Effects of hydrogen-bonded biological molecules on photochemical properties of tryptophan enantiomers probed by temperature-dependent ultraviolet photodissociation spectroscopy in the gas phase.

The effects of hydrogen-bonded cysteine (Cys), cystine, and serine (Ser) on the photochemical properties of tryptophan (Trp) enantiomers were investigated at low temperatures in the gas phase. The difference in the relative intensity at 270-280 nm in the ultraviolet photodissociation spectra at 8 K showed that the higher excited states of H⁺(L-Trp) (L-Cys) were red-shifted by stronger intermolecular interactions compared to H⁺(D-Trp) (L-Cys). Photoexcitation of protonated Trp enantiomers hydrogen-bonded to Cys and cystine led to detachment of the hydrogen-bonded molecules. The relative abundance of the NH₂CHCOOH loss from H⁺(D-Trp) (L-Ser) was higher than that of the loss from H⁺(L-Trp) (L-Ser) in the product ion spectra obtained by 285 nm photoexcitation, indicating that hydrogen bonding with L-Ser induces the enantiomer-selective photodissociation of Trp. The photodissociation spectra for the S₁-S₀ transition of H⁺(L-Trp) (L-Ser) at 8-100 K showed the conformational changes to the conformers with weaker intermolecular interactions at high temperatures. In terms of their influence on the photochemical properties of Trp enantiomers at low temperatures in the gas phase, Cys and Ser can be classified as the same type of alanine and leucine, respectively.