On the Formation of Radical Dications of Protonated Amino Acids in a “Microsolution” of Water or Acetonitrile and Their Reactivity Towards the Solvent

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Abstract
In high‐energy collisions (50 keV) between O2 and protonated amino acids AH+, radical dications AH2+. are formed for A=Phe, His, Met, Tyr, and Trp. When solvated by water or acetonitrile (S), AH2+.(S)1,2 are formed for A=Arg, His, Met, Tyr, and Trp. The stability of the hydrogen‐deficient AH2+. in the “microsolution” depends on the energetics of the electron transfer reaction AH2+.+S→AH++S+., the hydrogen abstraction reaction AH2+.+S→AH22++[S−H]., and the proton transfer reaction AH2+.+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=Tyr 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 TyrH2+.(S) to overcome the Coulomb barrier, which renders the complex observable with a life‐time larger than 5 μs. The ionization energy, IE, of TyrH+ is calculated to be 11.1 eV in agreement with an experimental measurement of 10.1±2.1 eV ([IE(CH3CN)+IE(Tyr)]/2); hydration 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.