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Continuous wave (cw) X-band EPR spectra at approximately 90 K were obtained for iron-transferrin-anion complexes with 18 anions. Each anion had a carboxylate group and at least one other polar moiety. As the second polar group was varied from hydroxyl to carbonyl to amine to carboxylate, the EPR spectra changed from a dominant signal at g' approximately 4.3 with a second smaller peak at g' approximately 9 to a broad signal with intensity between g' approximately 5 and 7. Computer simulation indicated that the changes in the EPR spectra were due to changes in the zero field splitting parameter ratio, E/D, from approximately 1/3 for carbonate anion to approximately 0.04 for malonate anion. Observation of iron-13C coupling in the electron spin echo envelope modulation (ESEEM) for iron transferrin [1-13C]pyruvate indicated that the carboxylate group was bound to the iron. It is proposed that all of the anions behave as bidentate ligands, with coordination to the iron through both the carboxylate and proximal groups, and the carboxyl group serves as a bridge between the iron and a positively charged group on the protein.

Abstract. The orientation dependence of the electron spin phase memory relaxation time (T m ) was measured for vanadyI5-(4-carboxyphenyl)-10,15,20-tritolylporphyrin at 22, 50, and 100 K, copper(II) bis(diethyldithiocarbamate) at 50 and 100 K, and copper(II) 5,10, 15,20-tetratolylporphyrin at 50, 70, and 100 K in frozen solution. Tm was determined by fitting a single exponential to two-pulse electron spin echo data. The values of Tm were strongly dependent on the orientation of the molecule in the magnetic field. Longer values were obtained when the magnetic field was along a principal axis or along a non-canonical turning point in the spectrum. Shorter values of T were observed at intermediate orientations. The orientation dependence of T is attributed to molecular motion. The EPR spectra for the three systems examined~e approximately axial, so the relevant motion is motion of the molecular z axis with respect to the external magnetic field. Longer values of T m (slower relaxation) occur for orientations at which the resonant condition is less sensitive to a change in orientation of the molecular z axis. Shorter values of Tm (faster relaxation) occur at orientations for which the resonant condition is more sensitive to a change in orientation of the molecular z axis. INTRODUCTIONThe effects of motion on EPR lineshapes and electron spin phase memory time (T m ) of organic radicals have been well documented both in fluid solution and in partially immobilized samples. I. 2 In fluid solution it has been shown that the larger anisotropy of transition metal EPR spectra, as compared to organic radicals, provides a longer time-scale for the study of incomplete motional averaging.' However the effect of motion on T m for transition metals in frozen solution does not appear to have been examined. Due to the large anisotropy in transition metal EPR spectra, it is expected that T m for transition metals will be more sensitive to slower motion than that for organic radicals. In this paper we report the orientation dependence of T m for a vanadyl complex and two copper(II) complexes.

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