Temperature Dependence of D-Amino-Acid Selectivity of L-Tryptophan Probed by Ultraviolet Photodissociation Spectroscopy

Temperature effects on differentiating d-amino acids using the molecular recognition ability of l-tryptophan were investigated by ultraviolet photodissociation spectroscopy in the gas phase. Temperature-dependent ultraviolet photodissociation spectra of hydrogen-bonded protonated clusters of l-tryptophan with arginine, lysine, asparagine, and glutamine enantiomers, generated <i>via</i> electrospray ionization, were obtained using a tandem mass spectrometer containing a variable-temperature ion trap. The spectra at 8 K differed between the amino acids and their enantiomers, indicating that l-tryptophan recognized amino acids and their enantiomers through its hydrogen bonding and electronic structure. The spectral differences observed at 100 K were significantly smaller than those at 8 K. Hot bands and entropic effects at liquid nitrogen cooling temperature prevented the differentiation of d-amino acids. To avoid these contributions in the spectra, cooling of the hydrogen-bonded clusters using a cryogenic refrigerator was necessary to distinguish amino acids and their enantiomers based on the molecular recognition of l-tryptophan.