| The relaxation rates in frozen aqueous solutions of whale ferri-myoglobin azide, bovine ferri-hemoglobin azide, cupric azurin (P. aeruginosa) and cupric spinach plastocyanin were measured at 9.5 GHz using the pulse-saturation recovery method. Measurements covered a temperature range of 1.4 K to as high as 22 K, with corresponding relaxation rates up to 10;The effect on the relaxation rate of several cosolvents and solutes is also examined. The effect on the direct process is much more pronounced than on the Raman region. The differences are shown to be consistent with changes in the velocity of sound at room temperature caused by the addition of cosolvents and solutes.;Finally, the EPR recovery form is analyzed. We propose that the deviations in the recovery from an exponential form are due to a distribution of relaxation rates. The source of the distribution is most likely sample heating in the lower temperatures and a distribution of conformations frozen in near the paramagnetic site in the higher temperatures. It is not likely that it is caused by spin-spin interactions. The exact form of the distribution is unclear, but the most successful functional form for the recoveries is a stretched exponential with an exponent ranging from 0.5 to 1.0. However, a simple exponential fit to a limited portion of the recovery was the experimentally most viable and consistent method of extracting a rate. |
