Gas-phase IR spectroscopy of deprotonated amino acids.

TLDR: In this article, gas-phase infrared multiple photon dissociation (IRMPD) spectra have been recorded for the conjugate bases of a series of amino acids (Asp, Cys, Glu, Phe, Ser, Trp, Tyr) and showed strong symmetric and antisymmetric carboxylate stretching modes around 1300 and 1600 cm−1, respectively.

Abstract: Gas-phase infrared multiple photon dissociation (IRMPD) spectra have been recorded for the conjugate bases of a series of amino acids (Asp, Cys, Glu, Phe, Ser, Trp, Tyr). The spectra are dominated by strong symmetric and antisymmetric carboxylate stretching modes around 1300 and 1600 cm−1, respectively. Comparison of the experimental spectra with spectra calculated at the DFT level suggests a carboxylate structure for all species investigated, which is in contrast with what has recently been suggested in this journal for deprotonated cysteine [J. Am. Chem. Soc. 2007, 129, 5403−5407]. In addition, the IR spectrum of the conjugate base of tyrosine is also unambiguously that of a carboxylate ion and not that of a phenoxide ion. In sharp contrast with the conjugate bases of other amino acids investigated here, the aspartate and glutamate anions show very broad, hardly resolved spectral features. We present qualitative experimental evidence indicating that this can be attributed to the formation of a proton br...