Solid-substrate room-temperature phosphorescence study on zinc(II) and tin(IV) protoporphyrins and their interaction with DNA

The solid-substrate room-temperature phosphorescence (SS-RTP) of two commercially available metalloporphyrin compounds, zinc(II) protoporphyrin (ZnPP) and tin(IV) protoporphyrin (SnPP) has been studied. Strong and stable RTP signals of the two metalloporphyrins in neutral to weakly basic solutions can be simply induced on filter paper without addition of external heavy atom perturbers. Their emission bands appeared at 723 nm for ZnPP and 718 nm for SnPP at an excitation wavelength of 417 nm. Compared with SnPP, ZnPP is a better RTP probe for DNA because its RTP enhancement effect is much higher under the same experimental conditions. The interaction of ZnPP with DNA at pH 8.5 gives an apparent binding constant of 9.1 x 10(3) which is similar to that of the cationic porphyrin absorption probe CuTMPyP (copper (II)- tetrakis(4-N-methylpyridyl)porphine complex). Hydrogen bonding between the monocarboxylic acid substituent of ZnPP and the base pairs of DNA plays a crucial role in the binding.