Intercalation of Proflavine and a Platinum Derivative of Proflavine into Double-Helical Poly(A)

Abstract

The equilibria and kinetics of the interactions of proflavine (PR) and its platinum-containing derivative [{PtCl(tmen)} 2{HNC 13H 7(NHCH 2CH 2) 2}] + (PRPt) with double-stranded poly(A) have been investigated by spectrophotometry and Joule temperature-jump relaxation at ionic strength 0.1 M, 25°C, and pH 5.2. Spectrophotometric measurements indicate that base-dye interactions are prevailing. T-jump experiments with polarized light showed that effects due to field-induced alignment could be neglected. Both of the investigated systems display two relaxation effects. The kinetic features of the reaction are discussed in terms of a two-step series mechanism in which a precursor complex DS I is formed in the fast step, which is then converted to a final complex in the slow step. The rate constants of the fast step are k 1 = (2.5 ± 0.4) × 10 6 M −1 s −1, k −1 = (2.4 ± 0.1) × 10 3 s −1 for poly(A)-PR and k 1 = (2.3 ± 0.1) × 10 6 M −1 s −1, k −1 = (1.6 ± 0.2) × 10 3 s −1 for poly(A)-PRPt. The rate constants for the slow step are k 2 = (4.5 ± 0.5) × 10 2 s −1, k −2 = (1.7 ± 0.1) × 10 2 s −1 for poly(A)-PR and k 2 = 9.7 ± 1.2 s −1, k −2 = 10.6 ± 0.2 s −1 for poly(A)-PRPt. Spectrophotometric measurements yield for the equilibrium constants and site size the values K = (4.5 ± 0.1) × 10 3 M −1, n = 1.3 ± 0.5 for poly(A)-PR and K = (2.9 ± 0.1) × 10 3 M −1, n = 2.3 ± 0.6 for poly(A)-PRPt. The values of k 1 are similar and lower than expected for diffusion-limited reactions. The values of k −1 are similar as well. It is suggested that the formation of DS I involves only the proflavine residues in both systems. In contrast, the values of k 2 and k −2 in poly(A)-PRPt are much lower than in poly(A)-PR. The results suggest that in the complex DS II of poly(A)-PRPt both proflavine and platinum residues are intercalated. In addition, a very slow process was detected and ascribed to the covalent binding of Pt(II) to the adenine.