Structure of a beta-alanine-linked polyamide bound to a full helical turn of purine tract DNA in the 1:1 motif.

Polyamides composed of N-methylpyrrole (Py), N-methylimidazole (Im) and N-methylhydroxypyrrole (Hp) amino acids linked by beta-alanine (beta) bind the minor groove of DNA in 1:1 and 2:1 ligand to DNA stoichiometries. Although the energetics and structure of the 2:1 complex has been explored extensively, there is remarkably less understood about 1:1 recognition beyond the initial studies on netropsin and distamycin. We present here the 1:1 solution structure of ImPy-beta-Im-beta-ImPy-beta-Dp bound in a single orientation to its match site within the DNA duplex 5'-CCAAAGAGAAGCG-3'.5'-CGCTTCTCTTTGG-3' (match site in bold), as determined by 2D (1)H NMR methods. The representative ensemble of 12 conformers has no distance constraint violations greater than 0.13 A and a pairwise RMSD over the binding site of 0.80 A. Intermolecular NOEs place the polyamide deep inside the minor groove, and oriented N-C with the 3'-5' direction of the purine-rich strand. Analysis of the high-resolution structure reveals the ligand bound 1:1 completely within the minor groove for a full turn of the DNA helix. The DNA is B-form (average rise=3.3 A, twist=38 degrees ) with a narrow minor groove closing down to 3.0-4.5 A in the binding site. The ligand and DNA are aligned in register, with each polyamide NH group forming bifurcated hydrogen bonds of similar length to purine N3 and pyrimidine O2 atoms on the floor of the minor groove. Each imidazole group is hydrogen bonded via its N3 atom to its proximal guanine's exocyclic amino group. The important roles of beta-alanine and imidazole for 1:1 binding are discussed.