Formation and Reduction Reactions of α-Amino Radicals Derived from Methionine and Its Derivatives in Aqueous Solutions

Article abstract of DOI:10.1021/j100242a022

Absolute rate constants and yields have been measured for the reaction of α-amino radicals derived via an oxidative decarboxylation process from various sulfur-containing amino acids with suitable electron acceptors.The measurements were carried out with aqueous solutions and by using optical and conductivity pulse radiolysis methods.Bimolecular rate constants have been measured for the reactions of CH3SCH2CH2.CHNH2 with Fe(CN)6(3-) (k = (3.5+/-0.4)E9 M^-1 s^-1), C(NO2)4 (4.2+/-0.5)E9), cytochrome(III) c (6.6+/-0.6)E8), 4-hydroxy-2,2,6,6-tetramethylpiperidinyl-1-oxy (TMPN), (5.4+/-0.5)E8), lipoate ((1-2)E8), O2 ((1.8+/-0.4)E9), p-nitroacetophenone (PNAP) ((3.9+/-0.4)E9), methylviologen (MV(2+) ((3.6+/-0.3)E9), and nicotinamide adenine dinucleotide (NAD⁺) ((8.5+/-3)E8).The reduction potential of this α-amino radical is more negative than -0.94 V.The protonated form CH3SCH2CH2.CHNH3(1+) reacts more slowly and k = (1.0+/-0.2)E7 and (2.3+/-0.2)E8 M^-1 s^-1 have been evaluated for the respective electron transfer reactions with MV(2+) and TMPN.The pK of the equilibrium CH3SCH2CH2.CHNH3(1+) = CH3SCH2CH2.CHNH2 + H_aq(1+) has been determined to be 3.85.Further rate constants have been determined for hydrogen atom abstraction from cysteine by CH3SCH2CH2.CHNH2 (k>=(1-2)E9 M^-1 s^-1), and for the electron transfer processes CH3SCH2CH2.CHNHCOCH3 + MV(2+) (k ca. (1-2)E7 M^-1 s^-1), and CH3CH2CH2.CHNH2 + PNAP ( k = 4.1+/-0.4)E9) M^-1 s^-1).