[Investigation of aggregates of dyes by the method of resonance light scattering: correction of spectra]

Resonance light scattering, a phenomenon of an abrupt enhancement of Rayleigh light scattering in close proximity to an absorption band, is easily detectable in solutions of strongly absorbing chromophores, which form large aggregates with strong pi-pi-electronic coupling among the chromophores. Resonance light scattering spectra need to be corrected for the sensitivity of spectrofluorimeter as well as for the effects of internal light filter. A method for correcting the measured resonance light scattering is described. It was shown by the method that the addition of KCl induces the formation of extended supramolecular aggregates (probably of H-type) of the anionic dye merocyanine 540 in water. The spectra of the resonance light scattering of the photosensitizer meta-tetra(hydroxyphenyl)chlorin (mTHPC, foscan ) indicate the formation of J-aggregates of this dye in aqueous medium.

[Photohemolysis sensitized by psoralen: dependence on pH]

The effect of pH on the hemolysis of erythrocytes photosensitized (366 nm, 23 Wt/m2) by psoralen has been studied. The dependence of the photohemolysis rate (V) on irradiation dose (D) was described by the equation V = Vo + kD, where Vo is the rate of hemolysis without irradiation (dark), and k is the constant. The index of the power at dose x was approximately equal to 2, and its value did not change as the pH of the erythrocyte suspension was changed. It was found that changes in pH led to a sharp change in the value of coefficient k and correspondingly V. The lowest rate of photohemolysis was observed in the pH range from 8.0 to 8.4. As pH was changed from 3.4 to 9.0 or from 8.0 to 7.4, the V value increased approximately twofold. At pH below 7.4, an abrupt increase (approximately fourfold) in V was observed, with the pK value being equal to 7.3. The psoralen molecule lacks titratable acidic and basic groups; therefore, the effects of pH can hardly be assigned to changes in the photophysical properties of the sensitizer. The increase in V in the alkaline region is prohably related to the acceleration of photooxidation of reduced glutathione, whereas the jump of V at pH of about 7.3 may be due to the titration of the product of psoralen photooxidation. The latter assumption is confirmed by the data of hign performance liquid chromatography. In these experiments, psoralen was oxidized in ethanol and mixed with the phosphate buffer at different pH values followed by a qualitative and quantitative analysis by high performance liquid chromatography of photoproducts. Several photoproducts of psoralen have been identified whose content depended on pH. The curve of titration of one photoproduct was similar in shape to the pH dependence of psoralen-photosensitized hemolysis.

[Photobiophysics of furanocoumarins]

Furocoumarins (psoralens) are photosensitizers of plant origin, which increase the sensitivity of biological objects to near ultraviolet (UV-A, 320-400 nm). In combination with UV-A, they are successfully used for treating many dermal and autoimmune diseases (PUVA therapy and photophoresis). Along with therapeutic effects, the furocoumarin photochemotherapy induces a number of side-effects (erythema, edema, hyperpigmentation, and premature aging of skin). All photobiological effects of furocoumarins result from their photochemical reactions. Therefore, in order to advance the therapy, it is necessary to know the photochemical mechanisms of induction of both side- and therapeutic effects. The types of photoreactions of furocoumarins classified with respect to reactive photoproducts interacting with substrate were considered. Primary emphasis was placed on reactions proceeding with the participation of photooxidation products of furocoumarins. Among these photoproducts, at least two types can be distinguished. Some of them possess membranotoxic properties, others produce the immunosuppressory action in vivo. The photochemical mechanisms of the formation of the photoproducts of furocoumarins are different. It was found that, by varying the illumination conditions (intensity of UV-A radiation or the concentration of the photosensitizer), it is possible to obtain the photoproducts of furocoumarins that have either membranotoxic or immunosuppressory properties. It was found that the mechanisms of the immunosuppressive action of the photooxidation products of furocoumarins have some features in common with those underlying the PUVA therapy and photophoresis. It is assumed that the photochemical basis of the therapeutic action of furocoumarins is the reactions with the involvement of the products of their photooxidation.

[Photo oxidative reactions of psoralens and their role in therapy of dermatoses]

Psoralenes (furocoumarins) in combination with ultraviolet (UV) A radiation (320-400 nm) are used in the treatment of vitiligo, psoriasis, cutaneous T-cell lymphoma and other skin and autoimmune diseases (PUFA therapy). The mechanism of psoralene-photosensitive modification of biologically significant molecules, such as DNA, protein unsaturated lipids, antioxidants, etc. is considered in the paper. Particular emphasis is laid on the mechanism and biomedical significance of photo reactions proceeding via the stage of formation of psoralene photo oxidation products (POP). POP causes oxidative damage to many molecules in in vitro experiments and mediates the T-cell immunity cell in vivo, as appeared as suppression of delayed hypersensitivity and contact sensitivity in mice. The use of randomized double blind control has indicated that POP has therapeutical effects on eczema. A new modality of psoralene photo chemotherapy that consists in the UV-A radiation of psoralene solution followed by its administration (POP therapy) to patients. Unlike conventional PUFA therapy, POP therapy does not require the patients' skin to be exposed to UF-A radiation, which allows adverse effects, such erythema, skin hyperpigmentation, etc. to be avoided.

[Death of cells from mouse peritoneal exudate induced by products of reaction of Fe2+ ions with previously photo-oxidized psoralen]

Toxic effect of previously photooxidized in water psoralen on survival of mouse peritoneal exudate cells was investigated. Cell survival was determined by trypan exclusion test. It was shown that photooxidized psoralen itself did not induce trypan-positive cells while decrease in cell survival was observed in the case of simultaneous addition of photooxidized psoralen and Fe2+ ions to cell suspension. To evaluate the quantity of psoralen photoproducts which react with Fe2+ photooxidized psoralen Fe2+ mixtures were titrated by different cell concentrations. Then parameter c50 (cell concentration such that 50% trypan-positive cells were observed) was determined. It was shown that c50 increased with the increase in preirradiation fluence of psoralen from 4.5 to 45 kJ/m2. But further increase in preirradiation fluence resulted in decrease of c50. The photodestruction of psoralen photoproducts which react with Fe2+ ions was proposed. Simultaneous addition of photooxidized psoralen, Fe2+ ions and antioxidant butylated hydroxytoluene significantly increased cell survival. That indicated free radical nature of reaction products of photooxidized psoralen with Fe2+ ions.

[Effect of Fe2+ ions on the hemolysis induced by products of psoralen photo-oxidation]

The effects of Fe2+ ions on haemolysis induced by previously photooxidized psoralen (POP-haemolysis) were investigated. It was shown that POP-haemolysis was strongly activated by Fe2+ ions when ferrous ions were added to erythrocytes just after addition of POP. If POP was preincubated with Fe2+ before mixing with erythrocytes, then POP completely lost its ability to induce haemolysis. These data indicate the peroxidic nature of POP products responsible for haemolysis.

[Psoralen sensitized thermoactivated photodamage to leukocyte membranes]

Psoralen sensitized photodamage of rat peritoneal exudate cells was investigated. Irradiation of cells induced latent lesions in membranes which during thermal activation at the post-irradiation stage were transformed into permeability channels for trypan blue. The effect linearly increased with fluence of irradiation which indicates one hit production of thermolabile psoralen photoproducts in the membranes.