Enhancing the Orienting Properties of Poly(γ-benzyl-L-glutamate) by means of Additives

Residual dipolar couplings (RDCs) have recently become increasingly important in organic structure determination due to their unique information content. One main limitation for the use of RDCs in organic compounds is the orientation that needs to be induced to be able to measure RDCs. So far, there are very few possibilities to modulate the orientational properties of organic solutes and even less when chiral media are considered. Based on our recent findings that the critical concentration of the liquid-crystalline phase of homopolypeptides depends on their molecular weight, we sought for further ways to modulate the orienting properties. We were especially interested in seeing whether we could not only influence the induced degree of orientation, but whether we could also change the solute's preferred orientation and even enhance enantio-differentiation. We thus tried different aprotic and protic additives and were successful in all of the above-mentioned aspects by using CCl4 as the additive. Furthermore. we consider DMSO to be a very useful additive. The LC phase of low MW poly(gamma-benzyl-L-glutamate) (PBLG) is usually unstable when DMSO is added. The high MW PBLG used in this study, however, remained stable up to a DMSO/CDCl3 ratio of 1:2. By using this combination of solvents, the alignment of the two enantiomers of a compound, which is insoluble in CDCl3, namely, the HCl salt of a tryptophane ester, was possible leading to high-quality spectra. The two enantiomers of the tryptophane ester showed different couplings, thus indicating that enantiodifferentiation is taking place. Thus we were able to modulate the orienting properties (degree of orientation, preferred orientation and enantiodifferentiation) of PBLG by using additives and to increase the accessible solvent and solute range significantly.