Proton wide-line nuclear magnetic resonance spectra of hydrated proteins

Abstract

The wide-line proton magnetic resonance spectra of ribonuclease and bovine serum albumin have been measured in the hydrated and vacuum-dried states over the temperature range −140° to −180°. A single, broad signal attributed to the protein protons was observed for the dried proteins up to 120°. Little variation in line-widths or second moments was noted over the entire temperature range. Above 120° the spectra showed an additional weak, narrow signal due to residual adsorbed water molecules. The spectra for hydrated proteins are essentially the same as for the dried proteins below −40°. Above −40° the spectra consist of a narrow signal superimposed upon a broad signal; the narrow signal can be removed by prolonged pumping in vacuo and is due to weakly adsorbed H 2O molecules. The broad signals for both hydrated proteins undergo line-narrowing transitions in the temperature range −60° to −10°. A further more marked transition of the broad component is noted at approx. 60° for ribonuclease and approx. 20° for bovine serum albumin. From the magnitudes of the line-widths and second moments of the broad signals it is concluded that the hydrated and dried proteins have considerable motional freedom in the solid state at all of the temperatures studied. The present measurements also indicate that the extent of thermally induced configurational changes in the proteins at elevated temperatures is strongly linked to the amount and structural properties of the adsorbed water.