The aim of this work is to estimate the degradation and change in antioxidant activity of quercetin in the presence of two different mixtures of phospholipids in methanol solution, under continuous UV-irradiation from three different sub-ranges (UV-A, UV-B and UV-C), in the presence and in the absence of selected UV-absorbing photosensitizer, benzophenone. Quercetin is employed to control lipid peroxidation process generated by UV-irradiation, by absorbing part of the UV-incident light, or/and by scavenging the involved, created free radicals. The results show that quercetin undergoes to irreversible destruction, highly dependent on UV-photons energy input, more expressed in the presence than in the absence of benzophenone. In the same time quercetin expresses suppression effect on lipid peroxidation processes in UV-irradiated phospholipid mixtures in both cases - absence or presence of benzophenone (more or less effective, respectively). In UV-C-irradiated mixtures, benzophenone photosensitizing function is significantly reduced due to its strong absorption in the same UV-C spectral range, therefore affecting less antioxidant activity of the remained quercetin

Stability of Zn(II) and Cu(II) complexes of porphyrin derivatives (pheophytin and mesoporphyrin) to UV-B -irradiation has been studied by absorbance spectroscopy in 95% ethanol. The chosen porphyrins as well as their heavy metal complexes undergo photochemical decomposition obeying first-order kinetics. In general, pheophytin is more stable than mesoporphyrin to UV-B irradiation. On the other hand, stability of Zn(II)-complex is smaller than Cu(II)-complex both for pheophytin and mesoporphyrin; however while Cu(II)-complex with pheophytin is more stable than the one with mesoporphyrin, with Zn(II)-complex the situation is vice versa

The aim of this study was to obtain a more accurate insight into the interaction of the major photosynthesis pigment, chlorophyll (Chl), with copper(II) and zinc(II) in solution using flow injection analysis combined with electrospray ionization mass spectrometry (FIA-ESI-MS), as well as combined with ultra high performance liquid chromatography with DAD detection (UHPLC-DAD). These interactions may potentially, but not necessarily, lead to the formation of Cu-Chl and Zn-Chl complexes of two different types, which has a large number, at least, disfunctional implications in the plant world. The results based on analysis of full-scan and MS/MS spectra, with and without UHPLC chromatograms, confirmed the formation of a "central type" Cu-Chl complex and a "central type" Zn-Chl complex, as well as proved the formation of a "peripheral" Zn-Chl complex, the latter one originating from a very weak coordinative interaction at the edge of the Chl structure. The employed techniques appeared to be efficient and reliable tools for studying the formation and stability of heavy metals complexes with chlorophyll, at least in vitro, with a considerable possibility for an assessment of real bioenvironmental behavior.

Horseradish peroxidase (HRP), a highly-investigated member of the peroxidase family has been known, among many other biological activities, to catalyze the oxidation of flavonoids and phenolic substrates overall, including quercetin. On the other hand, quercetin is very well known for its antioxidant activities, which in the case of UV external radiation is exibited partly in a preventive manner since it is an excellent UV-absorber. Therefore the aim of this investigation is to study quercetin oxidation by HRP in phosphate buffer under the conditions of UV-stress, i.e. continuous, prolonged UV-B irradiation. The results show that while UV-B irradiation affects the activity of HRP, and the overal rate of quercetin oxidation by HRP, it probably has very little effect on it for longer UV-B-irradiation periods (>30 min). [Acknowledgements. This work was supported by the Ministry of Education and Science of the Republic of Serbia under Project No.TR-34012 and OI-172044

The goal of this work was to study the possibility of the occurrence of radical- type lipid peroxidation of the lipid constituents on biomembranes, in compressed monolayers, having lipoidal benzophenone photosensitizers incorporated. The triplets of the photosensitizer abstract allylic and doubly-allylic hydrogen atoms from anticonjugated moities of the lipid molecules. The results simultaneously confirmed the occurrence of H-abstraction (and so the initiation of the peroxidizing chain mechanism), and the absence of the formation of lipid peroxides. The reason lies in "cage effect": the highly restricted spacial area of compressed lipid monolayers limits the mobility of the created radicals (lipid radicals and ketyl radicals) and leads to their recombination, thus preventing the propagation step of the chain mechanism. With certain reservations it may be concluded that these results have a clear implication on real biomembranes: the structure of which is one of themain factors preventing the spread of the chain reaction, and the formation of lipid peroxides.