A RESONANCE ENHANCEMENT OF THE PHASE RELAXATION IN THE ELECTRON-SPIN ECHO OF NITROXIDE COVALENTLY ATTACHED TO CYTOCHROME-C

The spin lattice T-l and phase memory T-M relaxation times were measur ed by an electron spin echo technique for the nitroxide spin label att ached covalently to horse heart cytochrome c below 80 K for oxidized ( Fe3+) and reduced (Fe2+) samples. T-l decreases on heating and below 1 0 K is governed by the direct relaxation process for the reduced sampl e. The spin-lattice relaxation is enhanced by a cross-relaxation to Fe 3+ in an oxidized sample. In the T-M 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 T-M of nitroxide at low temperature producing a minimum in T-M wh en the T-l value corresponds to the spin packet width of excited spins .