IRMPD spectroscopy of deprotonated selenocysteine - The 21st proteinogenic amino acid

The effect of deprotonation on the structure and stability of the 21st proteinogenic amino acid selenocysteine, generated as bare deprotonated species by electrospray ionization, has been investigated by infrared multiple photon dissociation (IRMPD) spectroscopy over an extended frequency range (700-3600 cm- 1 ), encompassing both the fingerprint and X-H (X = C, N, O) stretching ranges. IRMPD spectra, interpreted by anharmonic DFT calculations, provide evidence of a thermally averaged ion population of two types of low-lying canonical conformers deprotonated at the selenol (Se-H) group and involved in different H-bonding motifs. The broadened and diffuse band structures observed in the H-stretching range are well interpreted by Born- Oppenheimer molecular dynamics computations that provide a valuable description of the flexible backbone arrangements of Sec and of the proton sharing dynamics in the Se-HO and Se-HN moieties. Other prototropic isomers, deprotonated at the carboxylic group or with a zwitterionic structure, should not be significantly populated, according to their higher free energy and calculated IR spectra inconsistent with experimental evidence.