BINDING-ENERGIES OF THE PROTON-BOUND AMINO-ACID DIMERS GLY-CENTER-DOT-GLY, ALA-CENTER-DOT-ALA, GLY-CENTER-DOT-ALA, AND LYS-CENTER-DOT-LYS MEASURED BY BLACKBODY INFRARED RADIATIVE DISSOCIATION

Arrhenius activation energies in the zero-pressure Limit for dissociat ion of gas-phase proton-bound homodimers of N,N-dimethylacetamide (N,N -DMA), glycine, alanine, and lysine and the heterodimer alanine glycin e were measured using blackbody infrared radiative dissociation (BIRD) . In combination with master equation modeling of the kinetic data, bi nding energies of these dimers were determined. A value of 1.25 +/- 0. 05 eV is obtained for N,N-DMA and is in excellent agreement with that reported in the literature. The value obtained from the truncated Bolt zmann model is significantly higher, indicating that the assumptions o f this model do not apply to these ions. This is due to the competitiv e rates of photon emission and dissociation for these relatively large ions. The binding energies of the amino acid dimers are similar to 1. 15 +/- 0.05 eV and an indistinguishable despite the difference in thei r gas-phase basicity and structure, The threshold dissociation energie s can be accurately modeled using a range of dissociation parameters a nd absorption/emission rates. However, the absolute values of the diss ociation rates depend more strongly on the absorption/emission rates. For N,N-DMA and glycine, an accurate fit was obtained using frequencie s and transition dipole moments calculated at the ab initio RHF/2-31G and MP2/2-31G level, respectively. In order to obtain a similar accu racy using values obtained from AM1 semiempirical calculations, it was necessary to multiply the transition dipole moments by a factor of 3. These results demonstrate that in combination with master equation mo deling, BIRD can be used to obtain accurate threshold dissociation ene rgies of relatively small ions of biological interest.