Spin-Probe Studies of Poly(γ-benzyl L-glutamate)

Abstract

Spin-probe techniques have been employed to study the molecular motion of poly(γ-benzyl L-glutamate) (PBLG) in the solid state. Electron spin resonance (ESR) spectra of an organic nitroxide radical, 2, 2, 6, 6-tetramethyl-4-hydroxy-piperidine-1-oxyl, dissolved in PBLG have been measured as a function of temperature in the range from −180 to 130°C. Rotational correlation times were estimated on the basis of recent theories of motional effects on ESR spectra. Two motional processes were observed, below and above room temperature. Activation energies, calculated by the Arrhenius plots of the correlation times, were 3.5 and 10.5 kcal/mol for the lower and higher temperature motional processes, respectively. These motional processes have a strong correlation with the side-chain relaxations observed by other measurements, such as dielectric ones. It was found that the tumbling rate of the nitroxide radical dissolved in PBLG in the solid state is as rapid as in the liquid above room temperature, indicating that the side-chain region surrounding the α-helical backbone is essential in the liquid-like state.