Sidechain Order Parameters via 2h Nmr in Polypeptide Liquid Crystals

We have selectively deuterated the sidechain of the synthetic polypeptide poly (-y-benzyl-L-glutamate), PBLG. In magnetically oriented nematic solutions of PBLG, the helical polymer rods are aligned parallel to the field. The Brownian motion of the flexible PBLG sidechain is anisotropic, resulting in quadrupole splittings of the 2H NMR resonances. The order parameters of C-D bond directions in the sidechain can be determined. These studies permit new insights into the nature of organization of sidechains on the periphery of the a-helix. The general features of lyotropic polypeptide liquid crystals have been reviewed recently [l]. The chirality of the cc-helical polymer imposes a cholesteric texture in the mesophase. However, the diamagnetic anisotropy of the helix is positive and an oriented nematic phase persists in sufficiently strong magnetic fields (H, 2 5 kOe). Solvent molecules and guest molecules added to this mesophase exercise anisotropic rotational and diffusional motion within this oriented array of hexagonally packed helices. While the details of the anisotropic motion are complex [a uniaxial dispersion force field superposed on a chemical exchange process involving a bound guest (solvent) proximate to the helix exchanging with nearly isotropic molecules removed from the helix solvation shell], it permits NMR spectroscopy experiments in liquid crystals wherein the species observed is characterized by very low orientational order parameters (usually two to three orders of magnitude smaller than thermotropic mesophases). The low degree of order facilitates the observation of quadrupolar splittings (AV,) in the 'H NMR spectra of deuterated guest (solvent) molecules [2]. Herein we describe quadrupolar splittings for specifically 2H labeled sidechains of the polypeptide component of the liquid crystal. The magnitude of the Avi and their response to changes in the polymer concentration in the liquid crystal provide some general insights into the structural-dynamical organization of the sidechains on the periphery of the polypeptide helix. Two 'H labeled derivatives of the synthetic polypeptide polyf-y-benzyl-L-glutamate) (PBLG), were studied : PBLG-d, with deuterium at the para position on the phenyl ring of the benzyl ester and, PBLG-d, with the benzyl ester fully deuterated. In the oriented nematic phase, the director is parallel to the spectrometer magnetic field (z-axis) ; the quadrupolar splitting is given by where Qi is the quadrupole moment and Vizz is the appropriate component of the electric field gradient tensor. The tensor components gap in a molecule-fixed axis system (a, b, c) are related to qzz via the elements of the Saupe order matrix : (*) This research was supported in part by a grant from the In practice the average orientation or order parameter National Institute of Arthritis, Metabolism and Digestive Diseases of the C-D bond @-direction) can be extracted from (AM 19 497). AV if the quadmpole coupling constant qb, is known. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979393 C3-472 E. T. SAMULSKI (The asymmetry parameter q = (g,, qc,)/gbb is assumed to be negligible.) The small-molecule analogue toluene-d, provides a set of g,,. The g,, are derived from the 'H NMR spectrum of toluene-d, oriented in the PBLG liquid crystal (Fig. 1). The orientation of the toluene can FIG. 1. The 'H NMR spectrum of toluene-d, dissolved in an oriented PBLG-CH,CI, solution (30 % by wt. polymer). be specified with two order matrix elements, S,, and (Sz2-S,,). The former is obtained from the dipolar splittings in the 'H NMR spectrum of toluene methyl group. The results are based on the assumption of identical ordering and geometries for toluene and toluene-d, in the liquid crystal (Table I). There is good agreement with earlier determinations of the 9bb PINext we contrast the ordering of the free probe molecule benzyl alcohol-d, dissolved in the polypeptide liquid crystal with the bound probe, the benzyl ester in PBLG-d,. It is readily apparent from the difference in the magnitude of the Avi's (Fig. 2), that the deuterated probe covalently attached to sidechain is considerably more constrained than the free probe. The former exhibits AV's spanning a spectral width of nearly 25 kHz whereas the spectrum of the latter is limited to 5 kHz. Also, labeling the sidechain in itself does not appreciably influence the orientation ; spectra of PBLG-d, and PBLG-d, superpose (Fig. 3).