Perchlorate Binding to Cytochrome c

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Abstract
1. The effects of perchlorate on cytochrome c have been investigated by 1H and 35Cl NMR, electron paramagnetic resonance and optical spectroscopy. 2. The pK values for the formation and disappearance of the major alkaline conformation were found to be displaced from 9.3 to 8.3 and from 10.4 to 10.9, respectively. The displacement was dependent on the ClO4− concentration below 0.1 M. 3. Competition experiments between perchlorate and chloride show that ClO4− binds both to the neutral and alkaline forms but with a higher affinity for the latter. The appearance of a new binding site in the alkaline form accounts for the markedly enhanced relaxation rate of 35ClO4− in this pH range. Complex formation between cyanide and the alkaline species results in the loss of this binding site, which probably is located close to or within the heme crevice. 4. The neutral form of ferricytochrome c also binds perchlorate strongly as evidence by the unique appearance of a high‐spin signal dependent on pH and perchlorate concentration. This signal disappears with the same pK value as the neutral form. The effects of perchlorate on cytochrome c are due to specific binding of this ion.