Role of Sequence in Salt-Bridge Formation for Alkali Metal Cationized GlyArg and ArgGly Investigated with IRMPD Spectroscopy and Theory

The roles of hydrogen bonding, electrostatic interactions, sequence, gas-phase basicity, and molecular geometry in determining the structures of protonated and alkali metal-cationized glycyl-L-arginine (GlyArg) and L-arginylglycine (ArgGly) were investigated using infrared multiple photon dissociation spectroscopy in the spectral range 900-1800 cm(-1) and theory. The IRMPD spectra clearly indicate that GlyArg center dot M+, M = Li, Na, and Cs, form similar salt-bridge (SB) structures that do not depend significantly on metal ion size. In striking contrast, ArgGly center dot Li+ exists in a charge-solvated (CS) form, whereas ArgGly center dot M+, M = K and Cs, form SB structures. SIB and CS structures are similarly populated for ArgGly center dot Na+. Computed energies of low-energy structures are consistent with these results deduced from the experimental spectra. By comparison to Arg center dot M+, GlyArg center dot M+ and ArgGly center dot M+ have a greater and lesser propensity, respectively, to form SIB structures. The greater propensity for GlyArg to adopt SB structures in complexes with smaller metal cations than for ArgGly is due to the ability of alkali metal-cationized GlyArg to adopt a nearly linear arrangement of formal charge sites, a structure unfavorable for ArgGly complexes due to geometric constraints induced by its different amino acid sequence. The amide carbonyl oxygen solvates charge in both the SIB and CS form of both dipeptides. ArgGly is calculated to be slightly more basic than GlyArg, indicating that differences in intrinsic basicity do not play a role in the relative SB stabilization of these ions. Loss of a neutral water molecule from complexes in which SB structures are most stable indicates that CS structures are intermediates in the dissociation pathway, but these intermediates do not contribute to the measured IRMPD spectra.