RESUMO
Tandem mass spectrometry is routinely used for the structural analysis of organic molecules, but many fragmentation reactions are not well understood. Because several potential structures can correspond to a measured mass, the assignment of product ions is ambiguous using mass spectrometry alone. Here, we combine mass spectrometry with high-resolution gas-phase infrared spectroscopy and computational chemistry tools to identify product ion structures and derive collision-induced fragmentation mechanisms of the chromane derivatives Trolox and Methyltrolox. We find that protonated Trolox and Methyltrolox fragment identically via dehydration and decarbonylation, while deprotonated ions display substantially diverging reactivities. For deprotonated Methyltrolox, we observe unusual radical fragmentation reactions and suggest a [1,2]-Wittig rearrangement involving aryl migration in the gas phase. Overall, the combined experimental and theoretical approach presented here revealed complex proton dynamics and intramolecular rearrangement reactions, which expand our understanding on structure-reactivity relationships of isolated molecules in different protonation states.
RESUMO
How many solvent molecules are required to solvate an amino acid? This apparently simple question, which relates to the number of solvent molecules necessary to change the amino acid from its gas-phase neutral structure to the zwitterionic solvated structure, remains unanswered to date. Here we present experimental and theoretical (density functional theory: B3LYP/6-31+G**) infrared spectra for tryptophan-watern complexes where n = 1-6, which suggest that the zwitterionic structure becomes competitive in energy at the high end of the series. Compelling evidence for a gradual transition to zwitterionic structures comes from tryptophan-methanol complexes up to n = 9. Starting from n = 5, the infrared spectra show increasing intensity in the diagnostic asymmetric COO- stretch and in the weaker NH3+ bending modes as the cluster size increases. Moreover, convergence toward the Fourier transform infrared spectrum of a solution of tryptophan in methanol is clearly observed. For small solvent complexes (n = 1-4), the microsolvation by methanol and water is shown to behave very similarly. A detailed comparison of the experimental and the theoretical spectra allows us to determine both the preferred solvent binding sites on the amino acid and the evolution of conformational structures of tryptophan as the number of attached solvent molecules increases.
