Effect of Charge and Analyte Structure on Amino Acid-Water Droplets

Mass spectrometry (MS) has frequently been applied for the identification of molecules and structure determination due to its sensitivity and versatility. It is frequently coupled with soft ionization processes, such as electrospray ionization; however, the presence of microsolvated phases generated during these processes─intermediates between the condensed and gas phase─can alter the chemistry of analytes and therefore their detection and identification. To understand these processes, we use computational methods (semiempirical methods and density functional theory) to probe the ionization propensity of four small aliphatic amino acids─alanine, glycine, valine, and proline─in the gas phase and aqueous microsolvated clusters. We show that the tautomeric form of each amino acid is altered by the presence of water, with glycine requiring as little as three water molecules to stabilize the zwitterionic form and the other amino acids requiring four to five water molecules. We also show that the inclusion of ionic cosolutes can stabilize the zwitterion with even fewer (one or two) water molecules present. Ionization propensity as measured by gas phase basicity (GB) is also altered in the microsolvated state─distinct gas phase values in each of the four amino acids show remarkably similar GB values in the cluster form, revealing the sensitivity of the analyte to the solvation environment. The structural characteristics of these amino acid-water clusters are also determined, including symmetry, the total number of hydrogen bonds, and hydrogen bonding networks.