A 1H-NMR study of the DNA binding characteristics of thioformyldistamycin, an amide isosteric lexitropsin.
The interaction of thioformyldistamycin, an amide isostere of the naturally occurring antibiotic distamycin A, with a self-complementary decadeoxynucleotide duplex, d(CGCAATTGCG)2, has been examined using a variety of high-field 1H-NMR techniques. The ligand exhibits two forms in solution arising from geometric isomerism due to restricted rotation around the thioformamide bond. Only the thermodynamically more stable Z-form is shown to bind to the oligonucleotide along its minor groove at the central 5'-AATT segment with the end groups of the ligand extending into the flanking GC regions but without any close contact at the amidinium terminus. Cross-peaks involving characteristic intra- and interresidue proton connectivities in the 2D experiments (COSY and NOESY) were employed to assign individual resonances of both strands in the asymmetric DNA-drug complex. The solution structure of the complex was constructed by molecular mechanics calculations based upon initial estimates of drug-DNA NOE contacts and further refined through energy minimization. These results complement previous structural studies on distamycin and other lexitropsins with oligonucleotides. The exchange of the ligand between two equivalent binding sites on the DNA sequence was estimated to occur at 40 s-1 with a free energy of activation of 16.5 kcal.mol-1 at 321-326 K. There was no evidence of formation of a 2:1 drug-oligomer complex, in contrast to the case of the natural product, which is attributed to steric demands of the larger sulfur atom.