TIME-DOMAIN ELECTRON-PARAMAGNETIC-RESONANCE AS A PROBE OF ELECTRON-ELECTRON SPIN-SPIN INTERACTION IN SPIN-LABELED LOW-SPIN IRON PORPHYRINS

Nitroxyl free radical electron spin relaxation times for spin-labeled complexes of low-spin iron(III) porphyrins were measured between 8 and 70 K by two-pulse spin-echo spectroscopy and between 8 and 120 K by s aturation recovery. Relaxation times for low-spin Fe-III(TPP)(MeIm)(2) (TPP = tetraphenylporphyrin; MeIm = methylimidazole) were measured be tween 10 and 28 K by saturation recovery and between 10 and 25 K by el ectron spin-echo. At low temperature the iron electron spin relaxation rates are slow relative to the electron-electron spin-spin splitting. As temperature is increased, the relaxation rates for the Fe(III) bec ome comparable to and then greater than the spin-spin splitting, which collapses the splitting in the continuous wave EPR spectra and causes an increase and then a decrease in the nitroxyl spin-echo decay rate. Throughout the temperature range examined, interaction with the Fe(II I) increases the spin-lattice relaxation rate (1/T-1) for the nitroxyl . The measured relaxation times for the Fe(III) were used to analyze t he temperature-dependent changes in the spin-echo decays and in the sa turation recovery (T-1) data for the interacting nitroxyl and determin e the interspin distance, r. The values of r for the four complexes th at were examined were between 10.5 and 15 Angstrom, with good agreemen t between values obtained by spin-echo and saturation recovery. For ea ch of the compounds the value of r is consistent with other data for t hat spin-labeled porphyrin. Analysis of the nitroxyl spin-echo and sat uration recovery data also provide values of the iron relaxation rates at temperatures where the rates are too fast to measure directly by s aturation recovery or electron spin-echo spectroscopy. These data demo nstrate the power of time-domain EPR measurements as a probe of distan ce between a slowly-relaxing spin and a relatively rapidly relaxing me tal ion such as low-spin Fe(III).