Solution structure of a steroid-DNA complex with cholic acid residues sealing the termini of a Watson-Crick duplex.

The three-dimensional structure of a covalent hybrid between cholic acid and the self-complementary DNA hexamer 5'-TGCGCA-3' was solved via two-dimensional NMR and restrained torsion angle molecular dynamics. In the complex, refined to a pairwise rmsd of 0.64 A, the steroid binds to the terminal T:A base pairs via extensive van der Waals contacts but without any hydrogen bonds or detectable dipole-dipole interactions. The contacts involve the methyl groups as well as one edge of the streoid's sterane skeleton and both nucleobases and the deoxyriboses of the terminal base pair of the DNA. The surprising shape complementarity between steroid and the undisturbed DNA termini explains the increase in fidelity and affinity observed for hybridization probes bearing bile acid residues. Since the hydroxyl groups of the steroid do not contribute to the binding of the DNA, they may be derivatized, potentially giving access to a new class of specific binders for blunt ends of Watson-Crick duplexes.