Infrared multiple photon dissociation spectroscopy of protonated histidine and 4-phenyl imidazole
Özet
The gas-phase structures of protonated histidine (His) and the side-chain model, protonated 4-phenyl
imidazole (PhIm), are examined by infrared multiple photon dissociation (IRMPD) action spectroscopy
utilizing light generated by the free electron laser FELIX. To identify the structures present in the experimental studies, the measured IRMPD spectra are compared to spectra calculated at a B3LYP/6–311+G(d,p)
level of theory. Relative energies of various conformers are provided by single point energy calculations
carried out at the B3LYP, B3P86, and MP2(full) levels using the 6–311+G(2d,2p) basis set. On the basis of
these experiments and calculations, the IRMPD action spectrum for H+(His) is characterized by a mixture
of [N,N] and [N,CO] conformers, with the former dominating. These conformers have the protonated
nitrogen atom of imidazole adjacent to the side-chain (N) hydrogen bonding to the backbone amino
nitrogen (N) and to the backbone carbonyl oxygen, respectively. Comparison of the present results to
recent IRMPD studies of protonated histamine, the radical His•+ cation, H+(HisArg), H2
2+(HisArg), and
M+(His), where M+ = Li+, Na+, K+, Rb+, and Cs+, allows evaluation of the vibrational motions associated
with the observed bands.
