Changes In UV Absorbance Research Articles

Mechanism of Decarboxylation of Pyruvic Acid in the Presence of Hydrogen Peroxide

The purpose of this work was to probe the rate and mechanism of rapid decarboxylation of pyruvic acid in the presence of hydrogen peroxide (H2O2) to acetic acid and carbon dioxide over the pH range 2–9 at 25°C, utilizing UV spectrophotometry, high performance liquid chromatography (HPLC), and proton and carbon nuclear magnetic resonance spectrometry (1H, 13C-NMR). Changes in UV absorbance at 220 nm were used to determine the kinetics as the reaction was too fast to follow by HPLC or NMR in much of the pH range. The rate constants for the reaction were determined in the presence of molar excess of H2O2 resulting in pseudo first-order kinetics. No buffer catalysis was observed. The calculated second-order rate constants for the reaction followed a sigmoidal shape with pH-independent regions below pH 3 and above pH 7 but increased between pH 4 and 6. Between pH 4 and 9, the results were in agreement with a change from rate-determining nucleophilic attack of the deprotonated peroxide species, HOO−, on the α-carbonyl group followed by rapid decarboxylation at pH values below 6 to rate-determining decarboxylation above pH 7. The addition of H2O2 to ethyl pyruvate was also characterized.

Synthesis, optimization and structural characterization of a chitosan–glucose derivative obtained by the Maillard reaction

Chitosan (Chit) was submitted to the Maillard reaction (MR) by co-heating a solution with glucose (Glc). Different reaction conditions as temperature (40, 60 and 80°C), Glc concentration (0.5%, 1%, and 2%, w/v), and reaction time (72, 52 and 24h) were evaluated. Assessment of the reaction extent was monitored by measuring changes in UV absorbance, browning and fluorescence. Under the best conditions, 2% (w/v) of Chit, 2% (w/v) of Glc at 60°C and 32h of reaction time, a chitosan–glucose (Chit–Glc) derivative was purified and submitted to structural characterization to confirm its formation. Analysis of its molecular weight (MW) and the degree of substitution (DS) was carried out by HPLC–Size Exclusion Chromatography (SEC) and a colloid titration method, respectively. FT-IR and 1H NMR were also used to analyze the functional groups and evaluate the introduction of Glc into the Chit molecule. According to our objectives, the results obtained in this work allowed to better understand the key parameters influencing the MR with Chit as well as to confirm the successful introduction of Glc into the Chit molecule obtaining a Chit–Glc derivative with a DS of 64.76±4.40% and a MW of 210.37kDa.

Insight into photocatalytic degradation of dissolved organic matter in UVA/TiO2 systems revealed by fluorescence EEM-PARAFAC

Photocatalytic degradation of dissolved organic matter (DOM) using TiO2 as a catalyst and UVA as a light source was examined under various experimental settings with different TiO2 doses, solution pH, and the light intensities. The changes in UV absorbance and fluorescence with the irradiation time followed a pseudo-first order model much better than those of dissolved organic carbon. In general, the degradation rates were increased by higher TiO2 doses and light intensities. However, the exact photocatalytic responses of DOM to the irradiation were affected by many other factors such as aggregation of TiO2, light scattering, hydroxyl radicals produced, and DOM sorption on TiO2. Fluorescence excitation-emission matrix (EEM) coupled with parallel factor analysis (PARAFAC) revealed that the DOM changes in fluorescence could be described by the combinations of four dissimilar components including one protein-like, two humic-like, and one terrestrial humic-like components, each of which followed well the pseudo-first order model. The photocatalytic degradation rates were higher for protein-like versus humic-like component, whereas the opposite order was displayed for the degradation rates in the absence of TiO2, suggesting different dominant mechanisms operating between the systems with and without TiO2. Our results based on EEM-PARAFAC provided new insights into the underlying mechanisms associated with the photocatalytic degradation of DOM as well as the potential environmental impact of the treated water. This study demonstrated a successful application of EEM-PARAFAC for photocatalytic systems via directly comparing the kinetic rates of the individual DOM components with different compositions.

Combination of UV absorbance and electron donating capacity to assess degradation of micropollutants and formation of bromate during ozonation of wastewater effluents

In this study, the changes in UV absorbance at 254 nm (UVA254) and electron donating capacity (EDC) were investigated as surrogate indicators for assessing removal of micropollutants and bromate formation during ozonation of wastewater effluents. To measure the EDC, a novel method based on size exclusion chromatography followed by a post-column reaction was developed and calibrated against an existing electrochemical method. Low specific ozone doses led to a more efficient abatement of EDC than of UVA254. This was attributed to the abatement of phenolic moieties in the dissolved organic matter (DOM), which lose their EDC upon oxidation, but are partially transformed into quinones, which still absorb in the measured UV range. For higher specific ozone doses, the relative EDC abatement was lower than the relative UVA abatement, which can be explained by the oxidation of UV absorbing moieties (e.g. non-activated aromatic compounds), which contribute less to EDC. The abatement of the selected micropollutants (i.e., 17α-ethinylestradiol (EE2), carbamazepine (CBZ), atenolol (ATE), bezafibrate (BZF), ibuprofen (IBU), and p-chlorobenzoic acid (pCBA)) varied significantly depending on their reactivity with ozone in the examined specific ozone dose range of 0–1.45 mgO3/mgDOC. The decrease of EE2 and CBZ with high ozone reactivity was linearly proportional to the reduction of the relative residuals of UVA254 and EDC. The abatement of ATE, BZF, IBU, and pCBA with intermediate to low ozone reactivities was not significant in a first phase (UVA254/UVA254,0 = 1.00–0.70; EDC/EDC0 = 1.00–0.56) while their abatement was more efficient than the degradation of the relative residual UVA254 and much more noticeable than the degradation of the relative residual EDC in a second phase (UVA254/UVA254,0 = 0.70–0.25; EDC/EDC0 = 0.56–0.25) because the partially destroyed UV absorbing and electron donating DOM moieties become recalcitrant to ozone attack. Bromate formation was pronounced for specific ozone doses >0.25 mgO3/mgDOC. At these ozone doses, the residual DOM competes less with bromide for ozone as the electron-rich DOM moieties are almost completely degraded. Overall, these results imply that a combination of the relative residual UVA254 and EDC more truly reflects the intrinsic reactivity of DOM associated with the removal of micropollutants and bromate formation than the single use of the UV absorbance-based surrogate indicators.

Fluorescence, absorbance, and ion exchange: coupling analysis with water treatment for improved insight on copper complexation with natural organic matter

Groundwater high in dissolved organic carbon (DOC) was spiked with 0–120 μmol/L of copper(II) (Cu) at pH 4, 6, and 8 to observe changes in fluorescence and UV absorbance of natural organic matter (NOM). The groundwater had a fluorescence excitation–emission matrix (EEM) characteristic of humic-like substances. Addition of Cu to the groundwater resulted in fluorescence quenching of EEM peak intensity and increase in UV absorbance at 254nm (UVA254). Both fluorescence quenching and increase in UVA254 suggested the complex formation between Cu and NOM. Importantly, DOC measurements showed that physical loss of NOM did not play a measureable role in contributing to the observed fluorescence quenching and increase in UVA254, which supports the complex formation of Cu–NOM. The conditional stability constant for Cu–NOM was estimated using both fluorescence quenching and UVA254 increase at each pH and ranged from 4.26 to 5.77. As a follow up, anion exchange and cation exchange batch experiments were conducted using the groundwater spiked with 120 μmol/L Cu at pH 8 to gain insight on the behavior of Cu–NOM, and attempt to give physical meaning to the fluorescence quenching and UVA254 increase. The ion exchange results suggested the possibility of a neutral or partially negatively charged Cu–NOM complex. The significance of this work resides in the idea that fluorescence quenching and UV absorbance of NOM can be used as a tool to evaluate the co-removal of contaminants through water treatment processes.

Interaction of Ketoconazole with Bovine Serum Albumin: Electrochemical, Spectroscopic and Molecular Modeling Studies

Formulation and Evaluation of Ketoconazole Polymeric Films for Topical ApplicationMohamed Salama, Mahmoud A. Mahdy, Amina Mohamed, Aref T. Mohamed, Eseldin I. Keleb, Assad A. Omar, Nagib A. Elmarzugi

Exploring the relationship between the optical properties of water and the quality and quantity of dissolved organic carbon in aquatic ecosystems: strong correlations do not always mean strong predictive power.

Optical absorbance and fluorescence spectroscopies have the potential to play an important role in monitoring aquatic ecosystems. This paper explores the relationship between the optical absorption and fluorescence characteristics of water samples taken from 18 sites (spanning a range of aquatic environments including lowland rivers, small and large dams, and floodplain wetlands) with their dissolved organic carbon (DOC) concentration and 28 days bioavailability Both optical absorbance in the ultraviolet region and fluorescence above excitation wavelengths of about 240 nm and emission wavelengths above about 350 were correlated with DOC concentration (r(2) > 0.8). The initial (pre-incubation) optical absorbance in the UV region (r(2) ≈ 0.7-0.8) and fluorescence attributed to both 'humic like' (r(2) = 0.84) and 'tryptophan-like' (r(2) = 0.87) fluorophores correlated with DOC bioavailability. These correlations were used to develop empirical linear models relating the initial optical properties of water with DOC quantity and quality. The robustness of these models was then tested against a second suite of water samples from 12 different sites, collected independently of those used in the model development. Although based on strong correlations, the empirical models were not particularly good at predicting the bioavailability of DOC in the model validation samples. We suggest that one of the reasons for the low predictive power of the models is that the strong correlations observed between DOC bioavailability and optical absorbance in the UV region or fluorescence and are co-incidental rather than causal. Changes in UV-absorbance or fluorescence during the incubation experiments are not consistent with changes in DOC concentration. One of the best predictors of DOC bioavailability is the initial concentration of DOC. We argue the strong correlation between DOC bioavailability and initial fluorescence intensity or UV absorption simply reflects the strong correlation between initial DOC concentration and initial fluorescence intensity or UV absorption. We argue that unless there is an underlying causal relationship between two components (the component of interest and a surrogate measure for that component) care should be taken in extrapolating correlative models beyond the data set used to create them.

Radiation induced decomposition of a refractory cefathiamidine intermediate

Diisopropylthiourea (DPT), an intermediate of a widely used cephalosporin, has been found to be one of the most refractory components in cephalosporin synthesis wastewater. This compound cannot be completely removed by conventional biological processes due to its antimicrobial property. Ionizing radiation has been applied in the decomposition of refractory pollutants in recent years and has proved effective. Therefore, the decomposition of DPT by γ-irradiation was studied. The compound was irradiated at the dose of 150–2000Gy before a change of concentration and UV absorption of the solutions was detected. Furthermore, the decomposition kinetics and radiation yield (G-value) of DPT was investigated. The results of radiation experiments on DPT-containing aqueous showed that the DPT can be effectively degraded by γ-radiation. DPT concentration decreased with increasing absorbed doses. G-values of radiolytic decomposition for DPT (20mg/L) were 1.04 and 0.47 for absorbed doses of 150 and 2000Gy, respectively. The initial concentration and pH of the solutions affected the degradation. As the concentration of substrate increased, the decomposition was reduced. The decrease of removal rate and radiation efficacy under alkaline condition suggested that lower pH values benefit the γ-induced degradation. UV absorption from 190 to 250nm decreased after radiation while that from 250 to 300nm increased, indicating the formation of by-products.

Structure and antimicrobial mechanism of ɛ-polylysine–chitosan conjugates through Maillard reaction

The aim of the study was to testify the formation and antimicrobial activity of ɛ-polylysine–chitosan conjugate through Maillard reaction. The results of UV absorbance, browning index and fluorescence changes of Maillard reaction products (MRPs) between ɛ-polylysine and chitosan indicated there existed Maillard reaction between ɛ-polylysine and chitosan and the formation of their conjugate. The conjugate showed strong antibacterial activity against Escherichia coli, Staphylococcus aureus, Bacillus subtilis and beer yeast. Morphologies of E. coli and S. aureus treated with the conjugate were observed by transmission electron microscopy (TEM). The results revealed that the conjugate of ɛ-polylysine and chitosan increased the permeability of the outer membrane (OM) and inner membrane (IM) and ultimately disrupted bacterial cell membranes, with the release of cellular cytoplasm.

Uncovering the self-assembly of DNA nanostructures by thermodynamics and kinetics.

CONSPECTUS: DNA nanotechnology is one of the most flourishing interdisciplinary research fields. DNA nanostructures can be designed to self-assemble into a variety of periodic or aperiodic patterns of different shapes and length scales. They can be used as scaffolds for organizing other nanoparticles, proteins, and chemical groups, leveraging their functions for creating complex bioinspired materials that may serve as smart drug delivery systems, in vitro or in vivo biomolecular computing platforms, and diagnostic devices. Achieving optimal structural features, efficient assembly protocols, and precise functional group positioning and modification requires a thorough understanding of the thermodynamics and kinetics of the DNA nanostructure self-assembly process. The most common real-time measurement strategies include monitoring changes in UV absorbance based on the hyperchromic effect of DNA, and the emission signal changes of DNA intercalating dyes or covalently conjugated fluorescent dyes/pairs that accompany temperature dependent structural changes. Thermodynamic studies of a variety of DNA nanostructures have been performed, from simple double stranded DNA formation to more complex origami assembly. The key parameters that have been evaluated in terms of stability and cooperativity include the overall dimensions, the folding path of the scaffold, crossover and nick point arrangement, length and sequence of single strands, and salt and ion concentrations. DNA tile-tile interactions through sticky end hybridization have also been analyzed, and the steric inhibition and rigidity of tiles turn out to be important factors. Many kinetic studies have also been reported, and most are based on double stranded DNA formation. A two-state assumption and the hypothesis of several intermediate states have been applied to determine the rate constant and activation energy of the DNA hybridization process. A few simulated models were proposed to represent the structural, mechanical, and kinetic properties of DNA hybridization. The kinetics of strand displacement reactions has also been studied as a special case of DNA hybridization. The thermodynamic and kinetic characteristics of DNA nanostructures have been exploited to develop rapid and isothermal annealing protocols. It is conceivable that a more thorough understanding of the DNA assembly process could be used to guide the structural design process and optimize the conditions for assembly, manipulation, and functionalization, thus benefiting both upstream design and downstream applications.

A new bis-pyrene derivative as a selective colorimetric and fluorescent chemosensor for cyanide and fluoride and anion-activated CO2 sensing

In the present study, we report the solvent controlled selectivity of cyanide. A newly synthesized bis-pyrene derivative 1 exhibited large “off–on” fluorescence enhancement with fluoride and cyanide in CH3CN-DMSO (99:1, v/v), which was accompanied by unique colorimetric changes. Likewise, in CH3CN-DMSO-EtOH (98:1:1, v/v) or CH3CN-DMSO-H2O (94:1:5, v/v), selective fluorescence enhancement with cyanide was observed among the various anions, which was attributed to a favourable interaction between the fluoride ion with ethanol or water. The binding modes, UV absorption and fluorescence changes of 1 upon CN− binding are also explained by DFT calculations. Probe 1 was successfully applied for imaging cyanide in the HeLa cells. Finally, anion-activated CO2 sensing via colour and fluorescence changes is demonstrated using chemosensor 1.

Detection of Dissolved CO2 Based on the Aggregation of Gold Nanoparticles

A sensitive colorimetric assay of dissolved CO2 (dCO2) was developed based on the plasmon shift of gold nanoparticles (AuNPs). A water-soluble random copolymer poly(dimethyl acrylamide-co-(N-amidino)ethyl acrylamide), or P(DMA-co-NAEAA), containing amidine groups was synthesized. In the presence of dCO2, the amidine groups in the NAEAA block protonate and convert the polymer from a neutral to a positive-charged state, hence triggering the negative-charged AuNPs to aggregate by the electrostatic interaction. The degree of AuNP aggregation is dependent on the charge density of polymer, which is related to dCO2 concentration. The aggregation of AuNPs results in a red shift of the AuNP plasmonic spectrum, or a color change from red to blue. In addition, dCO2 concentration can be quantitatively measured by the UV absorbance change of the AuNP solution. A linear relationship between 0.264 and 6.336 hPa of dCO2 with a limit of detection (LOD) of 0.04 hPa can be acquired. This is the first report to detect dCO2 using the optical properties of nanoparticles.

Inclusion of 4-pyrrolidinopyridine derivatives in a symmetrical α,α′,δ,δ′-tetramethyl-cucurbit[6]uril and a Ba2+-driven pseudorotaxane with characteristic UV absorption changes

Novel pseudorotaxane have been built from TMeQ[6] and 4-pyrrolidinopyridine derivatives, and the unthreading and rethreading of the TMeQ[6] ring can be reversibly driven by Ba2+/SO42−.

Characterization of interaction between different adsorbents and copper by simulation experiments using sediment-extracted organic matter from Taihu Lake, China

The complex capacity of different types of organic matters (OMs) for Cu was quantitatively studied by simulation experiments using different adsorbents prepared from the sediment in Taihu Lake. The free Cu was measured with ion selective electrode (ISE) and complex capacity was calculated using a conditional formation constant model. The result indicated that the complex capacity was 0.048 mmol·g−1, 0.009 and 0.005 mmol·g−1for raw sediment, sediment without DOM, sediment without insoluble organic matters but with DOM and sediment without OM. Insoluble organic matter played a major role in the sorption of Cu in sediment and it can adsorb most Cu from water column. In the solution, Cu mainly existed as a complex with DOM and the DOM-Cu complexation capacity was 327.87 mg·g−1. The change of TOC and pH indicated ion-exchange in the interaction between free Cu and DOM. When the Cu concentration in the experiment reached the complex capacity of DOM, precipitation was the major mechanism to remove Cu from water phase, which was observed from UV absorbance change of DOM, that is, its aromaticity increased while molecular weight decreased. The desorption result indicated that DOM was more capable of desorbing Cu from adsorbents without OM than adsorbent with OM. The desorbed quantity with DOM was 1.65, 1.78 and 2.25 times higher than that with water for adsorbents without OM, raw adsorbents (sediment) and adsorbents without DOM.

Maillard reaction products from chitosan–xylan ionic liquid solution

A facile method is reported to prepare Maillard reaction products (MRPs) from chitosan and xylan in co-solvent ionic liquid. UV absorbance and fluorescence changes were regarded as indicators of the occurrence of Maillard reaction. FT-IR, NMR, XRD and TG were used to investigate the structure of chitosan–xylan conjugate. The results revealed that when chitosan reacted with xylan in ionic liquid, the hydrogen bonds in chitosan were destroyed, the facts resulted in the formation of chitosan–xylan MRPs. Moreover, when the mass ratio of chitosan to xylan was 1:1, the Maillard reaction proceeded easily. In addition, relatively high antioxidant property was also noted for the chitosan–xylan conjugate with mass ratio 1:1. So the obtained chitosan–xylan MRP is a promising antioxidant agent for food industry.

Identification of novel protein domains required for the expression of an active dehydratase fragment from a polyunsaturated fatty acid synthase

Polyunsaturated fatty acids (PUFAs) are made in some strains of deep-sea bacteria by multidomain proteins that catalyze condensation, ketoreduction, dehydration, and enoyl-reduction. In this work, we have used the Udwary-Merski Algorithm sequence analysis tool to define the boundaries that enclose the dehydratase (DH) domains in a PUFA multienzyme. Sequence analysis revealed the presence of four areas of high structure in a region that was previously thought to contain only two DH domains as defined by FabA-homology. The expression of the protein fragment containing all four protein domains resulted in an active enzyme, while shorter protein fragments were not soluble. The tetradomain fragment was capable of catalyzing the conversion of crotonyl-CoA to β-hydroxybutyryl-CoA efficiently, as shown by UV absorbance change as well as by chromatographic retention of reaction products. Sequence alignments showed that the two novel domains contain as much sequence conservation as the FabA-homology domains, suggesting that they too may play a functional role in the overall reaction. Structure predictions revealed that all domains belong to the hotdog protein family: two of them contain the active site His70 residue present in FabA-like DHs, while the remaining two do not. Replacing the active site His residues in both FabA domains for Ala abolished the activity of the tetradomain fragment, indicating that the DH activity is contained within the FabA-homology regions. Taken together, these results provide a first glimpse into a rare arrangement of DH domains which constitute a defining feature of the PUFA synthases.

Assessment of Trihalomethane Formation in Chlorinated Raw Waters with Differential UV Spectroscopy Approach

In this study, the changes in UV absorbance of water samples were characterized using defined differential UV spectroscopy (DUV), a novel spectroscopic technique. Chlorination experiments were conducted with water samples from Terkos Lake (TL) and Büyükçekmece Lake (BL) (Istanbul, Turkey). The maximum loss of UV absorbance for chlorinated TL and BL raw water samples was observed at a wavelength of 272 nm. Interestingly, differential absorbance at 272 nm (ΔUV272) was shown to be a good indicator of UV absorbing chromophores and the formation of trihalomethanes (THMs) resulting from chlorination. Furthermore, differential spectra of chlorinated TL waters were similar for given chlorination conditions, peaking at 272 nm. The correlations between THMs and ΔUV272 were quantified by linear equations with R 2 values >0.96. The concentration of THMs formed when natural organic matter is chlorinated increases with increasing time and pH levels. Among all THMs, CHCl3 was the dominant species forming as a result of the chlorination of TL and BL raw water samples. The highest chloroform (CHCl3), dichlorobromomethane (CHCl2Br), and dibromochloromethane (CHBr2Cl) concentration were released per unit loss of absorbance at 272 nm at pH 9 with a maximum reaction time of 168 hours and Cl2/dissolved organic carbon ratio of 3.2.

Sensitive and selective detection of Hg2+ and Cu2+ ions by fluorescent Ag nanoclusters synthesized via a hydrothermal method

An easily prepared fluorescent Ag nanoclusters (Ag NCs) probe for the sensitive and selective detection of Hg(2+) and Cu(2+) ions was developed here. The Ag NCs were synthesized by using polymethacrylic acid sodium salt as a template via a convenient hydrothermal process. The as-prepared fluorescent Ag NCs were monodispersed, uniform and less than 2 nm in diameter, and can be quenched in the presence of mercury (Hg(2+)) or copper (Cu(2+)) ions. Excellent linear relationships existed between the quenching degree of the Ag NCs and the concentrations of Hg(2+) or Cu(2+) ions in the range of 10 nM to 20 μM or 10 nM to 30 μM, respectively. By using ethylenediaminetetraacetate (EDTA) as the masking agent of Cu(2+), Hg(2+) was exclusively detected in coexistence with Cu(2+) with high sensitivity (LOD = 10 nM), which also provided a reusable detection method for Cu(2+). Furthermore, the different quenching phenomena caused by the two metals ions such as changes in visible colour, shifts of UV absorbance peaks and changes in size of Ag NCs make it easy to distinguish between them. Therefore the easily synthesized fluorescent Ag NCs may have great potential as Hg(2+) and Cu(2+) ions sensors.

Synthesis of dithienosilole-based highly photoluminescent donor–acceptor type compounds

Highly photoluminescent acceptor-donor-acceptor (A-D-A) and donor-acceptor (D-A) type compounds with a dithienosilole unit as the donor and perfluorotolyl or dimesitylboryl group(s) as the acceptor were prepared by the reaction of lithiated dithienosilole derivatives with perfluorotoluene or fluorodimesitylborane, respectively. The resulting A-D-A and D-A type compounds showed red-shifted UV absorption and PL bands compared to those of simple dithienosiloles having no acceptor units, reported previously, and were highly photoluminescent in the solid state as well as in solution. Solvatochromic behaviour that would arise from the intramolecular donor-acceptor interaction were observed for the D-A type compounds with respect to the UV absorption and PL spectra. In addition, it was found that bis(dimesitylboryl)dithienosilole and (dimesitylboryl)(methylthio)dithienosilole responded to coexisting fluoride anions, leading to clear UV absorption and PL spectral changes in solutions.

Functional properties of chitosan–xylose Maillard reaction products and their application to semi-dried noodle

The objective of this research was to evaluate the antimicrobial, antioxidant and darkening inhibitory activities of Maillard reaction products (MRPs) prepared from chitosan and xylose, and their effects on the preservation and quality of semi-dried noodles. The development of brown color and UV absorbance changes indicated the proceeding of Maillard reaction. The antimicrobial activity, reducing power, DPPH radical scavenging activity, copper-chelating activity and PPO inhibitory effects of chitosan–xylose conjugates increased with the reaction. Addition of MRPs could improve the textural and cooking quality of semi-dried noodles. Incorporation of 0.35% MRPs (6h) into semi-dried noodle resulted in an extension of shelf life for more than 7 days than the control noodles (at room temperature). The discoloration caused by the addition of MRPs was limited, and MRPs could inhibit darkening of the noodle effectively. Results suggested that the MRPs could be used as a novel promising preservative for semi-dried noodles.