Gas phase protonation thermochemistry of phenylalanine and tyrosine.

Gas phase basicities of phenylalanine and tyrosine, GB(Phe) = 892.0 +/- 1.3(2.6) kJ.mol(-1) and GB(Tyr) = 894.9 +/- 2.8(5.9) kJ.mol(-1) (uncertainties are standard deviation and, in parentheses, 95% confidence limit), have been experimentally determined by the extended kinetic method using ESI-TQ tandem mass spectrometry. Proton affinities deduced from these experiments, PA(Phe) = 931.3 +/- 1.1(2.3) kJ.mol(-1) and PA(Tyr) = 934.8 +/- 2.5(5.2) kJ.mol(-1), are perfectly reproduced by theoretical calculations performed at the B3LYP/6-311++G(3df,2p)//B3LYP/6-31+G(d,p) level. An entropy loss of approximately -25 J.mol(-1).K(-1) occurs upon protonation of both Phe and Tyr. The origin of this entropy change is attributed (i) to the change in strength of the interaction between the amino group and the aromatic moiety in the neutral and protonated forms and (ii) to the larger entropy of mixing associated with the population of neutral conformers with respect to their protonated counterparts. Previous neglect of the protonation entropy term has led to underestimated tabulated PA values; the evaluated values proposed in the present study are PA(Phe) = 932 +/- kJ.mol(-1) and PA(Tyr) = 935 +/- kJ.mol(-1).