Photoactivated H/D Exchange in Tyrosine: Involvement of a Radical Anion Intermediate

Abstract

The aromatic hydrogen nuclei of tyrosine are photochemically labile and exchange with deuterons in neutral D(2)O solution. The site meta to the ring hydroxyl substituent is preferentially deuterated, exhibiting a meta/ortho deuteration rate of approximately 4:1. In contrast with acid-catalyzed H/D exchange and with nearly all of the reported photoactivated H/D exchange studies, the UV-induced H/D exchange of tyrosine is optimal at pH 9 and is effectively quenched at acid pH. Photochemical H/D exchange is strongly stimulated by the alpha-amino group (the aromatic hydrogens of p-cresol are far less subject to exchange) and by imidazole or phosphate buffers. On the basis of the results obtained here and on the previously identified cyclohexadienyl radical (Bussandri, A.; van Willigen, H. J. Phys. Chem. A 2002, 106, 1524-1532), we conclude that the exchange reaction involves a radical intermediate and results from two distinct roles of tyrosine: (1) as a phototransducer of light energy into solvated electrons (e(aq)(-)), and (2) as an acceptor of an electron to create a radical anion intermediate which is rapidly protonated, yielding a neutral cyclohexadienyl radical. Regeneration of the tyrosine can occur via a bimolecular redox reaction of the cyclohexadienyl and phenoxyl radicals to yield a carbocation/phenoxide pair, followed by deprotonation of the carbocation. The oxidation step is pH dependent, requiring the deprotonated form of the cyclohexadienyl radical. The H/D exchange thus results from a cyclic one-electron (Birch) reduction/protonation/reoxidation (by phenoxyl radical)/deprotonation cycle. Consistent with these mechanistic conclusions, the aromatic hydrogens of tyrosine O-methyl ether are photochemically inert, but become labile in the presence of tyrosine at high pH. The deuteration rate of O-methyl tyrosine is lower than that of tyrosine and shows a preference for the ortho positions. This difference is proposed to result from a variation in the oxidation step, characterized by a preferential oxidation of a cyclohexadienyl resonance structure with the unpaired electron localized on the oxygen substituent.