BINDING OF THE CATIONIC 5,10,15,20-TETRAKIS(4-N-METHYLPYRIDYL) PORPHYRIN AT 5'CG3' AND 5'GC3' SEQUENCES OF HEXADEOXYRIBONUCLEOTIDES - TRIPLET-TRIPLET TRANSIENT ABSORPTION, STEADY-STATE AND TIME-RESOLVED FLUORESCENCE AND RESONANCE RAMAN STUDIES

Intercalative binding of the cationic 5,10,15,20-tetrakis(4-N-methylpy ridyl) porphyrin (H(2)TMpyP(4+)) at 5'CG3 and 5'GC3' sequences in [d(T ACGTA)](2) and [d(TAGCTA)](2) hexadeoxyribonucleotides has been monito red through porphyrin ground-state and transient triplet-triplet absor ption, steady-state and time-resolved fluorescence as well as resonanc e Raman scattering. The porphyrin intercalation results in large red s hifts and hypochromicity of the Soret absorption band. Charge-transfer processes between guanine residues and intercalated porphyrins, leadi ng to an efficient quenching of the porphyrin S-1 excited singlet stat e, occur at both 5'CG3' and 5'GC3' sites. However! not all of the inte rcalated molecules are involved in these charge-transfer processes. Ox ygen accessibility to intercalated porphyrins is practically the same for both sequences, resulting in late constants of porphyrin triplet-s tate quenching by oxygen of 0.12 X 10(9) and 0.14 X 10(9) (M s)(-1) fo r 5'CG3' and 5'GC3' sites, respectively. Minor parts (11 and 16% for [ d(TACGTA)](2) and [d(TAGCTA)](2), respectively) of the porphyrin molec ules are externally bound to hexamers, resulting in a higher oxygen ac cessibility (k(q) = 0.5 X 10(9) and 0.6 X 10(9) (M s)(-1) for 5'CG3' a nd 5'GC3' sites, respectively). The photophysical properties of bound H(2)TMpyP(4+) molecules in hexamers and the local polarity at the bind ing sites are close to those found at the corresponding binding sites in polynucleotides. The resonance Raman spectra of the H(2)TMpyP(4+) p orphyrin moieties in both complexes mainly bear features characteristi c of an intercalative binding mode, but there is also clear evidence f or the existence of groove-bound complexes. (C) 1998 Elsevier Science S.A. All rights reserved.