A resonance enhancement of the phase relaxation in the electron spin echo of nitroxide covalently attached to cytochrome c

The spin lattice T1 and phase memory TM relaxation times were measured by an electron spin echo technique for the nitroxide spin label attached covalently to horse heart cytochrome c below 80 K for oxidized (Fe3+) and reduced (Fe2+) samples. T1 decreases on heating and below 10 K is governed by the direct relaxation process for the reduced sample. The spin-lattice relaxation is enhanced by a cross-relaxation to Fe3+ in an oxidized sample. In the TM temperature dependence an unusual deep minimum appears at about 25 K. This resonance type effect which vanishes completely for the reduced sample is due to a coupling to the Fe3+ spins. The spin-lattice relaxation rate of Fe3+ is comparable to the TM of nitroxide at low temperature producing a minimum in TM when the Ti value corresponds to the spin packet width of excited spins.