Thermodynamic and Magnetic Resonance Studies on the Hydration of Polymers: II. Protein-Water Interactions in Powdered Ribonuclease

Abstract

ESR studies on spin-labeled amorphous RNase A as a function of varying concentrations of sorbed H2O and D2O will be presented. A relaxation analysis of saturation transfer (ST-)ESR spectra of 14N(1H) nitroxide spin-label molecules essentially fixed at amino acid residue His-105 will be given. A characteristic correlation has been observed between the microdynamic behavior - expressed by the rotational correlation times of the paramagnetic label - and the macroscopic thermodynamic entropy for the sorption process of H2O and D2O at RNase. This correlation is particularly pronounced at low water concentrations, viz., nH₂O/nprotein ≤ 100. A significant difference in this concentration range exists between the two systems “RNase-H2O” and “RNase-D2O”, which is manifested not only by the thermodynamic data but also by the microdynamic behavior extracted from the corresponding non-linear ESR absorption line shapes.