The pH-dependence of the magnetic moment of a ferri-haem undecapeptide, produced by peptic digestion of cytochrome c, has been measured in aqueous solution using a nuclear magnetic resonance method.Below pH 3 the magnetic moment is consistent with the p.resence of hydroxo-bridged dimers of high-spin iron(II1). Above pH 7 the iron(II1) is low-spin, the spin crossover conforming to a simple titration curve with pK 6.3 (n = 1). This transition is discussed with reference to spectrophotometric studies of ligand binding at the haem.The influence of the protein on the metal ion at the active site of a metalloenzyme is central to the theory of entasis [l]. The influence of globin on haem has been studied using the properties of intact haemoglobin [2], while iron-sulphur proteins have been approached in a complimentary manner by studying the effects of binding small peptides to their chemically synthesized iron-sulphur cores [3]. An intermediate approach, which we are beginning to investigate, is to make use of metal-containing peptide fragments resulting from the enzymic digestion of native enzymes.Of particular interest is a haem-containing undecapeptide ( Fig. 1) derived from peptic hydrolysis of cytochrome c [4]. This provides an opportunity to study the haem group in aqueous solution relatively unshielded by protein, yet soluble over an appreciable pH range. In addition, only a limited number of donor groups may be expected to interact with

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Preparation and Crystal and Molecular Structure of [I -Chloro-2,2-bis-(p-chlorophenyl)vinyI] bis( dimethylglyoximato)pyridinecobalt( 111)

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