The equilibria and kinetics of the interactions of proflavine (PR) and its platinum-containing derivative [PtCl(tmen)(2)HNC(13)H(7)(NHCH(2)CH(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 degrees 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) 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 slow 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 equilibrium 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 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.

The equilibria and kinetics of the intercalation of ethidium bromide (EB) into poly(A)‚poly(U) have been studied in aqueous solution (pH ) 7.0, 1 M NaCl, 25 °C) by spectrophotometry, spectrofluorimetry, and stopped-flow techniques. The analysis of the binding isotherms yields the site binding constant K ) 2.6 × 10 4 M -1 and the site size n ) 2.2 base pairs, thus revealing the occurrence of intercalation according to the excluded site model. The salt dependence of the binding constant indicates that approximately one Na + counterion is displaced from the polymer by the binding of one ethidium ion. The kinetic curves are biexponential both in absorbance and in fluorescence. The results suggest the formation of two bound forms according to the reaction scheme D + S /where k 1 ) 5.9 × 10 4 M -1 s -1 , k -1 ) 8.7 s -1 , k 2 ) 1.8 × 10 5 M -1 s -1 , and k -2 ) 0.70 × 10 5 M -1 s -1 . The value of the equilibrium constant K, derived from kinetics as a combination of rate constants, agrees with that derived from equilibria. The behavior of the investigated RNA/ethidium system is compared with that of the DNAs/ethidium systems.

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