The ability of Rh(III) tetrakis-N-methylpyridyl porphyrin, Rh III (TMPyP) 5+ , to interact with and cleave DNA was, investigated by UV-visible, luminescence, circular dichroism (CD), electron spin resonance (ESR) and gel electrophoresis methods in the presence or absence of ascorbic acid. UV-absorption data showed that Rh III (TMPyP) 5+ is capable of interacting with DNA, as indicated by the appearance of a red shift and hypochromicity of the Soret band. The CD data revealed that Rh III (TMPyP) 5+ was capable of binding to DNA via an external binding mode. The Rh III (TMPyP) 5+ showed fluorescence and phosphorescence at room temperature. The phosphorescence increased in the presence of DNA and this could be attributed to the shielding of the metal-porphyrin by DNA. Gel electrophoresis studies revealed that Rh III (TMPyP) 5+ was only able to cleave DNA in the presence of the reducing agent ascorbic acid. ESR data indicated the formation of Rh II (TMPyP) 5+ by reduction of Rh III (TMPyP) 5+ with ascorbic acid. The involvement of Rh(III)/Rh(II) species in catalytic DNA cleavage and a possible DNA cleavage mechanism is discussed.

The Rh III (Br 8 TMPyP) 5? showed a catalytic DNA cleavage in the presence of ascorbic acid. The UV-visible, Cyclic Votlammetric (CV) and Electron Spin Resonance (ESR) data confirmed involvement of the reduced form of the Rh III -Br 8 TMPyP, Rh III (Br 8 TMPyP) 4?Á radical, in the catalytic cycle. Gel-electrophoresis, results revealed that, Rh III (Br 8 TMPyP) 5? could cleave DNA at 0.01-0.1 lM levels which is significantly higher than that of other metalloporphyrins.