Cationized phenylalanine conformations characterized by IRMPD and computation for singly and doubly charged ions

Abstract

Electrospray ionization produces phenylalanine (Phe) complexes of the alkali metal ion series, plus Ag⁺ and Ba²⁺. Infrared multiple photon dissociation (IRMPD) spectroscopy using the FELIX free electron laser light source is used to characterize the conformations of the ions, in conjunction with density functional theory (DFT) calculations giving thermochemical information and computed infrared spectra for likely candidate conformations. For complexes of small, singly charged ions (Li⁺, Na⁺, K⁺ and Ag⁺) a single tridentate, charge-solvated conformational theme (N/O/Ring) binding amino nitrogen, carbonyl oxygen and the aromatic ring to the metal ion accounts for all the observations. The larger alkalis Rb⁺ and Cs⁺ show clear spectroscopic evidence of mixed populations, containing substantial fractions of both tridentate and also bidentate chelation. For Rb⁺ the bidentate fraction is assigned as the (O/Ring) chelation pattern, while for Cs⁺ a mixture of (O/Ring) and (O/O) chelation patterns seems likely. All of the smaller ions with high positive charge density have a clear preference for cation–π interaction with the side-chain aromatic ring, but for the larger ions Rb⁺ and particularly Cs⁺ this interaction becomes sufficiently weak to allow conformations having the metal ion remote from the π system. The Ba²⁺ complex is unique in showing clear evidence of a major fraction of salt-bridge (zwitterionic) ions along with charge-solvated conformations. Plots of the frequency shifts of the two highly perturbed ligand vibrational modes (CO stretch and NH₂ frustrated inversion) give good linear correlations with the binding energy of the metal to the ligand.