The phosphate clamp: a small and independent motif for nucleic acid backbone recognition.

The 1.7 Å X-ray crystal structure of the B-DNA dodecamer, [d(CGCGAATTCGCG)]₂ (DDD)-bound non-covalently to a platinum(II) complex, {Pt(NH₃)₃}₂-µ-{trans-Pt(NH₃)₂(NH₂(CH₂)₆NH₂)₂}₆ (1, TriplatinNC-A,) shows the trinuclear cation extended along the phosphate backbone and bridging the minor groove. The square planar tetra-am(m)ine Pt(II) units form bidentate N-O-N complexes with OP atoms, in a Phosphate Clamp motif. The geometry is conserved and the interaction prefers O2P over O1P atoms (frequency of interaction is O2P > O1P, base and sugar oxygens > N). The binding mode is very similar to that reported for the DDD and {trans-Pt(NH₃)₂(NH₂(CH₂)₆(NH₃(+))}₂-µ-{trans-Pt(NH₃)₂(NH₂(CH₂)₆NH₂)₂}₈ (3, TriplatinNC), which exhibits in vivo anti-tumour activity. In the present case, only three sets of Phosphate Clamps were found because one of the three Pt(II) coordination spheres was not clearly observed and was characterized as a bare Pt²(+) ion. Based on the electron density, the relative occupancy of DDD and the sum of three Pt(II) atoms in the DDD-1 complex was 1:1.69, whereas the ratio for DDD-2 was 1:2.85, almost the mixing ratio in the crystallization drop. The high repetition and geometric regularity of the motif suggests that it can be developed as a modular nucleic acid binding device with general utility.