Kinetics of formation and stability of {Pt(dien)}(2+) complexes with octamer and 14-mer DNA oligonucleotides containing a GG sequence

Reaction of [Pl(dien)Cl](+) (1) with the 14-mer oligonucleotide 5'-d(ATACAT GGTACATA) (I) gave rise to two major species which corresponded to the 5'-G and 3'-G platinated monofunctional adducts, and a minor amount of the bis- platinated adduct Formed during the later stages of the reaction. The react ion of (I) with the related octamer 5'-d(ATACATGC) (II) was also investigat ed. Kinetic data obtained by HPLC showed that the 5'-G and 3'-G bases of th e 14-mer oligonucleotide were platinated at similar rates: the second-order rate constant is 53 x 10(-2) M-1 s(-1) at 298 K in 0.1 M NaClO4. However, the platination rate of 5'-G of the octamer (II) (k = 69 x 10(-2) M-1 s(-1) ) was enhanced by a factor of three compared to the rate of platination at 3'-G (k = 22 x 10(-2) M-1 s(-1)). All the adducts were separated by HPLC an d characterized by NMR spectroscopy, enzymatic digestion and MALDI-TOF mass spectrometry. H-1 and N-15 NMR shifts suggest that there are distinct conf ormational differences between 14-mer duplexes platinated at 5'-G (I5'(ds)) and 3'-G (I3'(ds),). Molecular mechanics modelling indicates that rotation around the Pt-N7 bond is more restricted in the case of the 5'-G adduct th an in that of the 3'-G adduct. The binding of {Pt(dien)}(2+) to 5 '-GN7 and 3'-GN7 in the monofunctional adducts of (I) was shown to be reversible upo n the addition of high concentrations of chloride ions.