The Structural Effect of Methyl Substitution on the Binding of Polypyridyl Ru–dppz Complexes to DNA

Polypyridyl
ruthenium complexes have been intensively studied and
possess photophysical properties that are both interesting and useful.
They can act as probes for DNA, with a substantial enhancement in
emission when bound, and can induce DNA damage upon photoirradiation.
Therefore, the synthesis and characterization of DNA binding of new
complexes is an area of intense research activity. While knowledge
of how the binding of derivatives compares to that of the parent compound
is highly desirable, this information can be difficult to obtain.
Here we report the synthesis of three new methylated complexes, [Ru­(TAP)2(dppz-10-Me)]­Cl2, [Ru­(TAP)2(dppz-10,12-Me2)]­Cl2, and [Ru­(TAP)2(dppz-11-Me)]­Cl2 (TAP = 1,4,5,8-tetraazaphenanthrene; dppz = dipyrido­[3,2-a:2′,3′-c]­phenazine), and
examine the consequences for DNA binding through the use of atomic-resolution
X-ray crystallography. We find that the methyl groups are located
in discrete positions with a complete directional preference. This
may help to explain the quenching behavior found in solution for analogous
[Ru­(phen)2(dppz)]2+ derivatives.