IRMPD spectroscopy of metal-ion/tryptophan complexes

Infrared multiple-photon dissociation (IRMPD) spectroscopy is employed to obtain detailed binding information on singly charged silver and alkali metal-ion/tryptophan complexes in the gas phase. For these complexes the presence of the salt bridge (i.e. zwitterionic) tautomer can be virtually excluded, except for perhaps a few percent in the case of Li+. Two low-energy structures having the charge solvation form are shown to be likely, where the metal cation is either in a tridentate N/O/Ring conformation or in a bidentate O/Ring conformation. These two structures can be conveniently discriminated and their relative quantities can be approximated by IR spectroscopy, based principally on diagnostic modes near ∼1150 (N/O/Ring) and 1400 (O/Ring) cm−1. Interestingly, the smaller cation complexes (i.e. Ag+ and Li+) display exclusively the N/O/Ring conformation, whereas the O/Ring conformer becomes progressively more abundant with increasing alkali metal size, eventually representing the majority of the ion population for Rb+ and Cs+. These spectroscopic findings are in excellent agreement with thermochemical density functional theory (DFT) calculations, giving support to the utility of high-level quantum-chemical calculations for such systems. Moreover, in contrast to other mass spectrometry-based techniques, IRMPD spectroscopy allows clear differentiation and semi-quantitative approximation of these metal–ligand binding motifs, thereby underlining its importance in thermochemical model benchmarking.