Methionine-Based Radicals: Time Scales and Species

l-Methionine is an amino acid, which provides anti-oxidative properties. We report on radicals and radical cations being likely (short-lived) intermediates formed upon photo-oxidation reactions of methionine. In this context, we present photo-CIDNP experiments indicating that the character of the photooxidants is decisive for the observation of CIDNP effects based on methionine. Based on calculated hyperfine data and pKa values and on our experimental observations, we suggest that CIDNP polarizations are produced by an overlay of at least three geminal radical pairs, i.e., two α-thio carbon-centered radicals D· and G·, aminyl radical N·, and, possibly, 2c–3e radical cation SN.+ as short-lived reaction intermediates.