Spin-lattice relaxation rates of iron-sulfur proteins and heme proteins affected by dysprosium complexes and temperature

Abstract

The method of progressive power saturation of the electron paramagnetic resonance spectrum has been utilized to determine the spin-lattice relaxation rate, 1/ T 1, in the high potential iron-sulfur protein from Chromatium vinosum, the binuclear ferredoxin, bovine adrenodoxin, cytochrome c 2 from Rhodospirillum rubrum and sperm whale myoglobin-NO complex. The effect of added dysprosium complexes on these structurally known proteins was studied to describe more accurately the dipolar interaction. Based on an analysis of the data it can be concluded that an exogenous paramagnetic relaxer such as dysprosium-EDTA can be used to determine the effective distance ( r −6, weighted average distance) of a paramagnetic center relative to the protein surface, which is somewhat longer than the closest approach. The temperature dependence of 1/ T 1 of adrenodoxin could be followed over a wide range and confirms the low-temperature T 2 phonon bottleneck mechanism and intermediate-temperature T 9 Raman process found in other iron-sulfur proteins. The C. vinosum high potential iron-sulfur protein (Hipip) gave anomalous results due to the onset of lower-temperature line-broadening, although the T 2 lowest-temperature mechanism is confirmed.