Parallel-stranded DNA with Hoogsteen base pairing stabilized by a trans-[Pt(NH3)(2)](2+) cross-link: characterization and conversion into a homodimerand a triplex

The oligonucleotides 5'-d(TTTTCTTTTG) and 5'-d(AAAAGAAAAG) were cross-linke d with a trans-[Pt(NH3)(2)](2+) entity via the N7 positions of the 3'-end g uanine bases to give parallel-stranded (ps) DNA. At pH 4.2, CD and NMR spec troscopy indicate the presence of Hoogsteen base pairing. In addition, temp erature-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 degr eesC). 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 hel ix with two distinct melting points at 38 degreesC (platinum crosslinked Ho ogsteen part) and 21 degreesC (Watson-Crick part), respectively.