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

Abstract

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.