Interaction between a DNA oligonucleotide and a dinuclear iron(II) supramolecular cylinder; an NMR and molecular dynamics study

A tetracationic supramolecular cylinder, [Fe(2)L(3)](4+) (L=C(25)H(20)N(4)), with a triple-helical architecture, is just the right size to fit into the major groove of DNA but too big to fit into the minor groove. A detailed NMR spectroscopic analysis supported by molecular dynamics (MD) calculations shows unambiguously the close fit between the cylinder and a duplex oligonucleotide, [d(GACGGCCGTC)(2)]. Furthermore, only the left-handed enantiomer of the cylinder seems to fit the groove geometry. With both free and complexed species of [Fe(2)L(3)](4+) and DNA in solution, the NMR spectra are too complicated for a detailed structure determination. Based on differences in chemical shifts and extensive MD calculations, a realistic qualitative picture of the DNA-cylinder adduct is presented. Several sets of chemical shifts assigned to the protons of the three ligand strands in the cylinder indicate that the iron complex situated in the major groove exhibits restricted rotation on the NMR timescale around the cylindrical axis. The NMR NOE data support a model where the cylinder undergoes both a translational and rotational oscillation in the major groove. The results of an NOE restrained MD calculation indicates that the cylinder induces a 40 degrees bend of the double helix, in accordance with linear dichroism measurements. Other distinct features to be noticed are the very low value of the helical twist (16 degrees) induced at the G(4)C(5) step.