Photooxidation of Glycylglycine. Two-Channel Reaction Mechanism as Studied by Time-Resolved FT EPR

The Fourier transform electron paramagnetic resonance (FT EPR) spectroscopy has been employed as a detection technique in the investigation of the photooxidation of glycylglycine dipeptide induced by triplet-sensitized electron transfer to 9,10-anthraquinone-2,6-disulfonate in aqueous solutions at pH 6−10. Spin-polarized (CIDEP) radical species +NH3/NH2−CH2−CONH-•CH2 and •NH−CH2−CONH−CH2−COO- were identified as transient products formed on the nanosecond time scale. The radicals have been found to originate from two different reaction channels depending on the pH. The first channel, leading to the decarboxylated radical product, occurs in the whole pH range and is ascribed to the oxidative attack of the triplet on the peptide and/or carboxylate functional groups. At pH > 8 where the peptide terminal amino group exists in deprotonated form, the second channel, electron transfer from the amino group nitrogen nonbonding electron pair followed by deprotonation, becomes possible. This reaction leads to the formation of the observed aminyl radical.