Water spin relaxation in colloidal systems. Part 2.—17O and 2H relaxation in protein solutions

Abstract

Oxygen-17 and deuteron longitudinal and transverse spin relaxation rates from aqueous protein solutions (mainly lysozyme and haemocyanin) have been measured as a function of temperature, pH and resonance frequency. A detailed comparison of the ¹⁷O and ²H data demonstrates that ²H relaxation has a dominant contribution which is not seen by ¹⁷O. This contribution is, at least partly, due to deuteron exchange with ionizable groups on the protein. Furthermore, we refute two widespread beliefs about solvent spin relaxation in protein solutions, namely that the discrete-exchange model leads to interpretational inconsistencies and that the low-frequency dispersion always reflects protein reorientation.