Interactions Of Porphyrins And Metalloporphyrins With Single-Stranded Poly(dA)

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Inorganic Chemistry


A study of the interactions of single-stranded poly(deoxyadenylic acid) (poly(dA)) with a number of metallo- and nonmetallo-porphyrins is reported. Among the metal derivatives considered is the newly synthesized platinum(II) complex of tetrakis(N-methylpyridinium-4-yl)porphine (PtTMpyP-4). This metalloporphyrin behaves like other four-coordinated species in that it intercalates into double-stranded (ds) calf thymus DNA, resulting in a large bathochromic shift (18 nm) and hypochromicity (46%) of its Soret maximum. In addition, an induced, negative circular dichroism (CD) feature, typical of porphyrin intercalators, is observed at 420 nm with a Δε of -13 M⁻¹ cm⁻¹. All the porphyrin⋅poly(dA) complexes studied yield considerable hypochromicity compared to the free porphyrin, suggesting extensive interaction of the π-system of the porphyrins with that of the adenine bases. Whereas intercalating porphyrins display a hypochromicity with poly(dA) that is comparable to that obtained with calf thymus (ct) DNA, ZnTMpyP-4, which is a groove-binder by virtue of its single-axial ligand (H₂O), produces a substantially greater hypochromicity with the single-stranded than with the double-stranded nucleic acid. These results are consistent with a pseudointercalation model for binding of porphyrins to poly(dA). In addition, undercertain conditions CuTMpyP-4⋅poly(dA) shows a large conservative CD feature in the Soret region (Δε ~ ±10² M−¹ cm−¹), unlike profiles observed for other CuTMpyP-4⋅nucleic acid complexes. This metalloporphyrin may by virtue of this CD signature prove to be a useful reagent in reporting the presence of single-stranded regions in a complex nucleic acid mixture.

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