Elimination of Ammonium Ion from the a-Hydroxyalkyl Radicals of Serine and Threonine in Aqueous Solution and the Difference in the Reaction Mechanism

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Abstract
The zwitterionic radicals HO-ĊH-CH(COO-)NH3 + (4a) and HO-Ċ(CH3)-CH(COO-)NH3 + (4b) are the main species produced upon OH· radical attack in aqueous solutions at pH 3-7 at the amino acids serine, HO-CH2-CH(COO-)NH3 +, or threonine, HO-CH(CH3)-CH(COO-)NH3 +, respectively. Both radicals undergo elimination of NH4 + ion to form the radicals O=CH-ĊH-COO- (7) or CH3-CO-ĊH-COO- (9) respectively. The pKa of the serine-derived cationic radical HO-ĊH-CH(COOH)NH3 + (3a) (3a ⇄ 4a + H+), was determined by ESR spectroscopy to 2.2 ± 0.1 at 276 K. From kinetic data the pKa(OH) of radical 4a (4a ⇄ O-ĊH-CH(COO-)NH3 + (5a) + H+) was calculated to 7.0. The elimination of NH3 takes place from the ketyl radical 5a (type-B mechanism), the rate constant was calculated from kinetic data to 2.4 × 106 s-1 at 290 K. The half-lives of radicals 4a and 4b were measured by time-resolved conductivity changes upon pulse radiolysis, 170 ± 10 μs for 4a and 26 ± 2 μs for 4b, at 290 K and pH 5.8 . With the threonine derived radicals elimination of NH3 takes place at the stage of the α-hydroxyalkyl radical 4b (type-A mechanism). In this series the pKa of the product radical CH3-CO-ĊH-COOH (8) (8 ⇄ 9 + H+), was determined by ERS spectroscopy to 2.7 ± 0.1. The reasons for the observed mechanistic differences (type-A versus type-B decay) are discussed. As further examples for a type-B decay some preliminary data on the elimination of HF from the radicals CF3-Ċ(OH)-CF3 and CF3-ĊH-OH have been added.

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