1. The absorption spectra of deutero- and proto-ferrihaem in aqueous solution at 25 degrees C show marked changes with concentration and pH in the Soret band region. Quantitative studies of these phenomena imply that they are associated with ferrihaem dimerization and with protolytic equilibria involving monomeric (M) and dimeric (D) ferrihaem species according to the scheme: [Formula: see text] 2. For deuteroferrihaem we obtain K=1.9x10(-2), pK(a(M))=7.1, pK(a(D))=7.4. Protoferrihaem has a much higher dimerization constant, K=4.5 and pK(a(D))=7.5 (pK(a(M)) is not accessible). 3. Possible structural relationships between monomeric and dimeric ferrihaem species in solution are discussed in relation to recent work on the oxo-bridged nature of crystalline ferrihaem dimers.
1. The specific stoicheiometric catalatic activity of deuteroferrihaem is 10-100-fold greater than that for protoferrihaem, depending on pH. It is suggested that the difference in activity may be related to quantitative differences in the extent of dimerization in aqueous solutions of proto- and deutero-ferrihaem (Brown, Dean & Jones, 1970b). 2. A quantitative comparison of the kinetic and equilibrium data implies that the catalytic activities of ferrihaems are determined by the proportion of monomer present. The specific activity of ferrihaem monomer calculated varies inversely with H(+) ion concentration and attains a value equal to the maximal activity of catalase at pH>pK(a)(H(2)O(2)). 3. A comparison of catalatic behaviour in the series of iron(III)-centred catalysts aqua-iron(III) ion, ferrihaem monomer and catalase suggests that the unique feature of catalase action resides in the pH-independence of the reaction.
Apparent quantitative discrepancies between cataIytic activity data for protohaemin reported in previous investigations have been examined. The effect has been traced to an influence of the conditions of dissolution on the activity of protohaemin solutions. Deuterohaemin shows a similar effect.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.