Binding of copper(ii) polypyridyl complexes to DNA and consequences for DNA-based asymmetric catalysis

The interaction between salmon testes DNA (st-DNA) and a series of Cu^II polypyridyl complexes, i.e. [Cu(dmbpy)(NO₃)₂] (1) (dmbpy = 4,4′-dimethyl-2,2′-bipyridine), [Cu(bpy)(NO₃)₂] (2) (bpy = 2,2′-bipyridine), [Cu(phen)(NO₃)₂] (3) (phen = phenanthroline), [Cu(terpy)(NO₃)₂]·H₂O (4) (terpy = 2,2′:6′,2″-terpyridine), [Cu(dpq)(NO₃)₂] (5) (dpq = dipyrido-[3,2-d:2′,3′-f]-quinoxaline) and [Cu(dppz)(NO₃)₂] (6) (dppz = dipyrido[3,2-a:2′,3′-c]phenazine) was studied by UV/Vis absorption, Circular Dichroism, Linear Dichroism, EPR, Raman and (UV and vis) resonance Raman spectroscopies and viscometry. These complexes catalyse enantioselective C-C bond forming reactions in water with DNA as the source of chirality. Complex 1 crystallizes as an inorganic polymer with nitrate ligands bridging the copper ions, which adopt essentially a distorted square pyramidal structure with a fifth bridging nitrate ligand at the axial position. Raman spectroscopy indicates that in solution the nitrate ligands in 1, 2, 3 and 4 are displaced by solvent (H₂O). For complex 1, multiple supramolecular species are observed in the presence of st-DNA in contrast to the other complexes, which appear to interact relatively uniformly as a single species predominantly, when st-DNA is present. Overall the data suggest that complexes 1 and 2 engage primarily through groove binding with st-DNA while 5 and 6 undergo intercalation. For complexes 3 and 4 the data indicates that both groove binding and intercalation takes place, albeit primarily intercalation. Although it is tempting to conclude that the groove binders give highest ee and rate acceleration, it is proposed that the flexibility and dynamics in binding of Cu^II complexes to DNA are key parameters that determine the outcome of the reaction. These findings provide insight into the complex supramolecular structure of these DNA-based catalysts.