Hydroxyl radical scavenging and chemical repair capabilities of positively charged peptides (PCPs): a pulse radiolysis study

Pulse radiolysis was employed to investigate fundamental radiation chemical reactions, which are essential in the radiation protection of DNA. Two positively charged peptides (PCPs), histidine-tyrosine-histidine (His-Tyr-His) and lysine-tyrosine-lysine (Lys-Tyr-Lys), as well as the amino acids that constitute them, were involved. The reaction rate constants for tyrosine (Tyr), histidine (His), lysine (Lys), His-Tyr-His, and Lys-Tyr-Lys with OH radicals (center dot OH) were (1.6 +/- 0.3) x 1010, (9.0 +/- 0.9) x 109, (1.4 +/- 0.3) x 109, (1.8 +/- 0.1) x 1010, and (1.0 +/- 0.1) x 1010 M-1s-1, respectively, indicating that formation of peptide bond can affect the reaction of amino acids with center dot OH. Observed transient absorption spectra indicated a shielding effect of the His or Lys residues at both ends of the PCPs on the centrally located Tyr. The measurement of chemical repair capabilities using deoxyguanosine monophosphate (dGMP) as a model for DNA demonstrated that the reaction rate constants of Tyr, His-Tyr-His, and Lys-Tyr-Lys with dGMP radicals were (2.2 +/- 0.5) x 108, (2.3 +/- 0.1) x 108, and (3.3 +/- 0.4) x 108 M-1s-1, respectively, implying that the presence of a positive charge may enhance the chemical repair process.