LONG-RANGE ELECTRON-TRANSFER IN PEPTIDES - TYROSINE REDUCTION OF THE INDOLYL RADICAL - REACTION-MECHANISM, MODULATION OF REACTION-RATE, AND PHYSIOLOGICAL CONSIDERATIONS

We have observed long-range electron transfer (LRET) across the oligoproline spacer from the tyrosine side chain to the indolyl cation radical derived from the 1-electron oxidation of 1-N-methyltryptophan (Metrp(.+)). On the basis of these results and measured bimolecular electron transfers in the model system p-cresol/N-methylindole, phenol O-H bond breaking can clearly accompany the 1-electron transfer under conditions in which H-atom transfer to the indole nitrogen is impossible. With Metrp(.+) as the electron acceptor, the rate of the LRET process across the oligoproline spacer is an order of magnitude higher than with the tryptophanyl radical (Trp(.)) as electron acceptor. However, the apparent distance dependence, as measured by the exponential constant beta, of the LRET process is the same with both radicals. We argue that these observations support our earlier conclusion, based on kinetic arguments, that proton transfer is not rate determining in the LRET between tyrosine and Trp(.). We have also shown that LRET rates are enhanced considerably in the electron transfer from the tyrosine phenolate side chain to Metrp(.+) and from tryptophan to Metrp(.+). These rates are fast enough to allow speculation about the design of peptide ''wires''. Finally, the reported results are the basis of a discussion on a possible mechanism for protein structural control of an LRET process.