Interaction of Copper(I) with Nucleic Acids

Summary

Poly(dG-dC) and poly(I) form particularly stable complexes with Cu(I): thus characteristic UV absorbance changes enabled demonstration of Cu(I) transfer from poly(dA-dT) to poly(dG-dC), or from DNA to poly(I). Using pulse radiolysis to generate Cu(I), a rate constant of ∼ 4 × 10⁷ dm³ mol⁻¹ s⁻¹ (per base unit) was estimated for association of Cu(I) to native DNA, and slightly higher values were found for poly(dA-dT), poly(C), poly(dG-dC) and poly(G). For native DNA and for the models poly(dA-dT) and poly(dG-dC) the addition of Cu(I) was followed by secondary absorbance changes in the time scale of 10 ms, probably due to internal Cu(I) transfer; such secondary reactions were not detectable in heat-denatured DNA or in the homopolymers of A, C, G, and I. Extraction of Cu(I) from the DNA by EDTA is slow, 0·019 s⁻¹, and independent of EDTA concentration, indicating that dissociation of the DNA–Cu(I) complex is the rate-determining step. A tentative value can hence be given for the DNA–Cu(I) stability constant: K = k (forward)/k (reverse) ∼ 2 × 10⁹ dm³ mol⁻¹. Addition of H₂O₂ to solutions of γ-radiolytically generated DNA–Cu(I), at Cu(I)/base < 0·1, resulted in DNA degradiation, comparable in yield to ·OH-induced degradation. In the case of poly(dA-dT) and poly(dG-dC) the reaction of H₂O₂ with the corresponding Cu(I) complexes produced even more damage than the reaction of ·OH. The formation of DNA-Cu(I), and the deleterious reaction with H₂O₂, were hardly affected by RNase or BSA, when added at equal (w/v) concentration. Dismutation of O₂·⁻ by (Cu,Zn)-SOD was partly inhibited by DNA and even more by poly(I) at pH 4·4, but not at pH 7, probably by competitive complexation of Cu(I), occurring in the catalytic cycle of SOD.