PARENT SESSION

Symposium S1A Proton-coupled electon transport and ATPase
Monday August 30th, 2004 10:20 AM-12:20 PM Room 511D
Chair: Colin Wraight
Co-Chair: Wolfgang Junge

Switching the mechanism of bi-directional proton coupled electron transfer from tyrosine and tryptophan. Martin Sjödin^*,1, Stenbjörn Styring², Björn Åkermark³, Licheng Sun³, Leif Hammarström¹, ¹ Department of Physical Chemistry, Uppsala, Sweden² Department of Biochemistry, Lund, Sweden³ Department of Chemistry, Stockholm, Sweden

ABSTRACT- Amino acid radicals are key redox intermediates in several natural enzymes including Cytochrome c peroxidase, DNA photolyase, ribonucletide reductase, cytochrome c oxidase and photosystem II. Electron transfer from amino acids is often coupled to deprotonation and this work concerns the coupling of photo-induced electron transfer from tyrosine and tryptophan to trisbipyridineruthenium(III) with deprotonation in model complexes. Specifically the mechanisms for these proton coupled electron transfer reactions have been studied and the controlling parameters has been identified, the possible mechanisms being stepwise electron transfer followed by deprotonation and deprotonation followed by electron transfer or concerted electron transfer/deprotonation. Proton coupled electron transfer reactions have been studied using nano-second flash photolysis in water solution and the effect of pH, temperature, reaction driving force, deuteration and the nature of the amino acid has been determined. I have shown that the rate constant for the concerted reaction depends intrinsically on the mixing entropy of the released proton and that the pH-dependence can be used as an experimental tool for mechanistic discrimination. Moreover I have shown that the concerted reaction inherently has a high reorganisation energy due to the coupling of the electron motion with deprotonation. Hydrogen bonding to the transferring proton however significantly reduces this reorganisation energy. Counteracting the high reorganisation energy in the competition between the concerted reaction and the stepwise electron transfer first reaction the concerted reaction also has a relatively high driving force. The relative importance of the high reorganisation energy and the high driving force for the concerted reaction determines the mechanistic outcome of the reaction, the stepwise reaction being favoured by high over-all driving forces and the concerted reaction by high pH.

KEY WORDS: amino acid radicals, tryptophan, proton coupled electron transfer, tyrosine