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Structural modeling of the distamycin A-d(CGCGAATTCGCG)2 complex using 2D NMR and molecular mechanics.

The structure of the distamycin A-d(CGCGAATTCGCG)2 complex has been determined through a combination of SKEWSKY and NOESY 2D NMR experiments and molecular mechanics calculations. NMR data provided upper bounds on many proton-proton pairs. The advantage of the SKEWSKY/NOESY method is that small groups of strongly coupled spins can be treated accurately as isolated systems. The AMBER molecular mechanics package, modified to include the NMR constraints, was used in energy refinements. Distamycin A fits snugly into the 5'-AATT-3' minor-groove binding site. Structural analysis revealed van der Waals contacts between A5, A6, and A18 C2H and drug H3 protons, potential three-center hydrogen bonding between drug amide protons and adenine N3 and thymine O2 atoms analogous to the spine of hydration in the crystal structure of the free DNA, and stacking of the sugar O1' atoms of A6-C21, T7-T20, and, T8-T19, over drug pyrrole rings 1, 2, and 3, respectively. In addition to hydrophobic effects, hydrogen bonding, and electrostatic interactions proposed by others, it is suggested that stacking interactions between DNA sugar O1' atoms and the three drug pyrrole rings contribute to the stability of the complex.