Besides its own interest, tryptophan oxidation by photogenerated Ru complexes is one of the several examples where concerted proton-electron transfer (CPET) to water as proton acceptor endowed with a pH-dependent driving force has been invoked to explain the data. Since this notion is contrary to the very basic principles of chemical physics, it was interesting to attempt uncovering the source of this contradiction with an easily accessible substrate. Careful examination of the oxidation of the tryptophan (ethyl ester derivative) bearing a NH3+/NH2 group showed that there is no trace of such an unconventional H2O - CPET with a pH-dependent driving force. The reaction mechanism simply consists, with both the NH3+ acid and NH2 basic forms of the tryptophan derivative, in a rate-determining electron transfer step followed by deprotonation steps. The same is true with the ethyl ester - methyl amide derivative of tryptophan. No such unconventional H2O-CPET was either found with phenol, another easily accessible substrate. It may thus be inferred that the same applies to less easily available systems in which electron transfer occurs intramolecularly. These observations help ridding the road of such artificial obstacles and improving present models of H2O-CPET reactions having particularly in mind the role of water chains in natural systems.References(1) J. Bonin, C. Costentin, M. Robert, M. Routier, J-M. Sav�ant, J. Am. Chem. Soc. 2013 , 135, 14359-14366. (2) J. Bonin, C. Costentin, M. Robert, J-M. Sav�ant, C. Tard, Acc. Chem. Res. 2012  , 45, 372-381. (3) C. Costentin, M. Robert, J-M. Sav�ant, Chem. Rev. 2010 , 110, PR1-PR40. (4) C. Costentin, M. Robert, J-M. Sav�ant, J. Am. Chem. Soc. 2007  , 129, 5870-5879.

Proton-coupled electron transfers: pH-dependent driving forces? Fundamentals and artifacts

248th National Meeting (2014)

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