OAK-B135 Our BES funded research is aimed at determining structure(s) of model gas-phase ions and understanding how structure influences unimolecular reactivity. The model gas-phase ions include positional isomers of di- and tri-amino acids synthesized in my laboratory, i.e., RGG, GRG, and GGR, to peptides derived from proteolytic digestion of biologically relevant proteins. We are especially interested in understanding the role of intramolecular interactions in the stabilizing ion structure and how changing the charge-site affects structure. The location of charge of gas-phase ions can be manipulated by changing the position of the charge carrying amino acid (basic vs. acidic side chains) and by derivatization of the N- and/or C-terminus. For example, the proton of(M+ H)+ ions is mobile and migrates over the entire molecule, whereas Li+, Na+, and to some extent K+ prefers to bind to the C-terminal or side-chain carboxylic acid groups, and Cu+ binds exclusively to the N-terminus and/or basic side-chains such as H, K, and R. The studies are carried out using tandem TOF mass spectrometry, viz. 193 nm (6.43 eV) photodissociation, low (Elab= 10-100 eV) and high kinetic energy (Elab= 1-10 keV) collision-induced dissociation (CID) and surface-induced dissociation (SID)(Elab= 20-70 eV). These techniques are used to probe the structure of model gas-phase ions, i.e., to determine the amino acid sequence of the peptide ions or metal ion (alkali metal and/or transition metal ions) binding site(s) or the site(s) of other charge-carrying functional groups, i.e., oxidized side-chains as well as phosphate or sulfate groups. We are especially interested in understanding how metal ion binding alters the secondary/tertiary (2o/3o) structure of the peptide, i.e., intra-molecular interactions. We have also combine these studies with solution-phase studies and ion mobility spectrometry (IMS), which can be used to study 2o/3o structure of low-internal energy (collisionally stabilized) ions. It is difficult to probe 2o/3o structure of gas-phase ions using fragmentation chemistry, because the energy barriers to inter-conversion of different structural forms lie below the fragmentation threshold, studies of low internal energy ions are more suited for these studies. A major challenge for gas-phase ion research is the design of experimental structural probes that can be used in parallel with computational chemistry, molecular modeling and/or classical structural diagnostic tools to aid interpretation of the experimental data. Our experimental design and selection of research problems is guided by this philosophy. The following section of the progress report focus on three main issues: (1) technique and instrument development, and (2) studies of ion structure and ion chemistry.
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