ELECTRON-PARAMAGNETIC-RESONANCE SPECTROSCOPY OF THE IRON-MOLYBDENUM COFACTOR OF CLOSTRIDIUM-PASTEURIANUM NITROGENASE

We report a computer simulation study of the electron paramagnetic res onance (EPR) spectral line shape of-the iron-molybdenum cofactor of ni trogenase. The unusually broad and asymmetric line shape of the EPR sp ectrum can be interpreted in terms of a distribution of zero-field spl itting parameters called D-strain. The best fit simulations were compu ted using D = 2.5 cm(-1) and E = 0.317 cm(-1) and distributions in D a nd E approximated by Gaussians of half-widths 0.446 cm(-1) and 0.108 c m(-1), respectively. The value of D estimated in the present work is s maller than previous estimates by others but consistent with the tempe rature dependence of the EPR spectrum. The large D-strain is most like ly caused by an ensemble of nearly isoenergetic conformational states and should not be considered as being indicative of chemical inhomogen eity.