The Reaction of the Hydrated Electron with Amino Acids, Peptides, and Proteins in Aqueous Solutions: III. Histidyl Peptides

The reaction rate constants of <tex-math>$e{}_{{\rm aq}}{}^{-}$</tex-math> with glycyl-histidine (Gly-His) and β-alanyl-histidine (Carnosine, β-Ala-His) were determined and compared to those of β-alanyl-alanine (β-Ala-Ala), alanyl-alanine [<tex-math>$({\rm Ala})_{2}$</tex-math>], and histidine (His). The rate constants were found to be pH dependent. Below the pK value of the imidazole ring, the rate constants of the histidyl peptides are similar to that of His. This indicates that the main site of the <tex-math>$e{}_{{\rm aq}}{}^{-}$</tex-math> reaction is the protonated ring. Above this pK value the pH dependent rate constants were less in the His amino acids than in the His peptides. This difference was attributed to the presence of the carbonyl group in the peptides. This group, which is known to react quite rapidly with <tex-math>$e{}_{{\rm aq}}{}^{-}$</tex-math>, exhibits its presence when the imidazole ring loses its reactivity after deprotonation. The difference in reactivity toward <tex-math>$e{}_{{\rm aq}}{}^{-}$</tex-math> between the α and β His peptides is explained by the relative position of the protonated amino groups with respect to the carbonyl groups. A similar difference was also found in <tex-math>$({\rm Ala})_{2}$</tex-math> and β-Ala-Ala. The transient absorption spectra resulting from the reaction of <tex-math>$e{}_{{\rm aq}}{}^{-}$</tex-math> with the His peptides were recorded and examined with respect to peptide concentration and pH dependence. Here again, at pH values below the pK of the imidazole, the transient absorption spectra are similar to that of histidine. In alkaline solutions, however, proper experimental conditions could be attained only for Gly-His. In His and β-Ala-His the interference of the OH radical reaction was observed. In Gly-His it was found that the band characterizing the imidazole transient (<tex-math>$\lambda _{{\rm max}}=360\ {\rm nm}$</tex-math>) disappears with a simultaneous appearance of a band at <tex-math>$\lambda _{{\rm max}}\simeq 410\ {\rm nm}$</tex-math>. The 410-nm band is suggested to characterize a deaminated radical produced via an internal electron transfer process from the carbonyl groups. These findings lead to the suggestion that protonated histidyl residues are important sites for the <tex-math>$e{}_{{\rm aq}}{}^{-}$</tex-math> reaction with proteins. In view of this study, earlier results of the pulse radiolysis of proteins are explained. The OH radical reaction with the His peptides was also investigated. It was shown that their reaction rate constants and their transients produced are similar to those observed with histidine.