Item

Abstract

The homochirality of the amino acid metabolism still puzzles biochemists. Vibrational spectroscopy of mass-
selected gas-phase amino acids and their clusters can precisely reveal their conformation and might ultimately help to decode the interactions responsible for chirality recognition. Infrared photo-
dissociation (IRPD) and conformer-selective IR–IR hole burning spectra of protonated glutamic acid dimers (LL-/LD-Glu₂H⁺) recorded in the fingerprint and XH stretch ranges (1100–1900 and 2600–3600 cm^-1) provide direct insight into their stereospecific interactions. Glu₂H⁺ dimers are generated by electrospray
ionization and stored in a cryogenic quadrupole ion trap held at 10 K. The assignment of the IRPD spectra is
supported by vibrational analysis using many-body dispersion-corrected hybrid density-functional theory.
Sampling of the conformational space is accomplished by basin hopping and replica-exchange molecular dynamics simulations. The most stable LD-Glu₂H⁺ dimer (LD1) is predicted to be more stable than the most stable LL-Glu₂H⁺ dimer (LL1) by DE₀
= 4.0kJmol^-1, which relies on stronger secondary interactions in LD1
as demonstrated by the noncovalent interaction method. IR–IR hole burning spectroscopy reveals the coexistence of at least four LD-Glu₂H⁺ and three LL-Glu₂H⁺
conformers. Their IR-dip spectra are assigned to the most stable conformers at room and cryogenic temperature, revealing incomplete thermalization of the ions by kinetic trapping in the cold trap. We observe different population ratios of LL and LD conformers of Glu₂H⁺, as revealed by specific ⁿNH₂ and ⁿ_CO
intensities (fingerprints of chirality recognition).