Cation-Size-Dependent Conformational Locking of Glutamic Acid by Alkali Ions: Infrared Photodissociation Spectroscopy of Cryogenic Ions.

Consolidated knowledge of conformation and stability of amino acids and their clusters is required to understand their biochemical recognition. Often, alkali ions interact with amino acids and proteins. Herein, infrared photodissociation (IRPD) spectra of cryogenic metalated glutamic acid ions (GluM+ , M = Li-Cs) are systematically analyzed in the isomer-specific fingerprint and XH stretch ranges (1100-1900, 2600-3600 cm-1 ) to provide a direct measure for cation-size-dependent conformational locking. GluM+ ions are generated by electrospray ionization and cooled down to 15 K in a cryogenic quadrupole ion trap. The assignment of the IRPD spectra is supported by density functional theory calculations at the dispersion-corrected B3LYP-D3/aug-cc-pVTZ level. In the global minimum of GluM+ , the flexibility of Glu is strongly reduced by the formation of rigid ionic CO···M+ ···OC metal bridges, corresponding to charge solvation. The M+ binding energy decreases monotonically with increasing cation size from D0 = 314 to 119 kJ/mol for Li-Cs. Whereas for Li and Na only the global minimum of GluM+ is observed, for K-Cs at least three isomers exist at cryogenic temperature. The IRPD spectra of cold GluM+ ions are compared to IR multiple-photon dissociation spectra measured at room temperature. Furthermore, we elucidate the differences of the impact of protonation and metalation on the structure and conformational locking of Glu.