Reversible Intramolecular Hydrogen Transfer between Cysteine Thiyl Radicals and Glycine and Alanine in Model Peptides:Absolute Rate Constants Derived from Pulse Radiolysis and Laser Flash Photolysis

摘要

The intramolecular reaction of cysteine thiyl radicals with peptide and protein ~(alpha)C-H bonds represents a potential mechanism for irreversible protein oxidation.Here,we have measured absolute rate constants for these reversible hydrogen transfer reactions by means of pulse radiolysis and laser flash photolysis of model peptides.For N-Ac-CysGly6 and N-Ac-CysGly2AspGly3,Cys thiyl radicals abstract hydrogen atoms from Gly with k_f=(1.0-1.1)X 10~5 s~(-1),generating carbon-centered radicals,while the reverse reaction proceeds with k_r=(8.0-8.9)X 10~5 s~(-1).The forward reaction shows a normal kinetic isotope effect of k_H/k_D=6.9,while the reverse reaction shows a significantly higher normal kinetic isotope effect of 17.6,suggesting a contribution of tunneling.For N-Ac-CysAla2AspAla3,cysteine thiyl radicals abstract hydrogen atoms from Ala with k_f=(0.9-1.0)X 10~4 s~(-1),while the reverse reaction proceeds with k_r=1.0 X 10~5 s~(-1).The order of reactivity,Gly>Ala,is in accord with previous studies on intermolecular reactions of thiyl radicals with these amino acids.The fact that k_fk_r.Despite k_f