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 approximately 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.
Polfer
Nick C
NC
FOM Institute for Plasma Physics Rijnhuizen, Edisonbaan 14, 3439MN, Nieuwegein, The Netherlands.
Oomens
Jos
J
Dunbar
Robert C
RC
D001665:Binding Sites; D002413:Cations, Divalent; D046911:Macromolecular Substances / Q000737:chemistry*; D008672:Metals, Alkali / Q000737:chemistry*; D008958:Models, Molecular; D012834:Silver / Q000737:chemistry*; D012997:Solvents / Q000737:chemistry; D013055:Spectrophotometry, Infrared; D013816:Thermodynamics; D014364:Tryptophan / Q000737:chemistry*
D002413:Cations, Divalent; D046911:Macromolecular Substances; D008672:Metals, Alkali; D012997:Solvents; D012834:Silver; D014364:Tryptophan