Electron spin relaxation measurements on the blue-copper protein plastocyanin: Deviations from a power law temperature dependence

Electron spin relaxation rates over the temperatue range 1.41–15.6 K are presented for the copper-containing protein plastocyanin. Measurements are described for two samples, each derived from a different preparation of equivalent purity, for which the ionic, redox, and protein compositions varied slightly. X-band data are analyzed in terms of a phonon-limited direct process and a Raman relaxation process, where the index of the Raman transport integral is treated as a fitting parameter. Both samples yield rate data at the highest temperatures that are characterized by small deviations from a simple T n power law dependence, with n in the range 4.8–5.2. These deviations are most easily quantified when the T n power law fits are compared with similar functions that allow for a finite cutoff in the phonon density of states corresponding to Debye temperatures between 90 and 100 K with n in the range 5.0–5.5.