... can be illustrated for example in blue copper proteins. Indeed, the protein ligands coordinating the copper hold the latter in a much distorted arrangement keeping it an entatic state. This irregular high energy arrangement of the metal largely resembles the transition state geometry between the tetrahedral and the square planar configurations of the two involved oxidation states Cu+ and Cu2+, respectively, resulting in a higher rate of electron transfer (e.g. within the photosynthetic apparatus). Similar intermediate geometries, anisotropy of the actual electron transfer and electron delocalization between the metal and the porphyrin or sulphur ligand have been found in cytochromes and Fe-S proteins. The protein component is also of great significance in electron transfer proteins that transfer an electron from one protein to anther, as is the case for example for the cytochrome c/ cytochrome c peroxidase pair. In a study of the oxidation by cyt. c peroxidase of reduced cyt. c, ...

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