On the formation of radical dications of protonated amino acids in a "microsolution" of water or acetonitrile and their reactivity towards the solvent

In high-energy collisions (50 keV) between O-2 and protonated amino acids A H(+), radical dications AH(2+.) are formed for A = Phe. His, Met, Tyr. and Trp. When solvated by water or acetonitrile (S). AH(2+.)(S)(1.2) are formed for A = Arg, His. Met, Tyr. and Trp. The stability of the hydrogen-deficie nt AH(2+.) in the "microsolution" depends on the energetics of the electron transfer reaction AH(2+.) + S --> AH(+) + S+., the hydrogen abstraction re action AH(2+.) + S --> AH(2)(2+) +[S-H](.), and the proton transfer reactio n AH(2+.) + S --> A(+.) + SH+.. Using B3LYP/ 6-311 + G(2d,p)//B3LYP/6-31 G(d) model chemistry, we describe these three reactions in detail for A = T yr and find that the first two reactions are unfavorable whereas the third one is favorable. However, energy is required for the formation of Tyr(+.) and SH+. from TyrH(2+.) (S) to overcome the Coulomb barrier, which renders the complex observable with a life-time larger than 5 mus. The ionization e nergy, IE, of TyrH(+) is calculated to be 11.1 eV in agreement with an expe rimental measurement of 10.1 +/- 2.1 eV ([IE(CH3CN) + IE(Tyr)]/ 2); hydrati on further lowers the IE by 0.3 eV [IE(TyrH(+)(H2O) = 10.8 eV, calculated]. We estimate the ionization energies of TrpH(+), HisH(+), and MetH(+) to be 10.1 +/- 2.1 eV, 12.4 +/- 0.2 eV, and 12.4 0.2 eV. and that of PheH(+) to be larger than 12.6 eV.