Proton relaxation studies of dynamics of proteins in the solid state

Abstract

Dynamical information concerning proteins and other macromole- cules in the solid state may be obtained from investigations of proton magnetic relaxation. Measurements have been made of T1 for protons at 18, 30 and 60 MHz on polycrystalline ct-chymotrypsin, insulin, lysozyme and ribonuclease A in the temperature range 10 to 300 K. Analysis identifies the predominant source of relaxation between 70 and 250 K as methyl group reorientation in amino acid sidechains. Activation parameters characterizing a distribution of methyl rotors are deduced. Deuteration gives information concerning the relaxation contribution of water and other exchangeable protons. Investigations were extended to solid polymers devoid of methyl groups such as polyglycine and poly-L-proline to give information concerning relaxation contributions from main chain motions and ring puckering. The measurement of dipolar relaxation enables slower motions to be investigated. Measurements of T1D in solid a-chymotrypsin and solid lysozyme exhibit behaviour similar to T1 but displaced about 100 K lower in temperature, and enable other dynamical effects to be observed at higher temperatures.