Hydration Isomers of Protonated Phenylalanine and Derivatives: Relative Stabilities from Infrared Photodissociation

摘要

Abstract: The binding sites of water molecules to protonated Phe and its derivatives are investigated usingninfrared photodissociation (IRPD) spectroscopy and kinetics as well as by computational chemistry.nCalculated relative energies for hydration of PheH at various sites on the N- and C-termini depend on thentype of theory and basis set used, and no one hydration site was consistently calculated to be most favorable.nInfrared photodissociation (IRPD) spectra between ∼2650 and 3850 cm 1 are reported for PheH (H2O)1 4nat 133 K and compared to calculated absorption spectra of low-energy hydration isomers, which do notnresemble the IRPD spectra closely enough to unambiguously assign spectral bands. The IRPD spectra ofnPheH (H2O)1 4 are instead compared to those of N,N-Me2PheH (H2O)1,2, N-MePheH (H2O)1 3, andnPheOMeH (H2O)1 3 at 133 K, which makes possible systematic band assignments. A unique bandnassociated with a binding site not previously reported for PheH (H2O), in which the water molecule acceptsna hydrogen bond from the N-terminus of PheH and donates a weak hydrogen bond to the π-system ofnthe side chain, is identified in the IRPD spectra. IRPD kinetics at laser frequencies resonant with specificnhydration isomers are found to be biexponential for N,N-Me2PheH (H2O), N-MePheH (H2O), andnPheH (H2O). Relative populations of ions with water molecules attached at various binding sites arendetermined from fitting these kinetic data, and relative energies for hydration of these competitive bindingnsites at 133 K are obtained from these experimental values.