Interactions of platinum(ll)-derivatized triplexforming oligonucleotides with DNA¹

Polypyrimidine oligonucleotides can bind to tracts of contiguous purines in double-stranded DNA to form triple-stranded complexes. The stability of the triplex is reduced significantly if the target purine tract is interrupted by a single pyrimidine. Previous studies have shown that incorporation of an N ⁴-aminoalkylcytosine into the triplexforming oligonucleotide (TFO), opposite a single CG interruption, facilitates triplex formation. Examination of molecular models suggested that further modification of the amino group of the aminoalkyl arm might enable adduct formation with the N7 of the guanine of the CG interruption. To test this, we prepared 2'-deoxyribo-and 2'-O-methylriboTFOs that contained cytosine (C), N⁴-(2-aminoethyl)cytosine (ae-C), or diethylenetriamineplatinum(II) (DPt-C) or cisaquodiammineplatinum(II) (cPt-C) derivatives of N⁴-(2-aminoethyl)cytosine, positioned opposite a CG interruption of a polypurine tract found in the pol gene of HIV-1 proviral DNA. Although the C- and ae-C-derivatized deoxyribo-TFOs formed triplexes of modest stability and the DPt-C-modified TFO failed to form a triplex, the C- and ae-C-derivatized 2/-O-methylribo-TFOs formed remarkably stable triplexes (T _m = 57 °C). The DPt-C- and cPt-C-modified 2-Omethylribo-TFOs also formed triplexes, although their stabilities were reduced (T_m = 33 °C), suggesting that the tethered platinum group may interfere sterically with TFO binding. Consistent with this hypothesis was the observation that triplex stability was restored (T_m = 57 °C) when the diethylenetriamineplatinum(II) group was tethered to the 5'end of the Z-O-methylribo-TFO via a 2-aminoethylcarbamate linkage. Taken together, these results suggest that 2′-Omethylribo-TFOs may be particularly useful in targeting purine tracts in DNA that have CG interruptions, and that further modification with platinum derivatives could lead to the design of TFOs that are capable of covalent binding to their target, thus increasing the effectiveness of the TFO.