Abstract
Two-dimensional NMR has been used to study the 2:1 distamycin A.d(CGCAAATTGGC).d(GCCAATTTGCG) complex. The nuclear Overhauser effect spectroscopy (NOESY) experiment was used to assign the aromatic and C1'H DNA protons and to identify drug-DNA contacts. These data indicate that two drug molecules bind simultaneously in the minor groove of the central 5'-AAATT-3' segment and are in close contact with both the DNA and one another. One drug binds with the formyl end close to the second adenine base of the A-rich strand, while the other drug binds with the formyl end close to the second adenine of the complementary strand. With this binding orientation, the positively charged propylamidinium groups are directed toward opposite ends of the helix. Molecular modeling shows that the minor groove must expand relative to the 1:1 complex to accommodate both drugs. Energy calculations suggest that electrostatic interactions, hydrogen bonds, and van der Waals forces contribute to the stability of the complex.