Intercalation of proflavine and a platinum derivative of proflavine into double-helical poly(A)

The equilibria and kinetics of the interactions of proflavine (PR) and its platinum-containing derivative [{PtCl(tmen)}(2){HNC13H7(NHCH2CH2)(2)}](+) ( PRPt) with double-stranded poly(A) have been investigated by spectrophotome try and Joule temperature-jump relaxation at ionic strength 0.1 M, 25 degre es C, and pH 5.2. Spectrophotometric measurements indicate that base-dye in teractions are prevailing. T-jump experiments with polarized light showed t hat effects due to field-induced alignment could be neglected. Both of the investigated systems display two relaxation effects. The kinetic features o f the reaction are discussed in terms of a two-step series mechanism in whi ch a precursor complex DSI is formed in the fast step, which is then conver ted to a final complex in the slow step. The rate constants of the fast ste p are k(1) = (2.5 +/- 0.4) x 10(6) M-1 s(-1), k(-1) = (2.4 +/- 0.1) x 10(3) s(-1) for poly(A)-PR and k(1) = (2.3 +/- 0.1) x 10(6) M-1 s(-1), k(-1) = ( 1.6 +/- 0.2) x 10(3) s(-1) for poly(A)-PRPt. The rate constants for the slo w step are k(2) = (4.5 +/- 0.5) x 10(2) s(-1), k(-2) = (1.7 +/- 0.1) x 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 equili brium constants and site size the values K = (4.5 +/- 0.1) x 10(3) M-1, n = 1.3 +/- 0.5 for poly(A)-PR and K = (2.9 +/- 0.1) x 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 DSI involves only the proflavine resi dues in both systems. ln 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 co mplex DSII of poly(A)-PRPt both proflavine and platinum residues are interc alated. In addition, a very slow process was detected and ascribed to the c ovalent binding of Pt(II) to the adenine.