No abstract

The gas-phase acidities of the 20 L-amino acids have been predicted at the composite G3(MP2) level. A broad range of structures of the neutral and anion were studied to determine the lowest energy conformer. Excellent agreement is found with the available experimental gas-phase deprotonation enthalpies, and the calculated values are within experimental error. We predict that tyrosine is deprotonated at the CO(2)H site. Cysteine is predicted to be deprotonated at the SH but the proton on the CO(2)H is shared with the S(-) site. Self-consistent reaction field (SCRF) calculations with the COSMO parametrization were used to predict the pK(a)'s of the non-zwitterion form in aqueous solution. The differences in the non-zwitterion pK(a) values were used to estimate the free energy difference between the zwitterion and nonzwitterion forms in solution. The heats of formation of the neutral compounds were calculated from atomization energies and isodesmic reactions to provide the first reliable set of these values in the gas phase. Further calculations were performed on five rare amino acids to predict their heats of formation, acidities, and pK(a) values.

No abstract

No abstract

Gas‐phase deprotonation reactions, hydrogen–deuterium exchange reactions and collision‐induced dissociation (CID) were used to distinguish between two isomeric forms of [M+12H]12+ produced from the protein ubiquitin. Ions were generated by electrospray ionization and studied in a Fourier transform ion cyclotron resonance mass spectrometer. For [M+12H]12+ formed directly from the electrospray process, deprotonation reactions with ammonia and 2‐fluoropyridine yield non‐linear pseudo‐first‐order kinetic behavior that indicates the presence of two ion structures. The fraction of ions that undergo the fastest deprotonation reactions, and is presumably the least energetically stable isomer, accounts for ∽60% of the [M+12H]12+ produced by electrospray. In reactions with D2O and CD3OD, the [M+12H]12+ which are deprotonated faster exchange the first 11±1 hydrogens more readily that the remaining [M + 12H]12+ population. Results from CID experiments, obtained as a function of reaction time with the amines, also indicate the existence of more than one [M+12H]12+ structure. The CID fragmentation patterns provide information about the general locations of the charge sites. Surprisingly, evidence for only one structure (the slow‐reacting, more stable species) is found for [M+12H]12+ that is produced by gas‐phase deprotonation of [M+13H]13+, which is the ‘fully protonated’ form of ubiquitin. These results are discussed in terms of ubiquitin isomers related to protonation site and three‐dimensional conformation.