Spectroscopic characterization and assessment of microbiological potential of 1,3,4-thiadiazole derivative showing ESIPT dual fluorescence enhanced by aggregation effects.

In the presented study, advanced experimental techniques, including electronic absorption and fluorescence spectroscopies [with Resonance Light Scattering (RLS)], measurements of fluorescence lifetimes in the frequency domain, calculations of dipole moment fluctuations, quantum yields, and radiative and non-radiative transfer constants, were used to characterize a selected analogue from the group of 1,3,4-thiadiazole, namely: 4-[5-(naphthalen-1-ylmethyl)-1,3,4-thiadiazol-2-yl]benzene-1,3-diol (NTBD), intrinsically capable to demonstrate enol → keto excited-states intramolecular proton transfer (ESIPT) effects. The results of spectroscopic analyses conducted in solvent media as well as selected mixtures were complemented by considering biological properties of the derivative in question, particularly in terms of its potential microbiological activity. The compound demonstrated a dual fluorescence effect in non-polar solvents, e.g. chloroform and DMSO/H2O mixtures, while in polar solvents only a single emission maximum was detected. In the studied systems, ESIPT effects were indeed observed, as was the associated phenomenon of dual fluorescence, and, as demonstrated for the DMSO: H2O mixtures, the same could be relatively easily induced by aggregation effects related to aggregation-induced emission (AIE). Subsequently conducted quantum-chemical (TD-)DFT calculations supported further possibility of ESIPT effects. The following article provides a comprehensive description of the spectroscopic and biological properties of the analyzed 1,3,4-thiadiazole derivatives, highlighting its potential applicability as a very good fluorescence probes as well as a compound capable of high microbiological activity.

Mechanisms shaping the synergism of zeaxanthin and PsbS in photoprotective energy dissipation in the photosynthetic apparatus of plants.

Safe operation of photosynthesis is vital to plants and is ensured by the activity of processes protecting chloroplasts against photo-damage. The harmless dissipation of excess excitation energy is considered to be the primary photoprotective mechanism and is most effective in the combined presence of PsbS protein and zeaxanthin, a xanthophyll accumulated in strong light as a result of the xanthophyll cycle. Here we address the problem of specific molecular mechanisms underlying the synergistic effect of zeaxanthin and PsbS. The experiments were conducted with Arabidopsis thaliana, using wild-type plants, mutants lacking PsbS (npq4), and mutants affected in the xanthophyll cycle (npq1), with the application of molecular spectroscopy and imaging techniques. The results lead to the conclusion that PsbS interferes with the formation of densely packed aggregates of thylakoid membrane proteins, thus allowing easy exchange and incorporation of xanthophyll cycle pigments into such structures. It was found that xanthophylls trapped within supramolecular structures, most likely in the interfacial protein region, determine their photophysical properties. The structures formed in the presence of violaxanthin are characterized by minimized dissipation of excitation energy. In contrast, the structures formed in the presence of zeaxanthin show enhanced excitation quenching, thus protecting the system against photo-damage.

Recycling of Energy Dissipated as Heat Accounts for High Activity of Photosystem II.

Photosystem II (PSII) converts light into chemical energy powering almost all life on Earth. The primary photovoltaic reaction in the PSII reaction center requires energy corresponding to 680 nm, which is significantly higher than in the case of the low-energy states in the antenna complexes involved in the harvesting of excitations driving PSII. Here we show that despite seemingly insufficient energy, the low-energy excited states can power PSII because of the activity of the thermally driven up-conversion. We demonstrate the operation of this mechanism both in intact leaves and in isolated pigment-protein complex LHCII. A mechanism is proposed, according to which the effective utilization of thermal energy in the photosynthetic apparatus is possible owing to the formation of LHCII supramolecular structures, leading to the coupled energy levels corresponding to approximately 680 and 700 nm, capable of exchanging excitation energy through the spontaneous relaxation and the thermal up-conversion.

Spectroscopic Studies of Fluorescence Effects in Bioactive 4-(5-Heptyl-1,3,4-Thiadiazol-2-yl)Benzene-1,3-Diol and 4-(5-Methyl-1,3,4-Thiadiazol-2-yl)Benzene-1,3-Diol Molecules Induced by pH Changes in Aqueous Solutions.

This paper presents the results of stationary fluorescence spectroscopy and time-resolved spectroscopy analyses of two 1,3,4-thiadiazole analogues, i.e. 4-(5-methyl-1,3,4-thiadiazol-2-yl)benzene-1,3-diol (C1) and 4-(5-heptyl-1,3,4-thiadiazol-2-yl)benzene-1,3-diol (C7) in an aqueous medium containing different concentrations of hydrogen ions. An interesting dual florescence effect was observed when both compounds were dissolved in aqueous solutions at pH below 7 for C1 and 7.5 for C7. In turn, for C1 and C7 dissolved in water at pH higher than the physiological value (mentioned above), single fluorescence was only noted. Based on previous results of investigations of the selected 1,3,4-thiadiazole compounds, it was noted that the presented effects were associated with both conformational changes in the analysed molecules and charge transfer (CT) effects, which were influenced by the aggregation factor. However, in the case of C1 and C7, the dual fluorescence effects were visible in a higher energetic region (different than that observed in the 1,3,4-thiadiazoles studied previously). Measurements of the fluorescence lifetimes in a medium characterised by different concentrations of hydrogen ions revealed clear lengthening of the excited-state lifetime in a pH range at which dual fluorescence effects can be observed. An important finding of the investigations presented in this article is the fact that the spectroscopic effects observed not only are interesting from the cognitive point of view but also can help in development of an appropriate theoretical model of molecular interactions responsible for the dual fluorescence effects in the analysed 1,3,4-thiadiazoles. Furthermore, the study will clarify a broad range of biological and pharmaceutical applications of these compounds, which are more frequently used in clinical therapies. Graphical Abstract Upper left corner - C7 molecule at high pH, right upper corner - fluorescence emission spectrum for C7 dissolved in H2O at high pH (7-12) - single fluorescence. Bottom left corner - C7 molecule at low pH (1-7), lower right corner - fluorescence emission spectrum for C7 dissolved in water at low pH - two fluorescence emissions. The circles indicate the group related to dissociation of molecules at low and high pH and the additional long circles indicate C1 or a molecule with a shorter acyl chain.

Molecular Organization of Dipalmitoylphosphatidylcholine Bilayers Containing Bioactive Compounds 4-(5-Heptyl-1,3,4-thiadiazol-2-yl) Benzene-1,3-diol and 4-(5-Methyl-1,3,4-thiadiazol-2-yl) Benzene-1,3-diols.

This article presents the results of spectroscopic studies of two compounds from the 1,3,4-thiadiazole group, that is, 4-(5-methyl-1,3,4-thiadiazole-2-yl)benzene-1,3-diol (C1) and 4-(5-heptyl-1,3,4-thiadiazole-2-yl)benzene-1,3-diol (C7), present at different molar concentrations in 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) liposome systems. In the case of both investigated compounds, fluorescence measurements revealed the presence of several emission bands, whose appearance is related to the molecular organization induced by changes in the phase transition in DPPC. On the basis of the interpretation of Fourier transform infrared spectra, we determined the molecular organization of the analyzed compounds in multilayers formed from DPPC and the 1,3,4-thiadiazoles. It was found that the compound with a longer alkyl substituent both occupied the lipid polar head region in the lipid multilayer and interacted with lipid hydrocarbon chains. In turn, the compound with a shorter alkyl substituent interacted more strongly with the membrane polar region. On the basis of the knowledge from previous investigations conducted using different solvents, the fluorescence effects observed were related to the phenomenon of molecular aggregation. The effects were strongly influenced by the structure of the compound and, primarily, by the type of the alkyl substituent used in the molecule. The substantial shortening of fluorescence lifetimes associated with the effect of long-wave emission (with a maximum at 505 nm) decay also confirms the model of aggregation effects in the analyzed systems. Similar effects can be very easily distinguished and associated with respective forms of the compounds in biologically relevant samples.

Solvent Effects on Molecular Aggregation in 4-(5-Heptyl-1,3,4-thiadiazol-2-yl)benzene-1,3-diol and 4-(5-Methyl-1,3,4-thiadiazol-2-yl)benzene-1,3-diol.

The article presents the results of spectroscopic studies of 4-(5-methyl-1,3,4-thiadiazol-2-yl)benzene-1,3-diol (C1) and 4-(5-heptyl-1,3,4-thiadiazol-2-yl)benzene-1,3-diol (C7) in organic solvent solutions. Depending on the concentration of the compound used, three bands were observed in the fluorescence emission spectra of the compounds in DMSO solutions. A single band was observed in methanol, propan-2-ol, or ethanol. The significantly shortened fluorescence lifetimes and the different shapes of circular dichroism (CD) spectra clearly indicate association of the fluorescence effects with the aggregation processes in the analyzed compounds. The differences in the course of the CD spectra also imply an effect of the substituent group structure on the molecule aggregation interactions. Therefore, it has been postulated that the occurrence of the different spectral forms induced by changes in the compound concentration may be related to the aggregation effects of C1 and C7 molecules, which are also induced by differences in the alkyl substituent structure.