Structural consequences of a 3'-->3' DNA interstrand cross-link by a trinuclear platinum complex: unique formation of two such cross-links in a 10-mer duplex.

Combined multidimensional nuclear magnetic resonance spectroscopy and electrospray mass spectrometry was used to analyze the platinated DNA adduct of the phase II anticancer drug [[trans-PtCl(NH(3))(2)](2)-mu-[trans-Pt(NH(3))(2)(NH(2)(CH(2))(6)NH(2))(2)]](NO(3))(4) (BBR3464) with [5'-d(ACG*TATACG*T)-3'](2). Two 1,2-interstrand cross-links were formed by concomitant binding of two trinuclear moieties to the oligonucleotide. The four DNA-bound platinum atoms coordinated in the major groove at N7 positions of guanines in the 3' --> 3' direction and the central platinum unit is expected to lie in the DNA minor groove. This is the first report of such a DNA lesion. The melting temperature of the adduct is 76 degrees C and is 42 degrees C higher than that of the unplatinated DNA. The sugar residues of the platinated bases are in the N-type conformation and the G9 nucleoside is in the syn orientation, while the G3 nucleoside appears to retain the anti configuration. The secondary structure of DNA was significantly changed upon cross-linking of the two BBR3464 molecules. Base destacking occurs between A1/C2 and C2/G3 and weakened stacking is seen for the C8/G9 and G9/T10 bases. The lack of Watson-Crick base pairing is also seen for A1-T10 and C2-G9 base pairs, whereas Watson-Crick base pairs in the central sequence of the DNA (T4 --> A7) are well maintained. While DNA repair proteins may "see" different platinated adducts as bulky "lesions", the subtle differences involved in base pairing and stacking, as summarized here, may extend to their role as a substrate for repair enzymes. Thus, differences in protein recognition and repair efficiency among the various interstrand cross-links are likely and a subject worthy of detailed exploration.