Does Chiral Sensitivity of a Structure Depend on the Metal Core? Alkali Ion Complexes of Cyclo(Tyr-Tyr)

Alkali metal complexes of cyclic dipeptide cyclo Tyr-Tyr have been studied under cryogenic ion trap conditions. Their structure was obtained by combining Infra-Red Photo-Dissociation (IRPD) and quantum chemical calculations. The structural motif strongly depends on the relative chirality of the tyrosine residues. For residues of identical chirality, the cation interacts with one amide oxygen and one of the aromatic rings only; the distance between the aromatic rings does not change with the nature of the metal. In contrast, for residues of opposite chirality, the metal cation is located in between the two aromatic rings and interacts with both of them. The distance between the two aromatic rings strongly depends on the metal. Electronic spectra obtained by Ultra Violet Photodissociation (UVPD) spectroscopy and analysis of the UV photo-fragments shed light on the excited state deactivation processes, which depend on both the chirality of the residue and that of the metal ion core. Na⁺ stands out by the presence of low-lying charge transfer states resulting in the broadening of the electronic spectrum.