, J. D.; Slaney, A. J.; Goodby, J. W. J. Muter. Chem. 1993.3. 399. (8) Radley, K.; Cattey, H. Mol. Cryst. Liq. Cryst., accepted. (9) Radley, K.; Lilly, G. J. Mol. Crysr. Liq. Crysf. 1993, 231, 183. (10) Achimis, M.; Reeves, L. W. Can. J. Chem. 1980, 58, 1533. (11) Grandjean, F. C. R. Acud. Sci. 1921, 172, 71. (12) Cano, F. Bull. Soc. Fr. Minerul. 1968, 91, 20. (13) Luque, F. J.; Orozco, H. J . Chem. Soc., Perkin Trum. 2 1993,683. (14) Impericili, B.; Fisher, S. L.; Moats, R. A.; Prins, T. J. J. Am. Chem. Soc. 1992, 114, 3183. (15) Grathwohl, C.; Wuthrich, Biopolymers 1976, IS, 2025. (16) Stewart, D. E.; Sarkar, A.; Wampler, J. E. J. Mol. Biol. 1990, 214, 253. (17) Tracey, A. S.; Zang, X. J . Phys. Chem. 1992,96, 3889.
