BINDING-ENERGIES OF PROTONATED BETAINE COMPLEXES - A PROBE OF ZWITTERION STRUCTURE IN THE GAS-PHASE

The dissociation kinetics of proton-bound dimers of betaine with molec ules of comparable gasphase basicity were investigated using blackbody infrared radiative dissociation (BIRD). Threshold dissociation energi es were obtained from these data using master equation modeling. For b ases that have comparable or higher gas-phase basicity, the binding en ergy of the protonated base betaine complex is similar to 1.4 eV. For molecules that are similar to 2 kcal/mol or more less basic, the disso ciation energy of the complexes is similar to 1.2 eV. The higher bindi ng energy of the former is attributed to an ion-zwitterion structure w hich has a much larger ion-dipole interaction. The lower binding energ y for molecules that are similar to 2 kcal/mol or more less basic indi cates that an ion-molecule structure is more favored. Semiempirical ca lculations at both the AM1 and PM3. levels indicate the most stable io n-molecule structure is one in which the base interacts with the charg ed quaternary ammonium end of betaine. These results indicate that the measurement of binding energies of neutral molecules to biological io ns could provide a useful probe for the presence of zwitterions and sa lt bridges in the gas phase. From the BIRD data, the gas-phase basicit y of betaine obtained from the kinetic method is found to be 239.2 +/- 1.0 kcal/mol. This value is in excellent agreement with the value of 239.3 kcal/mol (298 K) from ab initio calculations at the MP2/6-31+g* level. The measured value is slightly higher than those reported prev iously. This difference is attributed to entropy effects. The lower io n internal energy and longer time frame of BIRD experiments should pro vide values closer to those at standard temperature.