NMR Structure of a Cyclic Polyamide−DNA Complex

The solution structure of a cyclic polyamide ligand complexed to a DNA oligomer, derived from
NMR restrained molecular mechanics, is presented. The polyamide, cyclo-γ-ImPyPy−γ-PyPyPy*−, binds
to target DNA with a nanomolar dissociation constant as characterized by quantitative footprinting previously
reported. 2D 1H NMR data were used to generate distance restraints defining the structure of this cyclic
polyamide with the DNA duplex d(5‘-GCCTGTTAGCG-3‘):d(5‘-CGCTAACAGGC-3‘). Data interpretation
used complete relaxation matrix analysis of the NOESY cross-peak intensities with the program
MARDIGRAS. The NMR-based distance restraints (276 total) were applied in restrained molecular dynamics
calculations using a solvent model, yielding structures with an rmsd for the ligand and binding site of ∼1
Å. The resulting structures indicate some distortion of the DNA in the binding site. The constraints from
cyclization lead to altered stacking of the rings in the halves of the cyclic ligand relative to unlinked complexes.
Despite this, the interactions with DNA are very similar to what has been found in unlinked complexes.
Measurements of ligand amide and DNA imino proton exchange rates indicate very slow dissociation of
the ligand and show that the DNA can undergo opening fluctuations while the ligand is bound although the
presence of the ligand decreases their frequency relative to the free DNA.