Determination by electrospray mass spectrometry of the outersphere association constants of DNA/platinum complexes using 20-mer oligonucleotides and ([Pt(NH3)(4)](2+), 2Cl(-)) or ([Pt(py)(4)](2+), 2Cl(-))

The cytotoxic effects of cisplatin, cis-diamminedichloroplatinum(II), are g enerally ascribed to the formation of DNA adducts, Several in vitro as well as in vivo studies showed that cisplatin binds preferentially to guanines belonging to (G)(n) sequences (n greater than or equal to 2). After mono- o r diaquation of cisplatin, giving the cationic complexes cis-[PtCl(NH3)(2)( H2O)](+) and cis-[Pt(NH3)(2)(H2O)(2)](2+), DNA platination occurs in two st eps: the cationic complex gives an outersphere association with DNA and the actual coordination then occurs by substitution of one aqua ligand by guan ine-N7, For a better understanding of the (G;), selectivity of cisplatin gi ving the biologically active adducts, also necessary for the conception of new platinum drugs, the respective contribution of the outersphere associat ion and actual guanine platination must be investigated, To check the role of outersphere association In the overall platination process, we used elec trospray mass spectrometry (ESMS) to detect and quantify outersphere associ ation between 20-mer oligonucleotides and platinum complexes, The 20-mer ol igonucleotides were single- or double-stranded, with the same number of gua nines either isolated or adjacent to each other. To deal only with outersph ere association and check the influence of platinum ligands, the [Pt(NH3)(4 )](2+) and [Pt(py)(4)](2+) complexes were used, We characterized by ESMS al l the different outersphere association species formed during titration of each oligonucleotide with the various platinum complexes and evaluated thei r affinity constants. The ESMS results demonstrate that the outersphere ass ociation depends on electrostatic interactions and on the ability of the pl atinum ligands to participate to hydrogen bonding, particularly within the duplex form, (C) 2000 Academic Press.