DETERMINATION OF HIGH-SPIN IRON(III)-NITROXYL DISTANCES IN SPIN-LABELED PORPHYRINS BY TIME-DOMAIN EPR

Continuous wave EPR spectra of the nitroxyl signals for four spin-labe led high-spin (h.s.) Fe(III) porphyrins showed partially resolved spli ttings at temperatures near 4 K. Axial ligands were fluoride, chloride , or bromide. As temperature was increased to 20 to 30 K the iron-nitr oxyl splitting collapsed due to increasing rates of iron relaxation, E lectron spin-echo (ESE) spectroscopy showed that above about 6 K colla pse of the iron-nitroxyl spin-spin splitting caused a dramatic increas e in the nitroxyl phase memory relaxation rates. Electron spin relaxat ion rates were determined for Fe(tetratolylporphyrin)X, X = F, Cl, Br, in toluene solution by ESE or inversion recovery at 4.5 to 6 K and by analysis of the temperature-dependent contributions to the continuous wave EPR linewidths between about 10 and 120 K. Above about 10 K iron relaxation rates increase in the order X = F < CI < Br, which is the order of increasing zero-field splitting. Saturation recovery data for two spin-labeled h.s. iron(III) porphyrins between about 15 and 120 K and for two additional spin-labeled h.s. iron(III) porphyrins between about 85 and 120 K demonstrated that interaction with the h.s. iron e nhanced the electron spin relaxation rate of the spin label. The satur ation recovery curves for the nitroxyl were analyzed to determine inte rspin distances using a modified version of the Bloembergen equation a nd independently determined iron relaxation rates. Interspin distances were between 11.6 and 15.0 Angstrom, were independent of axial ligand , and were in good agreement with values obtained previously for low-s pin Fe(III) and Cu(II) analogs. (C) 1998 Academic Press.