Chemical Alterations in Native Histone Octamer Complexes Induced by the Attak of · OH and · N3 Radicals

·OH Radicals generated by short electron-beam pulses were allowed to attack histone octamer complexes (extracted from calf-thymus chromatin) in N₂O-saturated dilute solution (0.5–1.3 g/l, NaClO₄] = 1 – 2M , pH 9). They induced a volume contraction due to intra-complex cross-linking. In this process, essentially non-tyrosine moieties of the proteins were involved. Phenol coupling via tyrosyl radicals occurred mainly as an intramolecular reaction, i.e., it was restricted to single histone moelecules. Furthermore, it turned out that only about 55% of the tyrosine moieties were accessible to attacking ·OH and/or ·N₃ radicals. When ·N₃ radicals were generated via continuous irradiation of N₂O-saturated octamer solutions containing NaN₃ with ⁶⁰Co-γ-rays, dimers, trimers, and tetramers were detected by SDS gel electrophoresis, in contrast to pulse radiolysis where only dimers were found. These results were explained in terms of denaturation being induced by small chemical changes and causing partial or complete dissociation of the complexes thus permitting, in the course of the γ-irradiation, the attack and conversion of amino-acid moieties non-accessible in the native octamer complexes. Removal of steric restrictions for the combination of tyrosyl radicals may also play a role. By time-resolved absorption measurements, it was shown that, upon the attack of intact octamer complexes by ·OH radicals, tyrosyl radicals were formed which were converted to dityrosine groups according to two modes with half-lives of several 100 m?s and 1–2 ms, respectively. Cross-linking of histone molecules occurred with a definitely lower rate (1st half-life: 50 100 ms). This process was detectable both by optical absorption measurements at λ = 300 400 nm and by light-scattering measurements.