Binding of the Mn(III) complex of meso-tetrakis(4-N-methyl-pyridiniumyl)porphyrin to DNA. Effect of ionic strength

Interactions of the water-soluble Mn(III) complex of meso-tetrakis( 4-N-met hyl-pyridiniumyl)porphyrin (Mn(III) TMPyP) with DNA in aqueous solutions at low (0.01 M) and high (0.2 M) ionic strengths have been studied by optical absorption, resonance light scattering (RLS) and H-1 NMR spectroscopies. O ptical absorption and RLS measurements have demonstrated that in DNA soluti ons at low ionic strength the Mn(III)TMPyP forms aggregates, which are deco mposed at DNA excess. At high ionic strength the aggregation was not observ ed. We explain this effect by assuming that, upon increase in ionic strengt h, Mn(III)TMPyP dislocates from the DNA sites, which produces better condit ions for the porphyrin aggregation, to sites where the aggregation is hinde red. The H-1 NMR data demonstrated that the aggregation observed at low ion ic strength reduces the paramagnetism of Mn(III)TMPyP. This phenomenon was not observed at the high ionic strength in the absence of aggregation. (C) 1999 Elsevier Science Inc. All rights reserved.