The Study of the Effect of Dimethylsulfoxide (or Diethylsulfoxide) on Quinine Sulfate-DNA Binding by UV-Vis and Steady-State Fluorescence Spectroscopies

The effect of dimethylsulfoxide (DMSO) and diethylsulfoxide (DESO) on binding between quinine sulfate (QS) and DNA was studied by virtue of UV-Vis absorption, steady-state fluorescence spectroscopies, and fluorescence polarization measurements. The binding constant was determined at three different temperatures and the values of standard Gibbs energy change, enthalpy and entropy of binding were determined. The mechanism of binding and the effect of sulfoxides on this process was revealed. The values of binding constant, fluorescence polarization and iodide quenching studies confirmed that the main binding mode in QS-DNA system is groove binding. Addition of sulfoxides does not change the binding mechanism. Moreover, with addition of sulfoxides binding constant increases due to the removal of water molecules from DNA grooves making them more available for QS molecules. To explain the effect of DMSO and DESO on QS-DNA binding the photophysical properties of QS in aqueous solutions of DMSO and DESO were also studied. On the basis of quantum yield of QS in water, DMSO and DESO the types of intermolecular interactions were discussed. The obtained results show that quantum yield of QS in sulfoxides is lower compared with that in water and aqueous solution of 0.1 M H2SO4. QS forms ground state complexes with both DMSO and DESO that are stronger fluorophores compared with free QS molecules.

Active and passive chlorophyll fluorescence measurements at canopy level on potato crops. Evidence of similitude of diurnal cycles of apparent fluorescence yields

We performed active and passive measurements of diurnal cycles of chlorophyll fluorescence on potato crops at canopy level in outdoors conditions for 26 days. Active measurements of the stationary fluorescence yield (Fs) were performed using Ledflex, a fluorescence micro-LIDAR described in Moya et al. (Photosynth Res 142:1-15, 2019), capable of remote measurements of chlorophyll fluorescence under full sun-light in the wavelength range from 650 to 800 nm. Passive measurements of solar-induced fluorescence (SIF) fluxes were performed with Spectroflex, an instrument based on the method of filling-in in the O₂A and O₂B absorption bands at 760 nm (F760) and 687 nm (F687), respectively.Diurnal cycles of Fs showed significant variations throughout the day, directly attributed to changes in photosystem II yield. Contrasting patterns were observed according to illumination conditions. Under cloudy sky, Fs varied in parallel with photosynthetically active radiation (PAR). By contrast, during clear sky days, the diurnal cycle of Fs showed a "M" shape pattern with a minimum around noon.F687 and F760 showed different patterns, according to illumination conditions. Under low irradiance associated with cloudy conditions, F687 and F760 followed similar diurnal patterns, in parallel with PAR. Under high irradiance associated with clear sky we observed an increase of the F760/F687 ratio, which we attributed to the contributions in the 760 nm emission of photosystem I fluorescence from deeper layers of the leaves, on one end, and by the decrease of 687 nm emission as a result of red fluorescence re-absorption, on the other end.We defined an approach to derive a proxy of fluorescence yield (FYSIF) from SIF measurements as a linear combination of F687 and F760 normalized by vegetation radiance, where the coefficients of the linear combination were derived from the spectral transmittance of Ledflex. We demonstrated a close relationship between diurnal cycles of FYSIF and Fs, which outperformed other approaches based on normalization by incident light.

Development of multifunctional heterocyclic Schiff base as a potential metal chelator: a comprehensive spectroscopic approach towards drug discovery

Amyloid-β peptides and their metal-associated aggregated states have been implicated in the pathogenesis of Alzheimer's disease. The present paper epitomises the design and synthesis of a small, neutral, lipophilic benzothiazole Schiff base (E)-2-((6-chlorobenzo[d]thiazol-2-ylimino)methyl)-5-diethylamino)phenol (CBMDP), and explores its multifunctionalty as a potential metal chelator/fluorophore using UV-visible absorption, steady-state fluorescence, single molecule fluorescence correlation spectroscopic (FCS) techniques which is further corroborated by in silico studies. Some pharmaceutically relevant properties of the synthesized compound have also been calculated theoretically. Steady-state fluorescence and single molecule FCS reveal that the synthesized CBMDP not only recognizes oligomeric Aβ40, but could also be used as an amyloid-specific extrinsic fluorophore as it shows tremendous increase in its emission intensity in the presence of Aβ40. Molecular docking exercise and MD simulation reveal that CBMDP localizes itself in the crucial amyloidogenic and copper-binding region of Aβ40 and undergoes a strong binding interaction via H-bonding and π-π stacking. It stabilizes the solitary α-helical Aβ40 monomer by retaining the initial conformation of the Aβ central helix and mostly interacts with the hydrophilic N-terminus and the α-helical region spanning from Ala-2 to Val-24. CBMDP exhibits strong copper as well as zinc chelation ability and retards the rapid copper-induced aggregation of amyloid peptide. In addition, CBMDP shows radical scavenging activity which enriches its functionality. Overall, the consolidated in vitro and in silico results obtained for the synthesized molecule could provide a rational template for developing new multifunctional agents.

Exploring the affinity binding of alkylmaltoside surfactants to bovine serum albumin and their effect on the protein stability: A spectroscopic approach

Steady-state and time-resolved fluorescence together with circular dichroism (CD) spectroscopic studies was performed to examine the interactions between bovine serum albumin (BSA) and two alkylmaltoside surfactants, i.e. n-decyl-β-D-maltoside (β-C10G2) and n-dodecyl-β-D-maltoside (β-C12G2), having identical structures but different tail lengths. Changes in the intrinsic fluorescence of BSA from static as well as dynamic measurements revealed a weak protein-surfactant interaction and gave the corresponding binding curves, suggesting that the binding mechanism of surfactants to protein is essentially cooperative in nature. The behavior of both surfactants is similar, so that the differences detected were attributed to the more hydrophobic nature of β-C12G2, which favors the adsorption of micelle-like aggregates onto the protein surface. These observations were substantially demonstrated by data derived from synchronous, three-dimensional and anisotropy fluorescence experiments. Changes in the secondary structure of the protein induced by the interaction with surfactants were analyzed by CD to determine the contents of α-helix and β-strand. It was noted that whereas the addition of β-C10G2 appears to stabilize the secondary structure of the protein, β-C12G2 causes a marginal denaturation of BSA for a protein:surfactant molar ratio as high as 1 to 100.

Fluorescence assays for monitoring RNA-ligand interactions and riboswitch-targeted drug discovery screening

Riboswitches and other noncoding regulatory RNA are intriguing targets for the development of therapeutic agents. A significant challenge in the drug discovery process, however, is the identification of potent compounds that bind the target RNA specifically and disrupt its function. Essential to this process is an effectively designed cascade of screening assays. A screening cascade for identifying compounds that target the T box riboswitch antiterminator element is described. In the primary assays, moderate to higher throughput screening of compound libraries is achieved by combining the sensitivity of fluorescence techniques with functionally relevant assays. Active compounds are then validated and the binding to target RNA further characterized in secondary assays. The cascade of assays monitor ligand-induced changes in the steady-state fluorescence of an attached dye or internally incorporated 2-aminopurine; the fluorescence anisotropy of an RNA complex; and, the thermal denaturation fluorescence profile of a fluorophore-quencher labeled RNA. While the assays described have been developed for T box riboswitch-targeted drug discovery, the fluorescence methods and screening cascade design principles can be applied to drug discovery efforts targeted toward other medicinally relevant noncoding RNA.

Heat-induced fibrillation of BclXL apoptotic repressor

The BclXL apoptotic repressor bears the propensity to associate into megadalton oligomers in solution, particularly under acidic pH. Herein, using various biophysical methods, we analyze the effect of temperature on the oligomerization of BclXL. Our data show that BclXL undergoes irreversible aggregation and assembles into highly-ordered rope-like homogeneous fibrils with length in the order of mm and a diameter in the μm-range under elevated temperatures. Remarkably, the formation of such fibrils correlates with the decay of a largely α-helical fold into a predominantly β-sheet architecture of BclXL in a manner akin to the formation of amyloid fibrils. Further interrogation reveals that while BclXL fibrils formed under elevated temperatures show no observable affinity toward BH3 ligands, they appear to be optimally primed for insertion into cardiolipin bicelles. This salient observation strongly argues that BclXL fibrils likely represent an on-pathway intermediate for insertion into mitochondrial outer membrane during the onset of apoptosis. Collectively, our study sheds light on the propensity of BclXL to form amyloid-like fibrils with important consequences on its mechanism of action in gauging the apoptotic fate of cells in health and disease.

Heating of milk alters the binding of curcumin to casein micelles. A fluorescence spectroscopy study

Curcumin, a polyphenolic compound present in turmeric, is a hydrophobic molecule that has been shown to bind to casein micelles. The present work tested the hypothesis that surface changes in the casein micelles caused by heat-induced interactions with the whey proteins would affect the binding of curcumin. Binding was quantified by direct and tryptophan quenching fluorescence spectroscopy. Curcumin binds to the hydrophobic moieties of the casein proteins, with a 10nm blue shift in its fluorescence emission peak, and causes quenching of the intrinsic fluorescence spectra of the proteins. The fluorescence intensity of curcumin increased after heating of milk at 80°C for 10min; a similar trend in the binding constants was also observed with casein micelles separated from the soluble proteins by centrifugation. There was an increase in the non-specific interactions with heating milk at 80°C for 10min, both in milk as well as in casein micelles separated from the serum proteins. The increased capacity of milk proteins to bind curcumin after heat treatment can be attributed to whey protein denaturation, as whey proteins bind to the surface of casein micelles with heating.