Unusually high spin–orbit coupling in the methionine singlet–triplet transition and the role of magnetic perturbations in amino acids

Forbidden triplet-singlet transitions (T₁ ← S₀) in a few amino acids (methionine, glycine, tryptophan, cysteine) have been examined for their possible detection. This computational study is based on time-dependent density functional theory (TD-DFT) with spin-orbit coupling (SOC) account. The results of this study indicate that the possibility of detecting the absorption T₁ ← S₀ band is quite high for methionine and cysteine. The results demonstrate that the presence of sulfur in amino acids enhances the SOC effect, increasing the intensity of the singlet-triplet absorptions up to a measurable extinction coefficient. The importance of these SOC effects on the photochemical properties of proteins is emphasized together with the potential role of triplet-singlet transitions in photobiology.