Multifrequency ESR of copper(II)-(His)n (His = histidine). 1. Immobile phase

ESR studies of Cu(histidine), in the presence of excess histidine at two microwave frequencies were undertaken to support either the glycine-like or histamine-like complexes. Low-frequency, S-band data are well-resolved for the M, = -I/* line in the gH region, and the number of lines is attributed to four approximately equivalent nitrogen donor atoms. When the pH is increased from 6.8 to 7.3, a second form is detected from the MI= line in the g,,re gion but not in other portions of the spectrum. The change in ESR parameters does not appear to be large enough to account for a change from four nitrogen donor atoms to three nitrogen donor atoms in the square-planar configuration. This small change is more consistent with a change in axial coordination or a change from four nitrogen donor atoms to an altered sef of four nitrogen donor atoms. Further ESR analysis of frozen copper- histidine complexes in the presence of excess histidine where the I4N isotope in the imidazole ring is substituted with l5N is consistent with a structure for which four equivalent ISN nitrogen donor atoms are bound to cupric ion.