Electron spin-lattice relaxation rates for high-spin Fe(III) complexes in glassy solvents at temperatures between 6 and 298 K

The temperature dependence of spin-lattice relaxation rates was analyzed fo r four high-spin nonheme iron proteins between 5 and 20 K, for three high-s pin iron porphyrins between 5 and 118 K, and for four high-spin heme protei ns between 5 and 150 to 298 K. For the nonheme proteins the zero-field spli ttings, D, are less than 0.7 cm(-1), and the relaxation is dominated by the Orbach and Raman processes. For the iron porphyrins and heme proteins D is between 4 and 12 cm(-1) and the relaxation is dominated by the Orbach proc ess between about 5 and 100 K and by a local mode at higher temperatures. T he relaxation rates for the heme proteins in glassy matrices extrapolated t o values at room temperature that are similar to values obtained by NMR rel axivity in fluid solution. This similarity suggests that for high-spin Fe(I II) heme proteins with effective intramolecular spin-lattice relaxation pro cesses, the additional motional freedom gained when a relatively large prot ein goes from glassy solid to liquid solution at room temperature has littl e impact on spin-lattice relaxation, (C) 2000 Academic Press.