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 (^•OH) were (1.6 ± 0.3) × 10¹⁰, (9.0 ± 0.9) × 10⁹, (1.4 ± 0.3) × 10⁹, (1.8 ± 0.1) × 10¹⁰, and (1.0 ± 0.1) × 10¹⁰ M^-1s^-1, respectively, indicating that formation of peptide bond can affect the reaction of amino acids with ^•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) × 10⁸, (2.3 ± 0.1) × 10⁸, and (3.3 ± 0.4) × 10⁸ M^-1s^-1, respectively, implying that the presence of a positive charge may enhance the chemical repair process.