title: On the formation of radical dications of protonated amino acids in a "microsolution" of water or acetonitrile and their reactivity towards the solvent.
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 --> AH2(2+)+[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 micros. 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.
authors.affiliation: Institute of Physics and Astronomy, Aarhus Center for Atomic Physics, University of Aarhus, Denmark.
authors.forename: M
authors.initials: M
authors.lastname: Sørensen
authors.forename: J S
authors.initials: JS
authors.lastname: Forster
authors.forename: P
authors.initials: P
authors.lastname: Hvelplund
authors.forename: T J
authors.initials: TJ
authors.lastname: Jørgensen
authors.forename: S B
authors.initials: SB
authors.lastname: Nielsen
authors.forename: S
authors.initials: S
authors.lastname: Tomita
mesh_terms: D000097:Acetonitriles / Q000737:chemistry; D000596:Amino Acids / Q000737:chemistry*; D008956:Models, Chemical; D011522:Protons; D012996:Solutions / Q000737:chemistry; D014867:Water / Q000737:chemistry
chemical_list: D000097:Acetonitriles; D000596:Amino Acids; D011522:Protons; D012996:Solutions; D014867:Water; C032159:acetonitrile