The structures of proton-bound dimers of glycine with phenylalanine and pentafluorophenylalanine

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Infrared multiple photon dissociation spectra of the proton-bound heterodimers of phenylalanine/glycine (Phe·H+·Gly) and pentafluorophenylalanine/glycine (F5-Phe·H+·Gly) have been acquired in the 975–1975cm−1 region. Exhaustive basin hopping molecular dynamics searches and subsequent DFT calculations predict four low energy intermolecular binding motifs. The spectrum of F5-Phe·H+·Gly is assigned predominantly to isomers exhibiting a NH+⋯O intermolecular interaction between nitrogen-protonated Gly and the carbonyl oxygen atom of F5-Phe. In contrast, the spectrum Phe·H+·Gly is found to consist of a mixture of isomers exhibiting the NH+⋯O between nitrogen-protonated Gly and the carbonyl oxygen of neutral Phe, and isomers exhibiting a NH+⋯N interaction between nitrogen-protonated Phe and the neutral Gly moiety. Cation-π effects also seem to influence the cluster geometries, especially in the case of the Phe·H+·Gly global minimum structure where the phenyl ring orients with the site of protonation so as to maximize charge-quadrupole interactions.