Infrared multiphoton dissociation spectra of protonated p-aminobenzoic acid generated by electrospray ionization (ESI) from aqueous methanol and acetonitrile solutions were recorded in the gas phase from 2800-4000 cm(-1). The O-protonated ion is more stable than the N-protonated structure in the gas phase, whereas the opposite is true in both solutions. When CH(3)OH/H(2)O was used as the ESI solvent, only the O-protonated ion was observed. In contrast, a 70:30 mixture of the O- and N-protonated species were produced from CH(3)CN/H(2)O. These structural assignments are based on an assortment of experimental data (action spectra, photofragments, photofragmentation kinetics, and H/D exchange) and are fully supported by extensive computations. This work shows that ESI can lead to isomerization and that the ionization site may be varied by changing the solvent from which the substrate is analyzed.

Hydrogen bond interactions in small covalent model compounds (i.e., deprotonated polyhydroxy alcohols) were measured by negative ion photoelectron spectroscopy. The experimentally determined vertical and adiabatic electron detachment energies for (HOCH(2)CH(2))(2)CHO(-)(2a), (HOCH(2)CH(2))(3)CO(-) (3a), and (HOCH(2)CH(2)CH(OH)CH(2))(3)CO(-) (4a)reveal that hydrogen-bonded networks can provide enormous stabilizations and that a single charge center not only can be stabilized by up to three hydrogen bonds but also can increase the interaction energy between noncharged OH groups by 5.8 kcal mol(-1) or more per hydrogen bond. This can lead to pK(a) values that are very different from those in water and can provide some of the impetus for catalytic processes.

Phospholipid hydroperoxide (PLOOH) degrading activity of high density lipoprotein (HDL)-derived paraoxonase-1 (PON1) was investigated, using peroxidized 1-palmitoyl-2-oleoyl phosphatidylcholine (PCOOH) as substrate and high performance thin layer chromatography for quantitative peroxide analysis. Incubation of PCOOH with PON1 resulted in decay of the latter and reciprocal buildup of oleic acid hydroperoxide (OAOOH) at rates unaffected by GSH or other reductants. A serine esterase inhibitor blocked this activity and a recombinant PON1 was devoid of it, raising the possibility that the activity represents platelet-activating factor acetylhydrolase (PAF-AH), an esterase that co-purifies with PON1 from HDL. This was verified by showing that a recombinant PAF-AH recapitulates the ability of natural PON1 to hydrolyze PCOOH and release OAOOH while having essentially no effect on parental PC. Furthermore, recombinant PAF-AH and natural PON1 were shown to have similar K m values for PCOOH hydrolysis. Finally, we found that recombinant PAF-AH, but not PON1, catalyzes PLOOH hydrolysis in peroxidized low density lipoprotein. We conclude from this study that PON1 is neither a PLOOH peroxidase nor hydrolase and that the phospholipase A 2 -like activity previously attributed to PON1 in natural enzyme preparations was actually due to novel PLOOH hydrolytic activity of contaminating PAF-AH.

Enzymes and their mimics use hydrogen bonds to catalyze chemical transformations. Small-molecule transition state analogues of oxyanion holes have been characterized by computations, gas-phase IR and photoelectron spectroscopy, and determination of their binding constants in acetonitrile. A new class of hydrogen bond catalysts is proposed (donors that can contribute three hydrogen bonds to a single functional group) and demonstrated in a Friedel-Crafts reaction. The employed catalyst was observed to react 100 times faster than its rotamer that can employ only two hydrogen bonds. The former compound also binds anions more tightly and was found to have a thermodynamic advantage.

Infrared photodissociation (IRPD) spectra are reported for a proline-chloride anion cluster along with its d2- and d7-isotopomers. The spectral data indicate that proline is in its neutral form as opposed to a zwitterion, and computations are in agreement in that some neutral conformers are energetically low-lying and reproduce the observed spectra. Zwitterionic conformers are predicted to be essentially as stable as the neutral ones and should be significantly populated; however, there is no evidence for these structures in the IRPD spectra. An exploration of the potential energy surface for the loss of chloride anion, the observed fragmentation channel, reveals that it is 8.4 kcal mol(-1) more difficult to break apart the zwitterionic cluster ion. This is a reflection of the 15.8 kcal mol(-1) estimate for the gaseous proline zwitterion-neutral energy difference. Kinetic results suggest the presence of two photolabile populations in similar amounts (i.e., 56 vs 44%). The more abundant structure is also the more labile species, and the neutral form of proline is assigned to this cluster ion. The less abundant and slower fragmenting structure consequently is zwitterionic. As originally suggested by Evans et al. in general, it appears that in this instance both spectral and kinetic data are needed to determine the structure of the proline-chloride anion cluster.