The oligonucleotides 5'-d(TTTTCTTTTG) and 5'-d(AAAAGAAAAG) were cross-linked with a trans-[Pt(NH3)(2)](2+) entity via the N7 positions of the 3'-end guanine bases to give parallel-stranded (ps) DNA. At pH 4.2, CD and NMR spectroscopy indicate the presence of Hoogsteen base pairing. In addition, temperature-dependent UV spectroscopy shows an increase in melting temperature for the platinated duplex (35 degreesC) as compared to the non-platinated, antiparallel-stranded duplex formed from the same oligonucleotides (21 degreesC). A monomer-dimer equilibrium for the platinated 20mer is revealed by gel electrophoresis. At pH 4.2, addition of a third strand of composition 5'-d(AGCTTTTCTTTTAG) to the ps duplex leads to the formation of a triple helix with two distinct melting points at 38 degreesC (platinum crosslinked Hoogsteen part) and 21 degreesC (Watson-Crick part), respectively.