Efficient DNA Binding by Optically Pure Ruthenium Tris(bipyridyl) Complexes Incorporating Carboxylic Functionalities. Solution and Structural Analysis

Herein, we report on the binding of optically pure ruthenium complexes Δ- or Λ-[Ru(bpy)2(L−L)][PF6]2 [L−L =
Hcmbpy = 4-carboxy-4‘-methyl-2,2‘-bipyridine (1), L−L = H2dcbpy = 4,4‘-dicarboxy-2,2‘-bipyridine (2)] to DNA.
The binding constants of the two enantiomeric Δ-1 and Λ-1 complexes to DNA were estimated from titration
monitored by 1H NMR spectroscopy. 2D transferred NOESY (TRNOESY) experiments support the conclusion that
Δ-1 and Λ-1 bind to DNA and that an intermediate-to-fast exchange occurs between bound and free Ru(II) complex.
Further, evidence for enantioselective DNA cleavage by Δ-2 is provided by means of gel electrophoresis performed
in the presence and in the absence of light; in contrast, the Λ-2 enantiomer does not. The IR spectrum of enantiomer
Δ-2 (or Λ-2) compared to that of the racemate (rac-2) gives evidence that, in the latter form, the enantiomers are
strongly associated. Moreover the X-ray structure of rac-2 was also determined and exhibits as an outstanding
feature the formation of a one-dimensional supramolecular species in which the cohesion of the system is maintained
by strong hydrogen bonding between carboxylic acid groups of enantiomers Δ-2 and Λ-2 (cationic parts) with
d(O···O) = 2.6 Å in agreement with the infrared results. The conclusion that can be drawn from IR and X-ray
spectroscopies together is that the self-association in rac-2 is strong.