Binding of meso-tetrakis(N-methylpyridiniumyl)porphyrin isomers to DNA: quantitative comparison of the influence of charge distribution and copper(II) derivatization

Factors influencing the binding of tetracationic porphyrin derivatives to DNA have been comprehensively evaluated by equilibrium dialysis, stopped-flow kinetics, etc., for mesotetrakis (4-N-methylpyridiniumyl)porphyrin TMpyP (4)]. Technical difficulties have previously precluded a comprehensive study of metalloporphyrins. Since electrostatic interactions with the DNA and metal derivatization of the porphyrins have important consequences, we have investigated in greater detail two isomers of TMpyP (4) (meso-tetrakis(3-N-methylpyridiniumyl)porphyrin, TMpyP (3)] and meso-tetrakis(2-N-methylpyridiniumyl)porphyrin TMpyP (2)]) in which the position of the charged centers has been varied. A comprehensive study of the Cu(II) derivatives, e.g., CuTMpyP (4), was possible since the difficulties encountered previously with Ni(II) compounds were not a problem with Cu(II) porphyrins J. A. Strickland, L. G. Marzilli, M. K. Gay, and W. D. Wilson (1988) Biochemistry 27, 8870-8878]. At 25 degrees C, the apparent equilibrium constants Kobs] decreased with increasing Na+] for all porphyrins. The Kobs values were comparable for TMpyP (4) and TMpyP (3) binding to either polyd(G-C).polyd(G-C) polyd(G-C)2]] or polyd(A-T)].polyd(A-T)] polyd(A-T)2]]. For the copper(II) porphyrins, the Kobs values were about fivefold greater. The Kobs value for CuTMpyP (2) binding to polyd(G-C)2] was too small to measure under typical salt conditions; however, Kobs for binding to polyd(A-T)2] was about two orders of magnitude smaller than those found for CuTMpyP (4) or CuTMpyP (3). Application of the condensation theory for polyelectrolytes suggests about three charge interactions when CuTMpyP (4), CuTMpyP (3), and TMpyP (3) bind to polyd(G-C)2] or polyd(A-T)2], a result comparable to that reported for TMpyP (4). At 20 degrees C and 0.115 M Na+], incorporation of copper decreased the rates of dissociation from polyd(A-T)2] by a 100-fold compared to those reported for TMpyP (4) but had little effect on the rates of dissociation from polyd(G-C)2]. Also, movement of the H3CN+ group from the fourth to the third position of the pyridinium ring enhanced the rates of dissociation from polyd(A-T)2] but decreased the rates of dissociation from polyd(G-C)2]. From polyelectrolyte theory, the Na+] dependence of the dissociation rates from polyd(G-C)2] is consistent with intercalative binding, while that for polyd(A-T)2] is consistent with an outside binding model. For calf thymus CT] DNA at 20 degrees C, a greater decrease in the AT than in the GC imino 1H-nmr signal was observed upon addition of CuTMpyP (2), suggesting selective outside binding to the AT regions.(ABSTRACT TRUNCATED AT 400 WORDS)