Diffusional Behavior of Solvent in Polypeptide Liquid Crystalline and Gel States with Highly Oriented Chains As Studied by NMR Spectroscopy

Abstract

The 13C CP/MAS(cross polarization/magic angle spinning) NMR spectrum of solid poly(γ-benzyl l-glutamate) (PBLG) was measured at the slow spinning rate of 1.9 kHz. The exact principal values of the 13C chemical shift tensor (δ11, δ22, and δ33) for the main-chain carbonyl carbons of PBLG were obtained from an intensity analysis of the spinning sidebands, according to Hertzfeld−Berger method. Further, the 13C CP/MAS NMR spectra and static 13C CP NMR of PBLG were measured in a liquid crystalline solution, where PBLG takes the α-helix form. The experimental results show that there exists a linear relationship between the order parameter S and the observed main-chain carbonyl 13C chemical shift, δobs, such as S = 0.024 × δobs − 3.758. Using the δobs value and the S-δobs relation, the order parameter S of PBLG in the liquid crystalline state was determined to be 0.875 ± 0.025. The δobs value was then used as a monitor for determining the degree of orientation of PBLG in the PBLG gel. Furthermore, the diffusional behavior of 1,4-dioxane as solvent in the PBLG liquid crystalline solution, and in the highly oriented PBLG gel prepared in the magnetic field, was elucidated by the pulsed-field gradient spin−echo 1H NMR method.

Chemical shift tensor – The heart of NMR: Insights into biological aspects of proteins

“…Highly-oriented α-helical chains of poly(γ-benzyl L-glutamate) (PBLG) film can be prepared by evaporating slowly the solvent from the PBLG LC solution in a strong magnetic field of an NMR magnet [322]. It was demonstrated that there exists a linear relationship between the order parameter S of the α-helical polypeptide chains with respect to the applied magnetic field and the observed main-chain carbonyl 13 C chemical shift, δ obs , such that S = 0.024 × δ obs − 3.758.…”

Some aspects of the NMR chemical shift/structure correlation in the structural characterization of polymers and biopolymers

“…However, the C ¼ O chemical shifts do not seem to be affected by the residue structure and thus can be used to determine the main-chain conformation. This method has been applied for the structural characterization of collagen proteins, 42 wool keratin, 43,44 silk protein, 45 polypeptide liquid crystals, 46,47 polypeptide gels, 48,49 and polypeptide blends. 50,51 Other peptide systems analyzed by this method are reviewed in Saito et al 40,52 It is known that changes in the helix sense of polypeptides in the solid state often occur because of changes in external conditions, such as temperature and so on, and changes in the side-chain conformation also often induce changes in the helix sense of the main-chain or the other conformations.…”

Diffusional Behavior ofn-Alkanes in Polypeptide Gel System with Highly Oriented Chains As Studied by Pulsed Field Gradient Spin Echo1H NMR Method

“…From these studies it has been elucidated that poly(γ-benzyl Lglutamate) (PBLG) gel with highly oriented chains was successfully prepared in an NMR magnet, and solvents in the gel diffuse anisotropically in restricted molecular motion. 3 Further, in anisotropic systems [4][5][6][7] such as lipids, synthetic liquid crystalline polymers, etc., different from polypeptide liquid crystal the diffusion process has been elucidated. Under such a background, we have been attracted to a research program how is the diffusional process of n-alkanes with the relatively extended conformation in the gel with anisotropic structure because of the fact that n-alkanes are basic molecules for understanding structure and dynamics of linear polymer chains.…”