Intercalation of water-soluble bis-porphyrins into Poly(dA)-Poly(dT) double helix

The association constants (K) of nucleic acid monomers with a series of wat er-soluble bis-porphyrins (bisMC1, bisMC3, bisMC5, bisMC7, and bisMC11) in which two porphyrin units were linked by a methylene chain of various lengt hs were estimated spectrophotometrically. Among the bis-porphyrins. the K v alues are similar for each nucleic acid monomer, indicating that the bridgi ng chain length does not affect the association of the bis-porphyrins with the nucleic acid monomers. The melting curves of poly(dA)-poly(dT) in the p resence of bisMC3 or bisMC5 were found to be biphasic, suggesting that bisM C3 and bisMC5 are bound to poly(dA)-poly(dT) with a binding mode different from the groove binding exhibited by the corresponding porphyrin monomers. A negative-induced CD peak in the Soret region of bisMC3 and bisMC5 with po ly(dA)-poly(dT) is observed and the visible spectral changes of bisMC3 and bisMC5 upon addition of poly(dA)-poly(dT) are accompanied by a large red sh ift of the Soret band (bisMC3: 21 nm, bisMC5: 23 nm) with substantial hypoc hromicity (bisMC3: 49%. bisMC5: 40%). Therefore, it is reasonable to conclu de that both of the porphyrin units of bisMC3 and bisMC5 intercalate into p oly(dA)-poly(dT). In contrast to poly(dA)poly(dT). the melting curves of po ly(dA . dT)(2) in the presence of the bis-porphyrins did not show such biph asic behavior. Together with the CD and visible absorption data, it is cert ain that these bis-porphyrins do not intercalate into poly(dA . dT)(2). (C) 2001 Elsevier Science Ltd. All rights reserved.