Are Carboxyl Groups The Most Acidic Sites In Amino Acids?gas-phase Acidities, Photoelectron Spectra, And Computations On Tyrosine, P-hydroxybenzoic Acid, And Theirrnconjugate Bases

摘要

Deprotonation of tyrosine in the gas phase was found to occur preferentially at the phenolic site, and the conjugate base consists of a 70:30 mixture of phenoxide and carboxylate anions at equilibrium. This result was established by developing a chemical probe for differentiating these two isomers, and the presence of both ions was confirmed by photoelectron spectroscopy. Equilibrium acidity measurements on tyrosine indicated that △G°_(acid) = 332.5 ± 1.5 kcal mol~(-1) and △H°_(acid) = 340.7 ± 1.5 kcal mol~(-1). Photoelectron spectra yielded adiabatic electron detachment energies of 2.70 ± 0.05 and 3.55 ± 0.10 eV for the phenoxide and carboxylate anions, respectively. The H/D exchange behavior of deprotonated tyrosine was examined using three different alcohols (CF_3CH_2OD, C_6H_5CH_2OD, and CH_3CH_2OD), and incorporation of up to three deuterium atoms was observed. Two pathways are proposed to account for these results, and all of the experimental findings are supplemented with B3LYP/aug-cc-pVDZ and G3B3 calculations. In addition, it was found that electrospray ionization of tyrosine from a 3:1 (v/v) CH_3OH/H_2O solution using a commercial source produces a deprotonated [M - H]~- anion with the gas-phase equilibrium composition rather than the structure of the ion that exists in aqueous media. Electrospray ionization from acetonitrile, however, leads largely to the liquid-phase (carboxylate) structure. A control molecule, p-hydroxybenzoic acid, was found to behave in a similar manner. Thus, the electrospray conditions that are employed for the analysis of a compound can alter the isomeric composition of the resulting anion.