Characterization of Two Positional Isomers of the Cs⁺Gly Complex Using Two-Color, IR-IR Photobleaching of the...

Metal ion binding to amino acid residues is an important interaction motif that controls the tertiary structures of oligopeptides. Analyses of the vibrational band patterns displayed by the amino acid scaffolds are commonly used to characterize the local docking motifs. Here we carry out two-color, IR-IR photobleaching measurements to obtain isomer-selective vibrational spectra of the Cs⁺Gly ion-molecule complex isolated in a cryogenically cooled, radiofrequency ion trap. The distinct band patterns of two noninterconverting isomers are observed and traced to different bidentate binding motifs between Cs⁺ and the glycine scaffold. In one isomer, the ion attaches to the oxygen atoms of the carboxyl group whereas in the other it docks to the amino nitrogen and the carbonyl oxygen. Attachment to the acid headgroup yields a very diffuse absorption associated the OH group engaged in a strong intramolecular H-bond that closes a 5 membered ring. The band assignments, rearrangement pathways and electrostatic distortion of the electron density distributions in the glycine scaffold by the proximal ion are explored with electronic structure calculations and anharmonic theory.