The ability of Rh2(μ-O2CCH3)4 (1), cis-[Rh2(μ-O2CCH3)2(CH3CN)6]2+ (2), cis-[Rh2(µ-O2CCH3)2(dppz)(η1-O2CCH3)(CH3OH)]+ (dppz = dipyrido[3,2-a:2’,3’-c]phenazine) (3), and cis-[Rh2(µ-O2CCH3)2(bpy)(dppz)]2+ (bpy = 2,2’-bipyridine) (4) to transverse a membrane bilayer was determined through quenching of DNA-bound SYBR Green I (SG) encapsulated within the interior of a vesicle. For comparison, quenching studies were performed with the known DNA binders ethidium bromide (EtBr), methyl viologen (MV2+), Hoechst 33258 and 2-methyl-antrhacene (2-Me-An).
Emission quenching experiments were performed with DNA-bound SG (SG-DNA) free solution to determine the ability of the known DNA-binding compounds and the dirhodium series to affect the luminescence of the dye. The known minor groove binder Hoechst 3328 produced the most quenching of SG-DNA, likely due to displacement of the dye from the duplex. Within the dirhodium series, the degree of quenching correlates to the DNA binding constant of each complex. Complex 3, with the largest kb, produced the greatest quenching of SG-DNA. Each of the complexes in the dirhodium series can quench SG-DNA emission through various mechanisms, including displacement, energy transfer, and charge transfer. In each case, close proximity to the SG-DNA emissive species is a requirement. Therefore, a correlation between the association of each complex in the series with SG-DNA and its quenching ability is not unexpected.
When SG-DNA was encapsulated in POPC vesicles, compounds added to the outside of the vesicles can only quench the SG-DNA emission if they are able to penetrate the bilayer membrane. The vesicles with encapsulated DNA-bound SG, SG-DNA@POPC, were incubated for 4 hours and the emission was monitored at 520 nm. Quenching was retained for the known DNA-binders, EtBr and Hoechst 33258. In the dirhodium series, complexes 1, 2, and 4 maintained at least some of their quenching ability, showing that these complexes are able to transverse the lipid bilayer. In contrast, complex 3, which quenches the emission of SG-DNA in solution, does not when the system is encapsulated in POPC vesicles, indicating that this complex is not able to transverse the bilayer. These results are compared to partition coefficient data and the toxicity of each complex is discussed. Possible mechanisms for membrane penetration of the dirhodium complexes are considered.