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On the Formation of Radical Dications of Protonated Amino Acids in a “Microsolution” of Water or Acetonitrile and Their Reactivity Towards the Solvent
Author(s) -
Sørensen Martin,
Forster James S.,
Hvelplund Preben,
Jørgensen Thomas J. D.,
Nielsen Steen Brøndsted,
Tomita Shigeo
Publication year - 2001
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20010803)7:15<3214::aid-chem3214>3.0.co;2-7
Subject(s) - chemistry , protonation , reactivity (psychology) , acetonitrile , ionization , ionization energy , solvent , proton , hydrogen atom abstraction , photochemistry , hydrogen , computational chemistry , crystallography , organic chemistry , ion , medicine , physics , alternative medicine , pathology , quantum mechanics
In high‐energy collisions (50 keV) between O 2 and protonated amino acids AH + , 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‐deficient AH 2+ . in the “microsolution” depends on the energetics of the electron transfer reaction AH 2+ . + S →AH + + S + . , the hydrogen abstraction reaction AH 2+ . + S →AH 2 2+ +[ S −H] . , and the proton transfer reaction AH 2+ . + S →A + . + S H + . 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 S H + from TyrH 2+ . ( S ) to overcome the Coulomb barrier, which renders the complex observable with a life‐time larger than 5 μ s. 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(CH 3 CN)+IE(Tyr)]/2); hydration further lowers the IE by 0.3 eV [IE(TyrH + (H 2 O)=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.