Gas-phase basicity and acidity of tryptophan

Extensive exploration of the conformational space of neutral, protonated and deprotonated tryptophan has been conducted at various levels of theory including B3LYP, M06-2X, CBS-QB3 and G4MP2 methods. The two latter composite methods, CBS-QB3 and G4MP2, and the M06-2X functional provide very close results in term of relative H° ₂₉₈ and G° ₂₉₈ values between conformers. By contrast, B3LYP functional leads to less satisfying results. Theoretical protonation and deprotonation thermochemistry has been calculated using either the most stable conformers or an equilibrium population of conformers at 298 K. Gas-phase protonation thermochemistry of tryptophan has been experimentally determined by the extended kinetic method using ESI-TQ tandem mass spectrometry. Proton affinity and protonation entropy deduced from these experiments, PA(Trp) = 945.6 ± 2.0(4.3) kJ mol ^-1 and Δ _pS°(Trp) = -16 ± 2(5) J mol ^-1 K ^-1 (uncertainties are standard deviation and, into parentheses, 95% confidence limit). These figures lead to a gas-phase basicity value of GB(Trp) = 908.4 ± 2.1(4.6) kJ mol ^-1. These experimental data are perfectly reproduced by theoretical calculations performed at the G4MP2 level. Computational results provide also insights on possible re-interpretation of (i) IRMPD spectrum of deprotonated tryptophan, and (ii) mechanism of the NH ₃ loss from protonated tryptophan.