Photophysics and photodimerization of 6,5'-dimethylangelicin in different solvents

The photophysical and photochemical behaviour of 6,8-dimethyl-2H-furo[2,3-h]chromen-2-one (6,5'-dimethylangelicin, 6,5'-DMA) was studied by steady state and pulsed techniques and by semiempirical calculations. The fluorescence characteristics and the triplet state properties were determined in solvents of different polarity/proticity. The obtained values of fluorescence quantum yields and lifetimes together with the triplet formation quantum yield indicate that the energy gap between the two lowest excited states controls the decay pathways of the singlet. In non-polar and non-protic solvents the internal conversion is almost the sole decay process of the singlet, while in protic solvents the intersystem crossing reaches substantial quantum yields. The effect of ground state concentration on the fluorescence lifetime suggested the occurrence of aggregation while the triplet lifetime was quenched and the rate constant of self-quenching was determined. The 6,5'-DMA photodimerization was investigated in different solvents and in different experimental conditions. Two pyrone-pyrone and one furan-pyrone photodimers of 6,5'-DMA were the main photoproducts detected in the irradiated solution. The effects of the 6,5'-DMA concentration and of oxygen on the amounts of the dimers, obtained together with the photophysical behaviour gave indications oil the formation mechanism of different dimers

Photoinduced modifications by fluoroquinolone drugs in bovine serum albumin (BSA) and ribonuclease A (RNAse) as model proteins

Photosensitized protein oxidation by drugs, with the consequent modification of their structure is thought to be responsible for the occurrence of phototoxic phenomena such as photoallergy and loss of biological functions. In this paper we have investigated in detail the interaction of four fluoroquinolones namely ciprofloxacin, lomefloxacin, norfloxacin and ofloxacin with two proteins such as Bovine Serum Albumin (BSA) and Ribonuclease A (RNAse A) chosen as models. The interactions between the four drugs and the proteins, were studied by absorption and emission spectroscopy. Photophysical experiments were carried out in aqueous solutions by stationary and time-resolved fluorimetry and by laser flash photolysis, in the absence and in the presence of the proteins to obtain information on the various decay pathways of the excited states of the drugs and on transient species formed upon irradiation. In parallel we have investigated by a series of biochemical assays the photoinduced modifications exerted by the four drugs. The obtained results showed that the four drugs are able to photooxidize proteins with the formation of protein-protein cross-link. This effect was also confirmed in isolated erythrocyte membranes. Furthermore the effect of the fluoroquinolones was also evaluated on isolated aromatic aminoacids. In this context the four drugs are able to photodamage in particular tyrosine and histidine. These results are important in the light of the growing interest for the comprehension of the mechanism of phototoxicity induced by these antibacterial drugs

Photophysical properties of halo-derivatives of angelicins

The properties of the lowest excited singlet and triplet state of four halo-angelicins (HA) have been investigated by steady-state and time-resolved spectrometric techniques. The study has been performed in solvents with different polarity and proticity: cyclohexane, dioxane, acetonitrile, ethanol and 2,2,2-trifluoroethanol. Absorption and emission spectra showed that the nature of lowest singlet state is mainly pi,pi* and the transitions are allowed or partially allowed. Absorption coefficients, fluorescence quantum efficiency and lifetimes are also presented. Flash photolysis investigations have indicated the presence of a unique transient assignable to the lowest triplet state T-1. The triplet state has been characterized in terms of absorption spectra, decay kinetics, molar absorption coefficients and formation quantum yields. The racing of both intersystem crossing and fluorescence quantum yields going from a non-polar to a highly polar-protic solvent indicates the presence of a S-2 state, n,pi* in nature, close lying to the pi,pi* one. The quantum yields of the singlet decay pathways (fluorescence, intersystem crossing and internal conversion) depends markedly on the energy gap between S-1 and S-2 states in agreement with the manifestation of the "proximity effect". In fact in cyclohexane HA decay mainly through S-1 -> S-0 internal conversion, while in trifluoroethanol their fluorescence and intersystem crossing increase significantly. Singlet-oxygen quantum yields have been also determined in order to understand the possible application of the investigated drugs in photodynamic therapy. The absence of photochemical pathways allowed the indirect evaluation of the internal conversion decay bringing to the achievement of a complete decay pathways picture

1-thiopsoralen, a new photobiologically active heteropsoralen. Photophysical, photochemical and computer aided studies

1-Thiopsoralen (7H-thieno[3,2-g]benzofuran-7-one) I, a lead compound of a series of heteropsoralens, was investigated. The electronic transitions involved were studied. Fluorescence quantum yield is very low, while laser flash photolysis showed that the triplet state is practically the sole transient of 1. Fluorescence quantum yield (phi(F)) and triplet lifetime (tau(F)) as well as triplet quantum yield (phi(T)) and lifetime (tau(T)) were determined. The production of singlet oxygen was also evaluated by photophysical measurements. Photophysical data suggest that DNA photobinding of 1, owing to short fluorescence lifetime value and high triplet quantum yield, occurs likely through triplet mechanism. Interactions between 1 and DNA were studied both in the ground and the excited state. In the ground state 1 undergoes intercalation inside duplex DNA. This fact is also supported by molecular modeling studies. By UVA-light activation 1 photobinds covalently to DNA forming mono and diadducts. The furan side l-thymine monoadduct, isolated from DNA photomodified by thiopsoralen, shows a cis-syn stereochemistry, in agreement with quantum mechanics studies. Compound 1 photobinds also with linolenic acid, component of lecithins, giving a C-4-cycloaddition, and supporting that this compound also induces photolesions at the level of cell membrane, like psoralen. Compound 1 exhibits strong skin-phototoxicity

Photosensitization of DNA strand breaks by three phenothiazine derivatives

The interaction and the photosensitizing activity of three phenothiazine derivatives, fluphenazine hydrochloride (FP), thioridazine hydrochloride (TR), and perphenazine (PP), toward DNA were studied. Evidences obtained from various spectroscopic studies such as fluorimetric and linear dichroism measurements indicate that these derivatives bind to the DNA at least in two ways: intercalation and external stacking on the DNA helix, depending on their relative concentrations. Irradiation of supercoiled plasmid DNA in the presence of these phenothiazines leads to single strand breaks. The DNA photocleavage appears to be due to externally bound molecules rather than to those intercalated. The highest photocleavage activity was observed with PP and TR whereas FP was less efficient. The efficiency of the photocleavage in aerated and deaerated solutions does not change thus indicating that an involvement of singlet oxygen can be excluded. Primer extension analysis of plasmid DNA irradiated in the presence of phenothiazines indicates that photocleavage of DNA occurs predominantly at Gua and Cyt residues. Laser flash experiments carried out in the presence of 2'-deoxyguanosine 5'-monophosphate reveal an efficient electron transfer between the nucleotide and the radical cations produced by photoionization of the phenothiazines. In the presence of DNA, an electron transfer process takes place within the laser pulse from the lowest singlet state of phenothiazines to the DNA bases; the time-resolved measurements showed that the back-electron transfer is a negligible decay pathway for the charged species

DNA cleavage induced by photoexcited antimalarial drugs: A photophysical and photobiological study

The interactions and the photosensitizing activity of three antimalarial drugs quinine (Q), mefloquine (MQ) and quinacrine (QC) toward DNA was studied. Evidences obtained by absorption and emission spectroscopy and by linear dichroism measurements indicate that these derivatives bind the macromolecule with a high affinity (binding constants Ka approximately 10(5) M(-1)). The absorption characteristics of the drugs changed markedly by addition of DNA and their fluorescence was quenched with rate constants higher than that of diffusion. The geometry of binding involves predominantly the intercalation into the double helix. The DNA photocleavage properties of antimalarials was investigated using plasmid DNA as a model, at different [drug]/ [DNA] ratios. The results indicate that mainly MQ and Q are able to induce significant photodamage to DNA. In particular the marked effect of the former drug is evidenced after treatment of photosensitized DNA by two base excision repair enzymes, formamydo-pyrimidine glycosilase (Fpg) and Endonuclease III (Endo III). From a mechanistic point of view, experiments carried out in different experimental conditions indicate that these drugs photoinduce DNA damage through singlet oxygen and/or radical cation production. These findings are further supported by the determination of two photoproducts of 2'-deoxyguanosine, which are diagnostic for Type I and Type II pathways, namely 2,2-diamino(2-deoxy-beta-D-erythro-pentofuranosyl)-4-amino-5(2H)-oxazolone and (R,S)4-hydroxy-8-oxo-4,8-dihydro-2'-deoxyguanosine (4-OH-8-oxo-dGuo). Laser flash photolysis experiments carried out in the presence of DNA indicates that the excitation produces mainly the triplet state for Q and the triplet and radical cation for QC. Moreover the singlet and triplet states and radical cations of the drugs are quenched by 2'-deoxyguanosine monophosphate. The absorbances of these transients decrease with increasing DNA concentration