RATES OF PLATINATION OF AG AND GA CONTAINING DOUBLE-STRANDED OLIGONUCLEOTIDES - INSIGHTS INTO WHY CISPLATIN BINDS TO GG AND AG BUT NOT GA SEQUENCES IN DNA

The reactions of the self-complementary 14-base-pair duplexes 5'-d(AAT TAGTACTAATT)-3' (-AG-) and 5'-d(AATTGATATCAATT)-3' (-GA-) with N-15-ci splatin, (cis-[PtCl2((NH3)-N-15)(2)]) (pH 6.0, T = 298 K) and cis-[Pt( (NH3)-N-15)(2)(OH2)(2)](2+). (pH 4.9, T = 288 K), have been investigat ed using [H-1,N-15] HSQC 2D NMR spectroscopy. Reactions involving cisp latin progress via the hydrolysis product cis-[PtCl(NH3)(2)(OH2)](+). Two major -AG- monofunctional adducts, G(6)/Cl and A(5)/Cl, form at ra tes of 1.06 +/- 0.06 and 0.149 +/- 0.014 M-1 s(-1), respectively. The major Pt-GA- monofunctional adduct G(5)/Cl forms at a rate of 0.023 +/ - 0.002 M-1 s(-1), and several minor adducts, including A(6)/Cl (0.005 4 +/- 0.0010 M-1 s(-1)), are observed. Closure from the monofunctional /Cl adducts proceeds via an aquated species for both -AG- and -GA-. Th e rates of hydrolysis from the G/Cl adducts to the G/H2O species are ( 1.55 +/- 0.05) x 10(-5) s(-1) with -AG- and (0.198 +/- 0.008) x 10(-5) s(-1) with -GA-, and the rates of closure to the chelates are (9.8 +/ - 0.9) x 10(-5) and (0.69 +/- 0.17) x 10(-5) s(-1) for -AG- and -GA-, respectively. The rates of ring closure from A/Cl monofunctional speci es, treated as direct to chelate, are (0.16 +/- 0.06) x 10(-5) (-AG-) and (2.1 +/- 0.7) x 10(-5) s(-1) (-GA-). The rate constants found for the reactions between the oligonucleotides and cis-[Pt((NH3)-N-15)(2)( OH2)(2)](2+) are, for formation of the G/H2O monofunctional adducts, 0 .42 +/- 0.01 (-AG-) and 0.50 +/- 0.01 M-1 s(-1) (-GA-). Formation of A /H2O adducts is not observed for either -AG- or -GA-. Rates of ring cl osure from the G/H20 adducts are (3.71 +/- 0.05) x 10(-5) (-AG-) and ( 0.162 +/- 0.005) x 10(-5) s(-1) (-GA-). The rate of formation of monof unctional adducts from cis-[Pt((NH3)-N-15)(2)(OH2)(2)](2+) are similar for -AG- and -GA-, whereas for cisplatin a clear preference is observ ed for -AG- over -GA-. Closure to form the bifunctional adduct is more rapid in the case of -AG- than -GA- for both cisplatin and cis-[Pt((N H3)-N-15)(2)(OH2)(2)](2+), but the difference is greater for cis-[Pt(( NH3)-N-15)(2)(OH2)(2)](2+)(.) It is concluded that the bifunctional in trastrand adduct profile observed when cisplatin binds to DNA is subst antially controlled by the rate of formation of monofunctional adducts at the different X-purine-purine-X sequences. A slow rate of closure also contributes to the nonformation of the bifunctional GpA adduct.