Side Chain Dynamics of Poly(γ-[κ-2H1]benzyl L-glutamate) and Poly(γ-[ζ-2H2]benzyl L-glutamate) by Solid State 2H NMR

Abstract

In order to investigate side-chain dynamics of poly(γ-benzyl L-glutamate) in the solid state, 2H NMR spectra and spin-lattice relaxation-time T1 of poly(γ-[κ-2H1]benzyl L-glutamate) and poly(γ-[ζ-2H2]benzyl L-glutamate) were measured over a wide temperature range. Line shapes for both samples below room temperature showed static-like axially symmetric powder patterns whose quadrupolar splittings are slightly smaller than the rigid lattice values, and decreased gradually with temperature. This suggests the presence of a rapid, small-amplitude librational motions at both of κ- and ζ-positions. T1 analysis for such librational motion gave the correlation time of the order of 10−11—10−12 s. The spectral intensity started to decrease gradually from 208 K with an increase in temperature, and a remarkable intensity loss was observed at around 323 K. With a further increase in temperature, the spectrum recovered its intensity, and the motional averaged singlet-like spectra were observed for both samples, indicating the presence of rapid, large amplitude motions for the side chains. The T1 minimum was observed at 383 K. T1 and line shapes were fairly well simulated by use of the three-site jump model assuming a Gaussian and a log-Gaussian distributions for the angle between the C–2H bond and the C3 axis and the jump rate, respectively. The average jump rates are located on the relaxation map reported for poly(γ-benzyl L-glutamate). The motional mechanism at higher temperatures follows the WLF type.