12414
J. Phys. Chem. 1996, 100, 12414-12417
Trans-Cis Diastereorotamerization in Amphiphilic Cholesteric Liquid Crystals K. Radley,* N. McLay, and G. J. Lilly The Department of Chemical and Biological Sciences, The UniVersity of Huddersfield, Queensgate, Huddersfield HD1 3DH, U.K. ReceiVed: July 21, 1995; In Final Form: March 28, 1996X
Potassium hexadecanoyl-L-prolinate is investigated as a chiral dopant in the formation of amphiphilic cholesteric liquid crystals with several achiral hosts. When the host is potassium laurate, the sense of helical twist is positive. When the host is potassium dodecanoyl-DL-alanine or potassium tetradecanoyl-DL-alanine, the sense of helix twist is negative. The apparent reversal in the helical twist sense is explained in of terms trans and cis diastereorotamers formed by hindered rotation about the C-N amide link. 13C NMR is used to determine the relative populations of these diastereorotamers. The twisting powers of these trans and cis rotamers were calculated using 13C NMR population data to be -2000 ( 200 and +250 ( 20 cm-1, respectively.
Introduction
TABLE 1: Mesophase CompositionsAchiral Nematic Host Samples
Chiral detergents containing the peptide link, resulting from the acylation of amino acids, have been used as chiral hosts1-4 and chiral dopants5,6 in the formation of amphiphilic cholesteric liquid crystals (ACLCs). It is well-known that the peptide link can have trans and cis conformations derived from the hindered rotation of the constituents about the C-N amide bond. For many years the occurrence of rotamers in peptides has been of great interest in biological chemistry.7-10 This C-N amide link is the fundamental structure linking amino acids to form peptides. Rotamerization of the C-N link has bearing on the molecular stereochemistry and hence the biological activities of biologically important chemicals such as proteins and enzymes. In very simple amides such as dimethyl acetamide derivatives, two rotamers (with equal populations) have been revealed using 1H NMR.11-13 These rotamers are very good examples of NMR observable chemical exchange, where the room temperature doubled NMR peaks are observed followed by coalescence at higher temperatures.14 In fact these amide NMR studies are used as standard examples of NMR chemical exchange. In polypeptides the 1H NMR is more complex,15 but the 13C NMR spectra, which are less complex, have been used to investigate polypeptides and dipeptides in particular. 13C NMR has shown that the trans rotamer is preferred, when the peptide link is formed from amino acids with primary nitrogens such as alanine. The cis rotamer is preferred when secondary nitrogens are involved such as with proline. These results were confirmed using X-rays.16,17 The occurrence of rotamers in cholesteric liquid crystals has been proposed as a mechanism to explain reversals in bulk chirality in both amphiphilic and non amphiphilic chiral liquid crystals.18-21 This assertion has been confirmed for ACLCs in both amide and ester amino acid chiral detergent systems using 13C NMR. It was shown in a recent study of amino acid amides as chiral detergents in ACLCs derived from proline and thiaproline that the occurrence of rotamers formed by the hindered rotation about the C-N amide link is responsible for the reversal of the twist (bulk chirality).18 * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, July 1, 1996.
S0022-3654(95)02058-2 CCC: $12.00
composition, mg (wt %) host
detergent
decanol
H2O
KDD DL-KDDA DL-KTDA
300 (31.5) 200 (28.8) 130 (21.3)
50 (5.3) 45 (6.4) 30 (4.9)
600 (63.2)a 450 (64.8)b 450 (73.8)c
a 3 % KCl and 2 % K CO . b 10 % CsCl and 2 % K CO . c 10 % 2 3 2 3 CsCl.
The chiral center in an ACLC induces a spontaneously twisted superstructure, where the distance traveled during a 2π rotation of the director is called the pitch. The twist is the inverse of the pitch, where the twisting power is the differential of the twist with respect to concentration at zero concentration.22 The sense of the helical twist and the sign of optical rotation are opposite by convention. In the present study the rationalization of the relationship between the reversals in twisting senses in ACLCs and the hindered rotation about the C-N amide link in the chiral detergent dopants will be further investigated with potassium hexadecanoyl-L-prolinate (L-KHDP) using other achiral host detergents with 13C NMR. The total twist in a chiral nematic liquid crystal is the sum of the individual contributions of all the chiral centers. Experimental Section The chiral dopant L-KHDP and the achiral detergents potassium laurate (KDD), potassium dodecanoyl-DL-alanine (DLKDDA), and potassium tetradecanoyl-DL-alanine (DL-KTDA) were prepared as previously described.6 Routine 13C NMR spectra with a high S/N (usually 2%) were used to check the purity and the structure of the starting materials, the intermediates, and the final products. The 13C NMR and the 1H NMR spectra of the chiral detergent L-KHDP in deuterated methanol could be attributed to more than one species. The 13C NMR spectra of the rotamer carbonyls were taken in a nonroutine way as previously described23 in isotropic samples where the decanol had been left out of the chiral nematic sample. The acquistion time for the 13C NMR was 1 minute so as to ensure quantitative spectra The sample compositions are presented in Tables 1 and 2. The structures of the micelles in the ACLCs were inferred to be discotic ChD, using a polarizing microscope as previously © 1996 American Chemical Society
Trans-Cis Diastereoisomerization
J. Phys. Chem., Vol. 100, No. 30, 1996 12415
TABLE 2: Isotropic Phase CompositionsMinus Decanol + H2O: Achiral Host Samples composition, mg (wt %)
a
host
detergent
H2O
KDD DL-KDDA DL-KTDA
300 (23.1) 200 (16.7) 160 (13.8)
1000 (76.9)a 1000 (83.3)a 1000 (86.2)b
2 % KCl and 3 % K2CO3. b 10 % CsCl.
described.24,25 When the thin films of ACLC samples are allowed to dry out, a concentration gradient is set up between the chiral nematic and the neighboring dimensionally ordered phase. The planar texture of the middle soap phase and the oily streaks/pseudo isotropic textures in the lamellar phase indicated that the micelle structure in the neighboring chiral nematic phase was cylindrical ChC or disk ChD shaped, respectively. Under a polarizing microscope thin films of ACLC samples held in “CAMLAB” microslides gave rise to fingerprint textures, which could be used to determine the twist. In parts of the thin film under the polarizing microscope, Grandjean textures and Cano planes are formed, which could be used to determine the sign of the optical rotation.26,27 The optical rotation and the sense of helical twist have by convention opposite signs. The optical rotation sign was used to infer the sense of helical twist. The sense of helical twist was checked by mixing with samples of different dopants and checking for compensation or reinforcing of the twist magnitude. The compositions of the host ANLC samples are presented in Table 2. The twist is (magnitude > 500 cm-1) of the helix axis of amphiphilic cholesteric liquid crystal samples were measured using laser diffraction. The wavelength of the laser was 6.238 × 10-5 cm. The temperature of the samples was controlled to within 0.1 °C by placing in a brass block suitably drilled for water flow, sample placement, and optical path. The temperature was controlled by circulating water from a thermostated water bath. Samples were prepared by weighing out the components into a test tube and after heat sealing were homogeneously mixed by heating and centrifuging as previously described. The sample component decanol was specially purified by fractional distillation, and the H2O and D2O were double distilled. Results and Discussion The chiral detergent L-KHDP was doped into three host amphiphilic nematic liquid crystals, where the achiral host detergents were potassium laurate (KDD), potassium dodecanoyl-DL-alanine (DL-KDDA), and potassium tetradecanoylDL-alanine (DL-KTDA), to form amphiphilic cholesteric liquid crystals. The twist was determined as a function of concentration; see Figure 1. The data sets were fitted to linear regressions in each case. The twisting powers, calculated by extrapolating the data to zero concentration, were found to be KDD, +50 ( 5 cm-1; DL-KDDA, -17 ( 2 cm-1; and DL-KTDA, -200 ( 15 cm-1. The twisting power is negative with the racemized host detergents DL-KDDA and DL-KTDA but is reversed and positive with the achiral host KDD. Reversals of the sense of helical twist, which also involved one chiral center, have been found in thermotropic cholesteric systems. Two basically different hypotheses were proposed and discussed. The first relies on the effective partitioning of twisting powers for each individual chiral element in a molecule including handedness, which are added up to yield the macroscopically observed twisting powers of the cholesteric liquid
Figure 1. Addition of the chiral dopant L-KHDP to an achiral nematic host phase: (O) twist values when the achiral host is KDD; (b) twist values when the achiral host is DL-KDDA; 4 twist values when the achiral host is DL-KTDA at 25 °C.
crystal. No experiments to support the hypothesis will be presented in this paper. It has been suggested elsewhere that there is some direct relationship between molecular orientation and helical twist.28 In a parallel 2H NMR investigation with deuterated acylated amino acids as chiral dopants, no relationship has been found between molecular orientation and twist.29 Of course each rotamer will have its own molecular orientation and twisting power and make its own contribution to the twist, but there is no direct relationship without involving enantiomers. In the previous molecular orientation measurements,28 the 2H NMR parameters did not involve chirality, whereas the twist (1/P) is a measure of bulk chirality. The same NMR results would have been obtained in racemer or even achiral host systems. For such a study to be valid the dissymmetry in the molecular orientation must be measured in some way. It can only be measured through the visualization of enantiomers. The second hypothesis depends on the equilibrium between different rotamers. These rotamers are conformations resulting from the hindered rotation of the constituents about the C-N bond of the amide group. Each rotamer makes an opposite but not necessarily equal contribution to the twist. The twist values for each rotamer are unequal in magnitude, and thus the sense of helical twist is reversed for one rotamer with respect to the other. The bulk chirality is the result of dissymmetry in molecular polarization, which arises from the spatial chiral molecular stereochemistry. The reversal in the molecular polarization in the peptide link might be best imagined if the polarization is represented by four directors, each projected through a different corner of a tetrahedron. The directors intersect somewhere in the space of the tetrahedron but not in the center. If the directors intersected in the center, the polarization would be zero. Two of the directors lie in a common plane, while the other two lie in another common plane. These planes do not have to be perpendicular to each other. If one of the planes is rotated with respect to the other by a rotation of 180° along the intersecting axis of the directors in the director plane, a reversal in the molecular polarization is produced. The second polarization will be equal and reversed (enantiomeric) to the original polarization. Similar arguments by organic chemists are usually made with molecules containing asymmetric carbon centers, where the transformation is also enantiomeric. All these acylated amino acid derivatives contain a chiral center. In trans and cis rotamers the peptide link segment is usually recognized as having planar symmetry. Planar symmetry is not necessarily the case. If hindered rotation takes
12416 J. Phys. Chem., Vol. 100, No. 30, 1996
Figure 2. [Trans]/[cis] ratio in the chiral nematic phase samples resulting from the addition of chiral dopant L-KHDP to an achiral nematic phase, determined by 13C NMR, plotted as a function of the dopant concentration: (b) l twist values when the achiral host is KDD; (O) twist values when the achiral host is DL-KDDA; (4) twist values when the achiral host is DL-KTDA at 25 °C.
place, the molecular plane is destroyed and the peptide link no longer has any planar symmetry. This could involve the pyramidization of the bonds to nitrogen, where the nitrogen lone pair resides only partially with the C-N π bonding. The C-N bond will then become a chiral center. The peptide link will form diastereoisomers with the other chiral center, and hence two magnetically different (NMR chemical shift) species should be resolved in the NMR. The chirality induced by the trans and cis diastereo rotamers will not be equal in magnitude, but the sense of helix twist for one would be expected to be the reverse of the other. The cis and trans diastereo rotamers represent the low-energy level of the hindered rotation about the peptide bond. For the visualization of diastereoisomers in the NMR, the equilibrium cannot be planar for more than a very small part of the time. It is implied here that the equilibrium states of the trans and cis rotamers do not need to have planar symmetry.
13C NMR spectra were recorded on a series of samples for each chiral host as a function of the chiral dopant concentration. The decanol was excluded from the original samples, and some more D2O was added; see Tables 1 and 2. The observed spectra were regarded as being derived from two species, i.e., cis and trans diastereorotamers. The peaks in the NMR derived from the acylated carbonyl and the acids were recorded and the transcis ratio RTC was calculated. In Figure 2 the RTC is plotted as a function of percent chiral dopant. It can be seen that for the KDD host RTC was constant at 0.090 ( 0.008; for the DL-KTDA host RTC was also constant at 0.15 ( 0.01, and for the DL-KTDA the plot had a nonzero slope, where the average of the five data points was 0.24. A constant RTC might be expected if the data of Figure 1 for twist and dopant concentrations were straight lines. This is as found in the experiments for the host KDD and DL-KDDA but not with the host DL-KTDA. The explanation might be expected to lie in the compatibility of the chain lengths of the dopants and host. The C12 chains in the achiral host KDD and DL-KDDA would have been considerably less compatible with the long chain C16 L-KHDP chiral dopant than the achiral DL-KTDA. This is directly opposite to
Radley et al.
Figure 3. Twisting power for each achiral host taken from Figure 1 and plotted as a function of the trans and cis diastereorotamers population calculated from 13C NMR determined [trans]/[cis] ratio illustrated in Figure 2.
the observed experimental fact. In some way the micelles have a greater stability with the shorter host carbon chain than with a long chain chiral dopant. It is not clear why. If the basic assumption concerning the generation of twist in ACLCs already mentioned is that the total twist in an ACLC is the sum of the individual microtwists, the following equation could be derived
TT ) TPC + M(TPT - TPC)
(1)
where TPT and TPC are the twisting powers of the trans and cis diastereorotamers, respectively, and M is the mole fraction of the trans rotamer. The twisting power in each rotamer is the result of dissymmetrical pairwise interactions. In fact, it may be necessary to take into account two completely independent dissymmetrical interactions, the intermolecular interactions within micelles and the intermicellar between micelles. It is thought the latter leads to twist. The twisting powers were taken from Figure 1, and the mole fraction was calculated using RTC values derived from Figure 2 for each chiral host and plotted as illustrated in Figure 3. The linear regression was analyzed as a linear regression, and TPT and TTC were found to be -2000 ( 200 and 250 ( 20 cm-1, respectively. Conclusion It has been shown in the present study that with the chiral dopant L-KHDP the magnitude and the sense of the helical twist are dependent on the host achiral detergent. 13C NMR reveals that the magnitude of the twist is directly proportional to the relative trans-cis diastereorotamer populations. The twisting powers of the trans and the cis diastereorotamers were calculated, and twisting power senses were found to be opposite. It is well-known that the relative populations of the trans and cis rotamers with respect to the C-N link can be dependent on the substituent, molecular stereochemistry, and solvent effects. It would not be unexpected that the twist in an ACLC would be dependent on the host detergent if the transcis diastereorotamer populations are dependent upon the host detergent, as was found experimentally. Acknowledgment. The authors thank Professor M. I. Page at the Department of Chemical and Biological Sciences at the University of Huddersfield for providing research facilities. The authors thank the University for providing a small research grant
Trans-Cis Diastereoisomerization to buy chemicals and are also grateful for a postgraduate studentship to G.J.L. References and Notes (1) Covello, P. S.; Forrest, B. J.; Marcondes Helene, M. E.; Reeves L. W.; Vist, M. J. Phys.Chem. 1983, 87, 176. (2) Tracey, A. S.; Radley, K. J. Phys. Chem. 1984, 88, 6044. (3) Radley, K.; Tracey, A. S. Can. J. Chem. 1985, 63, 95. (4) Radley, K. Liq. Cryst. 1992, 11, 753. (5) Marcondes Helene, M. E.; Figueiredo Neto, A. M. Mol. Cryst. Liq. Cryst. 1988, 162B, 127. (6) Radley, K.; Lilly, G. J. Mol. Cryst. Liq. Cryst. 1992, 231, 195. (7) Vasquez, M.; Nernethy, G.; Scoraga, H. A. Chem. ReV. 1994, 94, 2183. (8) Fischer, S.; Dimbrack, R. L., Jr.; Karplus, M. J. Am. Chem. Soc. 1994, 116, 11931. (9) Luque, F. J.; Orozo, M. J. Chem. Soc., Perkin Trans. 2 1993, 683. (10) Sulzback, H. M.; Schleyer, P. R.; Schaefer, H. F., III. J. Am. Chem. Soc. 1994, 116, 3967. (11) Reeves, L. W.; Shaw,K. N. Can. J. Chem. 1970, 48, 3641. (12) Reeves, L. W.; Shaw, K. N. Can. J. Chem. 1971, 49, 3671. (13) Reeves, L. W.; Shaddick, R. C.; Shaw, K. N. Can. J. Chem. 1971×e2 49, 3684. (14) Stewart, W. E.; Siddall, T. H. Chem. ReV. 1970, 70, 517. (15) Rowe, J. J. M.; Hinton, J.; Rowe, K. L. Chem. ReV. 1970×e2 70, 1.
J. Phys. Chem., Vol. 100, No. 30, 1996 12417 (16) Stewart, D. E.; Sarkar, A.; Wampler, J. E. J. Mol. Biol. 1990, 214, 253. (17) Larive, C. K.; Rabenstein, D. L. J. Am. Chem. Soc. 1993, 115, 2833. (18) Radley, K.; Lilly, G. J.; Patel, P. R.; Cheema, H. K.; Rais, Z. M. Mol. Cryst. Liq. Cryst. 1995, 268, 107. (19) Radley, K.; McLay, N.; Gicquel, K. Submitted for publication in J. Am. Chem. Soc. (20) Dierking, I.; Giesselmann, F.; Zugenmaier, P.; Mohr, K.; Zaschke, H.; Kuczynski, W. Z. Naturforsch 1994, 49a, 1081. (21) Styring, P.; Vuijk, J. D.; Nishiyama, I.; Slaney, A. J.; Goodby, J. W. J. Matl. Chem. 1993, 3, 399. (22) Liquid Crystals, the fourth state of matter; Saeva, F. D., Ed.; Marcel Dekker: NY, 1979; p 439. (23) Radley, K.; McLay, N. J. Phys. Chem. 1994, 98, 3071. (24) Holmes, M. C.; Boden, N.; Radley, K. Mol Cryst. Liq. Cryst. 1983, 100, 93. (25) Boden, N.; Radley, K.; Holmes, M. C. Mol. Phys. 1981, 42, 493. (26) Grandjean, F. C. R. Acad. Sci. 1921, 172, 71. (27) Cano, F. Bull. Soc. Fr. Mineral. 1968, 91, 20. (28) Tracey, A. S.; Zhang, X. J. Phys. Chem. 1992, 96, 3889. (29) Lilly, G. J. Ph.D. Dissertation, Univeristy of Huddersfield, 1995.
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