Detailed studies of the kinetics of platination of the single‐stranded 14‐base DNA oligonucleotide d(ATACATGGTACATA) and the corresponding duplex by cis‐[Pt(NH3)2(H2O)2]2+ show that HPLC and NMR are complementary methods which provide similar results. The 5'‐G and 3'‐G monofunctional intermediates were trapped, separated and characterized by NMR (via 15NH3 labeling) and enzymatic digestion followed by mass spectrometry. The kinetic data are compared with those for the corresponding reactions of cis‐[PtCl2(NH3)2] (cisplatin) and its monohydrolysed analogue. For both single and double strands of the oligonucleotide, the aqua complex shows little selectivity for the 5'‐G or the 3'‐G in the initial platination step, whereas the chloro‐complex preferentially platinates the 3'‐G. The base on the 3' side of the GG sequence appears to play an important role in controlling this selectivity; replacement of T by C increases the selectivity of duplex platination by the diaqua complex by a factor of about 6, and the selectivity of chelation of the 3'‐G monofunctional adduct by a factor of about 3. In general the reactivity of the 5'‐G in a GG sequence appears to be enhanced in a duplex compared with a single‐strand. For both the aqua‐monoadduct and chloro‐monoadduct, cis‐[Pt(NH3)2(NG)(H2O or Cl)], the 5'‐G monoadduct is much longer lived (t½≈ 4 h at 288 K for aqua, 80 h at 298 K for chloro) than the 3'‐G monoadduct (t½≤ 45 min at 288 K for aqua, 6 h at 298 K for chloro). Inspection of molecular mechanics models of the end states of various monofunctional adducts provided insight into H‐bonding and destacking interactions in these adducts and the sequence selectivity observed in their formation. Such adducts may play an important role in the mechanism of action of platinum anticancer drugs.

The kinetics of the reactions between the GGcontaining double-stranded oligonucleotide d(TTGGCCAA), (II) and the platinum complexes cis-[Pt-(Hz0)lz+ (2) were studied and compared with those already determined for the reactions of the single-stranded octanucleotide d(CTGGCTCA) (I).['] The results were as follows: i) Complex 1 reacted faster than 2 with both I and 11. ii) Both complexes 1 and 2 reacted faster with I1 than with I. This acceleration was greater for 1 ( x 13) than for 2 ( x 4) and only due to the increase of the platination rate of the 5'-G of the GG sequence. iii) For both I and 11, the first platination by 1 and ( N H~) Z ( H Z~) Z~~~(1) and [Pt(NH3)3-2 was faster on the 5'-G than on the 3'43. This difference was more significant for the platination of I1 (ks,/k3, = 12 for 1 and 5 for 2) than of I (ks,/k3, 5 2). iv) The cyclization reaction of the monoadducts (G*) of 1 to yield the GG cis-Pt(NH,)$+ chelate (G*G*) was considerably slowed down in the duplex. This rate decrease was significantly larger for the chelation of the 5'-G* (factor of 16) than of the 3'-G* (factor of 4) monoadducts. v) The intrastrand chelation of the 3'-G* monoadducts (k3J was faster than that of the 5'-G* monoadducts ( k 5 J , both for I and I1 (k38c/ks,c = 3 and 13, respectively). vi) In addition to the intrastrand G*G* crosslink, we also observed the interstrand crosslink d(GG*CC)-d(GG*CC) between the two 3'43s of the central tetranucleotide. The rate constant for the interstrand crosslinking (k3,i) was half that of the intrastrand chelation (k3,c).vii) The 5' monoadduct, which was formed faster (ks. > k3.) and was chelated more slowly (k5,c @ k,.i c k3.c), exhibited a half-life of 3.2 h under our experimental conditions.

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