Equilibria and kinetics of the intercalation of Pt-proflavine and proflavine into calf thymus DNA Tarita Biver, Fernando Secco, * Maria Rosaria Tin e, and Marcella Venturini Dipartimento di Chimica e Chimica Industriale, Universit  a di Pisa, Via Risorgimento 35, 56126 Pisa, Italy Received 19 May 2003, and in revised form 22 July 2003 Abstract The interaction of the cis-platinum derivative of proflavine [{PtCl(tmen) 2 }{HNC 13 H 7 (NHCH 2 CH 2 ) 2 }] þ (PRPt) with CT- DNA is investigated by spectrophotometry and T-jump relaxation in 0.11 M NaCl, pH 7.0, and 25°C. The DNA–proflavine (PR) system is investigated under the same conditions. Static measurements indicate that base-dye interactions prevail and their analysis reveals that the site size for PRPt (n ¼ 2:6) is twice that found for PR (n ¼ 1:3). One relaxation effect is observed for the DNA/PR system and two effects for the DNA/PRPt system, the faster of them being similar to that of DNA/PR. The kinetics of the process are discussed in terms of the three-step sequence D þ S ¢ DS I ¢ DS II ¢ DS III , where PR and the aromatic residues of PRPt intercalate into DNA by the same mechanism. The third step represents the penetration of platinum residues between base-pairs and is associated to remarkable enthalpy and entropy changes. Further mechanistic details are discussed. Ó 2003 Elsevier Inc. All rights reserved. Keywords: DNA; Metal acridines; Anticancer drugs; Intercalation; Kinetics; Equilibria Acridines are known to interact with DNAs and double stranded RNAs mainly by intercalation [1–4]. The wide range of biological effects of intercalating drugs continues to stimulate investigations on this pro- cess, mainly in conjunction with the synthesis of new metal intercalators whose properties as drugs need to be tested. Actually, the activity of many anticancer, anti- malarial, and antibacterial agents and that of aromatic carcinogens find its origin primary in intercalation [5–7]. Studies on this issue have shown that intercalation is more complex than it was supposed to be and the details of the process strongly depend on the intercalate struc- ture. In this contest, particularly interesting appear to be bifunctional molecules bearing an aromatic residue and a metal containing residue [8–10]. The aromatic residue provides an anchorage for the molecule on the polymer chain by intercalation, whereas the metal containing residue may exert different functions. It can intercalate as well or it might remain outside of the cavity where the metal could interact with the polymer backbone. Studies in this sense are being performed by Barton and co-workers [11], who have synthesised special metal intercalators where the metal is intended to promote the cleavage of the phosphodiesteric bond at a selected point of the chain length. Other metal intercalators bear a platinum (II) com- plex appended to the aromatic residue. These substances could in principle be employed as anticancer drugs [8,12,13] and the knowledge of the mechanism of their binding to nucleic acids is of great importance, since this process does constitute a prerequisite for the subsequent slow attack of Pt(II) to the base nitrogen [14,15]. Recently, we have investigated the equilibria and ki- netics of the intercalation of a cis-platinum derivative of proflavine [{PtCl(tmen)} 2 {HNC 13 H 7 (NHCH 2 CH 2 ) 2 }] þ (PRPt) 1 into double stranded poly(A) [15]. We report here on the intercalation of the same molecule into CT- DNA. For comparison, the DNA–proflavine system has been investigated as well. Archives of Biochemistry and Biophysics 418 (2003) 63–70 www.elsevier.com/locate/yabbi ABB * Corresponding author. Fax: +39-050-918260. E-mail address: ferdi@dcci.unipi.it (F. Secco). 1 Abbreviations used: PRPt, Pt-proflavine; PR, proflavine. 0003-9861/$ - see front matter Ó 2003 Elsevier Inc. All rights reserved. doi:10.1016/S0003-9861(03)00384-9