Infrared Multiple Photon Dissociation Spectroscopy of Cationized Asparagine: Effects of Metal Cation Size on Gas-Phase Conformation

Gas-phase structures of cationized asparagine (Asn) including complexes with Li+, Na+, K+, Rb+, Cs+, and Ba2+, as well as protonated Asn, are examined by infrared multiple photon dissociation (IRMPD) action spectroscopy utilizing light generated by a free electron laser. Experimental spectra for the alkali metal cation complexes exhibit systematic trends, whereas spectra for Ba2+(Asn) and H+(Asn) are more distinct. To identify the structures formed experimentally, measured IRMPD spectra are compared to spectra calculated at a B3LYP/6-311+G(d,p) level with several effective core potentials and basis sets evaluated for the heavy metal systems. The dominant conformation ascertained for complexes with the smaller metal cations, Li+(Asn) and Na+(Asn). is a charge-solvated, tridentate [N,CO,CO] Structure that binds the metal cation with the amine group of the amino acid backbone and to the carbonyl oxygen atorns of the backbone and amino acid side chain. For the larger alkali metal cation complexes, K+(Asn), Rb+(Asn), and Cs+(Asn), ail additional charge-solvated, tridentate [COOH,CO] structure that binds the metal cation With the two oxygen atorns of the backbone carboxylic acid group and the carbortyl oxygen atorn of the Asn side chain may also be present. The Ba2+(Asn) spectrum is characteristic of a single charge-solvated [N,CO,CO] conformation, in contrast to Gly Trp, ArIg. Gin, Pro, Ser, Val, and Glu, which all take on a zwitterionic Structure when complexed to Ba2+. In no case do the cationized Asn complexes show definitive evidence of forming a zwitterionic Structure in the complexes studied here. For H+(Asn), a mixture of two [N,CO] structures, which differ only in the orientation the side chain and are calculated to be nearly identical in energy, explains the experimental spectrum well.