Vladimir S. Chirvony

Binding of the cationic 5-coordinate Zn(II)-5,10,15,20-tetrakis(4-N-methylpyridyl)porphyrin to DNA and model polynucleotides: Ionic-strength dependent intercalation in [poly(dG-dC)]2

Interactions of water-soluble cationic porphyrins and their metal- derivatives with DNA attract interest of investigators as for fundamental studies (for example, the use of photoexcited porphyrins as probes of DNA local structure and dynamics [1]) and for applied biomedical investigations as well (for example, the photodestruction of viral nucleic acids in blood and blood components by using visible light [2]). In spite of fairly well elaborated formal classification of the types of porphyrin-DNA interactions, the relative selectivity of GC vs AT binding, the nature of the charge interactions between the porphyrins and the polymers, the exact geometries of the porphyrin-polymer complexes and at last the porphyrin-induced DNA structural distortions are not yet fully understood and are subjects to very active discussions [3].

Binding of the cationic 5,10,15,20-tetrakis(4-N-methylpyridyl) porphyrin at 5′CG3′ and 5′GC3′ sequences of hexadeoxyribonucleotides: triplettriplet transient absorption, steady-state and time-resolved fluorescence and resonance Raman studies

Abstract Intercalative binding of the cationic 5,10,15,20-tetrakis(4- N -methylpyridyl) porphyrin (H 2 TMpyP 4+ ) at 5′CG3 and 5′GC3′ sequences in [d(TACGTA)] 2 and [d(TAGCTA)] 2 hexadeoxyribonucleotides has been monitored through porphyrin ground-state and transient triplettriplet absorption, steady-state and time-resolved fluorescence as well as resonance Raman scattering. The porphyrin intercalation results in large red shifts and hypochromicity of the Soret absorption band. Charge-transfer processes between guanine residues and intercalated porphyrins, leading to an efficient quenching of the porphyrin S 1 excited singlet state, occur at both 5′CG3′ and 5′GC3′ sites. However, not all of the intercalated molecules are involved in these charge-transfer processes. Oxygen accessibility to intercalated porphyrins is practically the same for both sequences, resulting in rate constants of porphyrin triplet-state quenching by oxygen of 0.12 × 10 9 and 0.14 × 10 9 (M s) −1 for 5′CG3′ and 5′GC3′ sites, respectively. Minor parts (11 and 16% for [d(TACGTA)] 2 and [d(TAGCTA)] 2 , respectively) of the porphyrin molecules are externally bound to hexamers, resulting in a higher oxygen accessibility ( k q = 0.5 × 10 9 and 0.6 × 10 9 (M s) −1 for 5′CG3′ and 5′GC3′ sites, respectively). The photophysical properties of bound H 2 TMpyP 4+ molecules in hexamers and the local polarity at the binding sites are close to those found at the corresponding binding sites in polynucleotides. The resonance Raman spectra of the H 2 TMpyP 4+ porphyrin moieties in both complexes mainly bear features characteristic of an intercalative binding mode, but there is also clear evidence for the existence of groove-bound complexes.

Dynamics of formation and decay of the exciplex created between excited Cu(II)-5,10,15,20-tetrakis(4-N-methylpyridyl)porphyrin and thymine CO groups in short oligothymidylates and double-stranded [poly(dA–dT)]2

Abstract Cationic water-soluble Cu(II)-5,10,15,20-tetrakis(4- N -methylpyridyl)porphyrin (CuTMpyP 4+ ) complexed with short oligothymidylates d(pT) n ( n =1, 2, 3, 4, 5, 9, 12–18) can form exciplexes simultaneously with both CO groups of thymine, forming [(CuP) * dd –CO], and with surrounding water molecules, forming [(CuP) * dd –H 2 O], where (CuP) * dd is the Cu(II)-porphyrin in its excited (d,d) state. The contribution of the CO exciplex in the complexes with d(pT) n increases from 10% for n =1 up to 100% for n =9 and 12–18. For all n , the water-exciplex rise time is of the order of 1–3 ps, its lifetime being as long as 30–160 ps and depending on n . The CO-exciplex lifetime (∼950 ps) is found to be independent of the length ( n ) of the oligothymidylate. The CO-exciplex rise time is found to be as long as ∼100 ps. This implies that the excited triplet CuTMpyP 4+ molecules, which form the CO exciplex, are protected during this time from the fast (∼1 ps) quenching by water molecules. It is assumed that the dependence of the water-exciplex lifetime on n found for CuP complexes with d(pT) n , as well as the difference of CuP-exciplex lifetimes found for various axial ligands and surrounding oligomers, may result from a dependence of the (d,d) state lifetime of five-coordinate (non-planar) CuP on the polarity of the microenvironment.

Dynamics of the exciplex formation and decay between excited CuTMpyP4+ and thymine C=O groups of short oligothymidylates

It was recently found that photoexcitation of the water-soluble cationic Cu(II)-5, 10, 15, 20-tetrakis(4-N-methylpyridyl)porphyrin (CuTMpyP4+) externally (groove) bound to DNA or [poly(dA-dT)]2 results in an exciplex formation between the Cu(II)-porphyrin (CuP) and the biopolymer [1]. It was shown that the exciplex is formed by excited CuTMpyP4+ in (d, d) state ligated by thymine C=O groups [2, 3]. Lifetime of such an exciplex, [(CuP)dd* — CO], is 3.2 ns [3]. It is known at the same time [4] that photoexcitation of free CuTMpyP4+ in water results in formation of short-lived (lifetime ∼10 ps) exciplex with water molecule, [(CuP)dd* — H2O]. Having in mind to study competitive axial ligation of excited CuP by water molecules and thymine C=O groups, comparative picosecond transient absorption (TA) studies were carried out of CuTMpyP4+ bound to short oligothymidylates (olygoT) of the common formula d(pT)n (n=1, 2, 3, 4, 6, 9, 12-18).