DNA interactions and photocatalytic strand cleavage by artificial nucleases based on water-soluble gold(III) porphyrins.

The novel gold porphyrin complex (5,10,15-tris(N-methylpyridinium-4-yl)-20-(1-pyrenyl)-porphyrinato)gold(III) chloride, [Au(III)(TMPy3Pyr1P)]Cl4, was prepared and characterized by optical spectroscopy, high-resolution nuclear magnetic resonance (NMR), and electrospray mass spectrometry. This cationic multichromophore compound exhibits excellent water solubility and does not form aggregates under physiological conditions. Binding interactions of this complex and related model compounds with nucleic acid substrates have been studied and characterized by NMR and circular dichroism spectroscopy. The photoreactivity of [Au(III)(TMPy3Pyr1P)]Cl4 was investigated under anaerobic and aerobic conditions in the presence of an excess of purine nucleoside, guanosine, and plasmid DNA. Photocatalytic oxidative degradation of guanosine and the change from supercoiled to circular plasmid DNA upon monochromatic irradiation and polychromatic blue-light exposure with a maximum at 420 nm was explored. The potential of the novel water-soluble cationic metallointercalator complex [Au(III)(TMPy3Pyr1P)]Cl4 to serve as a catalytic photonuclease for the cleavage of DNA has been demonstrated.

Significance of 8-oxoG in the spectrum of DNA damages caused by ionising radiation of different quality.

The yields and composition of DNA damages caused by ionising radiation depends on radiation quality. With increasing light energy transfer (LET), the proportion of isolated DNA damages with respect to cluster damaged sites decreases. Non-double strand break complex damages are induced by gamma radiation in mammalian cells at least four times more frequently that prompt DSB. The most important product of oxidative damage to DNA bases is 8-oxo-7,8-dihydroguanine (8-oxoG). The modelling of DNA damage induced by ionising radiation of different qualities was performed to assess frequencies and composition of complex damages containing 8-oxoG. The occurrence of clusters containing 8-oxoG increases from 6 to 11% for LET in the range 0.4-160 keV microm(-1). Distributions of single strand break (SSB) on opposite DNA strand around induced 8-oxoG have similar shape for different ionising radiations, but differ in their occurrence in the whole spectrum of DNA damages. The most probable configuration is a strand break localised at position +/-3 bases from 8-oxoG.

Synthesis and photodynamic activity of a cationic zinc monoazaporphyrin bearing a nitrogen atom at the peripheral position.

A new cationic monoazaporphyrin, zinc 2-aza-8,12,13,17-tetraethyl-2,3,7,18-tetramethylporphyrinium iodide 3 was synthesized. Photodynamic activity of 3 in degradation of 2',3'-isopropylideneguanosine 4 was compared with 2-aza-8,12,13,17-tetraethyl-3,7,18-trimethylporphyrin 1, zinc 2-aza-8,12,13,17-tetraethyl-3,7,18-trimethylporphyrin 2, and hematoporphyrin 5. The quarternary ammonium 3 showed a remarkable increase of photodynamic activity compared with 5, although no appreciable difference in the activity was observed between 1 and 5.

The degree of ultraviolet light damage to DNA containing iododeoxyuridine or bromodeoxyuridine is dependent on the DNA sequence.

The sequence selectivity of 300 nm ultraviolet light damage to DNA containing bromodeoxyuridine or iododeoxyuridine was examined on DNA sequencing gels. This was accomplished using a system where an M13 template was employed to direct synthesis of DNA in which thymidine was fully substituted with bromodeoxyuridine or iododeoxyuridine. The sites of damage corresponded to the positions of analogue incorporation. The extent of damage varied considerably at different sites of cleavage and ranged from the undetectable to over fifteen times the limit of detection (as assessed by laser densitometer scans). Strong damage sites had the "consensus" sequence CTT while sites of no detectable damage had the "consensus" sequence GTR. Bromodeoxyuridine and iododeoxyuridine had the same sites of damage although the extent of damage varied at different sites and bromodeoxyuridine damage was slightly greater than iododeoxyuridine. DNA containing thymidine was not damaged to any detectable level in this system with 300 nm ultraviolet light. The use of three closely related DNA sequences as targets for damage confirmed that (1) the sites of analogue incorporation are the cause of ultraviolet damage; and (2) that the neighbouring DNA sequence is an important parameter in determining the extent of damage. It is proposed that the microstructure of DNA--in particular the distance between the 5-carbon of the pyrimidine base (which is attached to the halogen) and hydrogen on the 2' carbon of the 5'-deoxyribose--ultimately determines the degree of cleavage with large distances giving a small degree of damage and smaller distances a large degree of damage.

Changes of fluorescence spectra of 2'-deoxyguanosine in aqueous solution by radiation.

Aqueous solution of 2'-deoxyguanosine (5 X 10(-4) M, pH 7.0) was irradiated with 60Co gamma-rays under O2, N2, N2O, and t-BuOH-N2, respectively. A marked increase in fluorescence emission intensity was observed under all atmospheric conditions as was observed in aqueous solutions of adenine and 2'-deoxyadenosine. However, the fluorescence yield from 2'-deoxyguanosine with radiation was lower under O2 and much higher under t-BuOH-N2 than that from 2'-deoxyadenosine though it was not so different both under N2 and N2O. Such high fluorescence yield from 2'-deoxyguanosine especially under t-BuOH-N2 suggests that guanine base has a specific reactivity with hydrated electron or t-butanol radical differing from the other nucleobases.