Probing Isomers and Conformers by Cryogenic Ion Vibrational Spectroscopy: Deprotonated States of Valine and Aminovaleric Acid

We present infrared photodissociation spectra of messenger-tagged, deprotonated valine and deprotonated aminovaleric acid, showing the difference in the vibrational spectra of isomers with the same functional groups. Through comparison of experimental results and density functional theory calculations, we find that the deprotonated states of both valine and aminovaleric acid adopt a configuration in which the carboxylate group forms a hydrogen bond with the amine group. Despite the similarities of the intramolecular hydrogen bonding, the spectra of the two molecules under study are sufficiently different to use the IR signatures of the carboxylate and amine functional groups as a means to distinguish between them. The conformational search in the case of aminovaleric acid is particularly challenging due to the large number of possible conformations induced by torsion around individual carbon-carbon bonds. For both valine and aminovaleric acid, their IR spectra in their deprotonated states suggests that two of the lowest energy conformers are likely to be populated.