Cross-linked succinyl chitosan as an adsorbent for the removal of Methylene Blue from aqueous solution

Removal of a basic dye (Methylene Blue) from aqueous solution was investigated using a cross-linked succinyl-chitosan (SCCS) as sorbent. The chemical structures of chitosan and its derivatives were testified by FT-IR. X-ray diffraction, DTG analysis and swelling measurements were conducted to clarify the characteristics of the chemically modified chitosan. The effect of process parameters, such as pH of the initial solution, and concentrations of dyes on the extent of Methylene Blue (MB) adsorption was investigated. The Langmuir isotherm model was used to fit the equilibrium experimental data, giving a maximum sorption capacity of 289.02 mg/g at 298 K. Kinetic studies showed that the kinetic data were well described by the pseudo-second-order kinetic model. Thermodynamic parameters such as enthalpy change (ΔH°), free energy change (ΔG°) and entropy change (ΔS°) were determined to be −25.32 kJ mol⁻¹, −6.76 kJ mol⁻¹ and −62.36 J mol⁻¹ K⁻¹, respectively, which leads to a conclusion that the adsorption process is spontaneous and exothermic.     

Potent non-hydroxamate inhibitors of histone deacetylase-8: Role and scope of an isoindolin-2-yl linker with an α-amino amide as the zinc-binding unit

A series of potent inhibitors of histone deacetylase-8 (HDAC8) is described that contains an α-amino amide zinc-binding unit and a substituted isoindolinyl capping group. The presence of a 2,4-dichlorophenyl unit located in the acetate-release cavity was shown to confer a gain of approx. 4.3 kJ mol⁻¹ in binding energy compared to a phenyl group, and the isoindoline linker has approx. 5.8 kJ mol⁻¹ greater binding energy than the corresponding tetrahydroisoquinoline ring system. In a series of 5-substituted isoindolin-2-yl inhibitors, a 5-acetylamino derivative was found to be more potent than the 5-unsubstituted lead HDAC8 inhibitor (increase in binding energy of 2.0 kJ mol⁻¹, ascribed to additional binding interactions within the N_ε-acetyl-l-lysine binding tunnel in HDAC8, including hydrogen bonding to Asp101. Tolerance of a 5-substituent (capping group) on the isoindoline ring has been demonstrated, and which in some cases confers improved enzyme inhibition, the HDAC8 substrate-binding region providing a platform for additional interactions.     

A cold-adapted endoglucanase from camel rumen with high catalytic activity at moderate and low temperatures: an anomaly of truly cold-adapted evolution in a mesophilic environment

Endoglucanases are important enzymes in plant biomass degradation. They have current and potential applications in various industrial sectors including human and animal food processing, textile, paper, and renewable biofuel production. It is assumed that the cold-active endoglucanases, with high catalytic rates in moderate and cold temperatures, can improve the cost-effectiveness of industrial processes by lowering the need for heating and, thus, energy consumption. In this study, the endoglucanase CelCM3 was procured from a camel rumen metagenome via gene cloning and expression in Escherichia coli BL21 (DE3). The maximum activity of the enzyme on carboxymethyl cellulose (CMC) was obtained at pH 5 and 30 °C with a V_max and K_m of 339 U/mg and 2.57 mg/ml, respectively. The enzyme with an estimated low melting temperature of 45 °C and about 50% activity at 4 °C was identified to be cold-adapted. A thermodynamic analysis corroborated that CelCM3 with an activation energy (E_a), enthalpy of activation (ΔH), and Gibb’s free energy (ΔG) of, respectively, 18.47 kJ mol⁻¹, 16.12 kJ mol⁻¹, and 56.09 kJ mol⁻¹ is a cold-active endoglucanase. In addition, CelCM3 was tolerant of metal ions, non-ionic detergents, urea, and organic solvents. Given these interesting characteristics, CelCM3 shows promise to meet the requirements of industrial applications.

     

Adduct formation of [(η-C7H7)Zr(η-C5H5)] with phosphines and N-heterocyclic carbenes: An experimental and theoretical study

The reaction of [(η⁷-C₇H₇)Zr(η⁵-C₅H₅)] with two Lewis bases, tetramethylimidazolin-2-ylidene and PMe₃, is reported and their stability probed via spectroscopic and theoretical methods. The strongly σ-basic N-heterocyclic carbene forms a stable adduct which has been structurally characterised, whilst the PMe₃ ligand coordinates weakly to the metal centre. Variable temperature ³¹P NMR spectroscopy has been used to determine the activation energy for this process (ΔG^‡ = 40.5 ± 1.9 kJ mol⁻¹). DFT calculations have been performed on both complexes and the structures discussed. In addition, the enthalpies for the formation of these compounds have been calculated [ΔH⁰(Zr-IMe) = −56.3 kJ mol⁻¹; ΔH⁰(Zr-PMe₃) = −2.3 kJ mol⁻¹] and show that the N-heterocyclic carbene forms a thermodynamically much more stable adduct than that with PMe₃.     

Kinetic study of dissociation of Eu(III) complex with H8dotp (H8dotp = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrakis(methylphosphonic acid))

The dissociation kinetics of the europium(III) complex with H₈dotp ligand was studied by means of molecular absorption spectroscopy in UV region at ionic strength 3.0 mol dm⁻³ (Na,H)ClO₄ and in temperature region 25-60 °C. Time-resolved laser-induced fluorescence spectroscopy (TRLIFS) was employed in order to determine the number of water molecules in the first coordination sphere of the europium(III) reaction intermediates and the final products. This technique was also utilized to deduce the composition of reaction intermediates in course of dissociation reaction simultaneously with calculation of rate constants and it demonstrates the elucidation of intimate reaction mechanism. The thermodynamic parameters for the formation of kinetic intermediate (ΔH⁰ = 11 ± 3 kJ mol⁻¹, ΔS⁰ = 41 ± 11 J K⁻¹ mol⁻¹) and the activation parameters (E_a = 69 ± 8 kJ mol⁻¹, ΔH^≠ = 67 ± 8 kJ mol⁻¹, ΔS^≠ = −83 ± 24 J K⁻¹ mol⁻¹) for the rate-determining step describing the complex dissociation were determined. The mechanism of proton-assisted reaction was proposed on the basis of the experimental data.     

Seasonal variations in photosynthetic irradiance response curves of macrophytes from a Mediterranean coastal lagoon

The main photosynthesis and respiration parameters (dark respiration rate, light saturated production rate, saturation irradiance, photosynthetic efficiency) were measured on a total of 23 macrophytes of the Thau lagoon (2 Phanerogams, 5 Chlorophyceae, 10 Rhodophyceae and 6 Phaeophyceae). Those measurements were performed in vitro under controlled conditions, close to the natural ones, and at several seasons. Concomitantly, measurements of pigment concentrations, carbon, phosphorous and nitrogen contents in tissues were performed. Seasonal intra-specific variability of photosynthetic parameters was found very high, enlightening an important acclimatation capacity. The highest photosynthetic capacities were found for Chlorophyceae (e.g. Monostroma obscurum thalli at 17 °C, 982 μmol O₂ g⁻¹ dw h⁻¹ and 9.1 μmol O₂ g⁻¹ dw h⁻¹/μmol photons m⁻² s⁻¹, respectively for light saturated net production rate and photosynthetic efficiency) and Phanerogams (e.g. Nanozostera noltii leaves at 25 °C, 583 μmol O₂ g⁻¹ dw h⁻¹ and 2.6 μmol O₂ g⁻¹ dw h⁻¹/μmol photons m⁻² s⁻¹ respectively for light saturated net production rate and photosynthetic efficiency). As expected, species with a high surface/volume ratio were found to be more productive than coarsely branched thalli and thick blades shaped species. Contrary to R_d (ranging 6.7–794 μmol O₂ g⁻¹ dw h⁻¹, respectively for Rytiphlaea tinctoria at 7 °C and for Dasya sessilis at 25 °C) for which a positive relationship with water temperature was found whatever the species studied, the evolution of P/I curves with temperature exhibited different responses amongst the species. The results allowed to show summer nitrogen limitation for some species (Gracilaria bursa-pastoris and Ulva spp.) and to propose temperature preferences based on the photosynthetic parameters for some others (N. noltii, Zostera marina, Chaetomorpha linum).     

New insights into the mechanism of dihydrodipicolinate synthase using isothermal titration calorimetry

Thermodynamic binding information, obtained via isothermal titration calorimetry (ITC), provides new insights into the binding of substrates, and of allosteric inhibitor interactions of dihydrodipicolinate synthase (DHDPS) from Escherichia coli. DHDPS catalyses the first committed step in (S)-lysine biosynthesis: the Schiff-base mediated aldol condensation of pyruvate with (S)-aspartate semi-aldehyde. Binding studies indicate that pyruvate is a weak binder (0.023 mM) but that (S)-ASA does not interact with the enzyme in the absence of a Schiff-base with pyruvate. These results support the assignment of a ping pong catalytic mechanism in which enthalpically driven Schiff-base formation (ΔH = −44.5 ± 0.1 kJ mol⁻¹) provides the thermodynamic impetus for pyruvate association. The second substrate, (S)-ASA, was observed to bind to a Schiff-base mimic (ΔH = −2.8 ± 0.1 kJ mol⁻¹) formed through the reduction of the intermediate pyruvyl–Schiff-base complex.     

Characterization of a recombinant Escherichia coli TOP10 [pQR239] whole-cell biocatalyst for stereoselective Baeyer–Villiger oxidations

This paper describes the kinetic characterization of a recombinant whole-cell biocatalyst for the stereoselective Baeyer–Villiger type oxidation of bicyclo[3.2.0]hept-2-en-6-one to its corresponding regio-isomeric lactones (−)-(1S,5R)-2-oxabicyclo[3.3.0]oct-6-en-3-one and (−)-(1R,5S)-3-oxabicyclo[3.3.0]oct-6-en-2-one. Escherichia coli TOP10 [pQR239], expressing cyclohexanone monooxygenase (CHMO) from Acinetobacter calcoaceticus (NCIMB 9871), was shown to be suitable for this biotransformation since it expressed CHMO at a high level, was simple to produce, contained no contaminating lactone hydrolase activity and allowed the intracellular recycle of NAD(P)H necessary for the biotransformation. A small-scale biotransformation reactor (20 ml) was developed to allow rapid collection of intrinsic kinetic data. In this system, the optimized whole-cell biocatalyst exhibited a significantly lower specific lactone production activity (55–60 μmol min⁻¹ g⁻¹ dry weight) than that of sonicated cells (500 μmol min⁻¹ g⁻¹ dry weight). It was shown that this shortfall was comprised of a difference in the pH optima of the two biocatalyst forms and mass transfer limitations of the reactant and/or product across the cell barrier. Both reactant and product inhibition were evident. The optimum ketone concentration was between 0.2 and 0.4 g l⁻¹ and at product concentrations above 4.5–5 g l⁻¹ the specific activity of the whole cells was zero. These results suggest that a reactant feeding strategy and in situ product removal should be considered in subsequent process design.     

Evaluation of the association of mercury(II) with some dicysteinyl tripeptides

The present study was undertaken to gain insight into the associations of mercury(II) with dicysteinyl tripeptides in buffered media at pH 7.4. We investigated the effects of increasing the distance between cysteinyl residues on mercury(II) associations and complex formations. The peptide–mercury(II) formation constants and their associated thermodynamic parameters in 3-(N-morpholino)propanesulfonic acid (MOPS) buffered solutions were evaluated by isothermal titration calorimetry. Complexes formed in different relative ratios of mercury(II) to cysteinyl peptides in ammonium formate buffered solutions were characterized by LTQ Orbitrap mass spectrometry. The results from these studies show that n-alkyl dicysteinyl peptides (CP 1–4), and an aryl dicysteinyl peptide (CP 5) can serve as effective “double anchors” to accommodate the coordination sites of mercury(II) to form predominantly one-to-one Hg(peptide) complexes. The aryl dicysteinyl peptide (CP 5) also forms the two-to-two Hg₂(peptide)₂ complex. In the presence of excess peptide, Hg(peptide)₂ complexes are also detected. Notably, increasing the distance between the ligating groups or “anchor points” in CP 1–5 does not significantly affect their affinity for mercury(II). However, the enthalpy change (ΔH) values (ΔH₁ ∼ −91 kJ mol⁻¹ and ΔH₂ ∼ −66 kJ mol⁻¹) for complex formation between CP 4 and 5 with mercury(II) are about one and a half times larger than the related values for CP 1, 2 and 3 (ΔH₁ ∼ −66 kJ mol⁻¹ and ΔH₂ ∼ 46 kJ mol⁻¹). The corresponding entropy change (ΔS) values (ΔS₁ ∼ −129 J K⁻¹ mol⁻¹ and ΔS₂ ∼ −116 J K⁻¹ mol⁻¹) of the structurally larger dicysteinyl peptides CP 4 and 5 are less entropically favorable than for CP 1, 2 and 3 (ΔS₁ ∼ −48 J K⁻¹ mol⁻¹ and ΔS₂ ∼ −44 J K⁻¹ mol⁻¹). Generally, these associations result in a decrease in entropy, indicating that these peptide–mercury complexes potentially form highly ordered structures. The results from this study show that dicysteinyl tripeptides are effective in binding mercury(II) and they are promising motifs for the design of multi-cysteinyl peptides for binding more than one mercury(II) ion per peptide.     

Structural studies of trans-N2S2 copper macrocycles

The X-ray crystal structures of two related trans-N₂S₂ copper macrocycles are reported. One was isolated with the copper in the divalent form and the other with copper in its univalent form affording a valuable insight into the changes of geometry and metrical parameters that occur during redox processes in macrocyclic copper complexes. A variable temperature NMR study of the copper(I) complex is reported, indicative of a chair-boat conformational change within the alkyl chain backbone of the macrocycle. It was possible to extract the relevant kinetic and thermodynamic parameters (ΔG^‡, 57.8 kJ mol⁻¹; ΔH^‡, 52.1 kJ mol⁻¹; ΔS^‡, −19.2 J K⁻¹ mol⁻¹) for this process at 298 K. DFT molecular orbital calculations were used to confirm these observations and to calculate the energy difference (26.2 kJmol⁻¹) between the copper(I) macrocycle in a planar and a distorted tetrahedral disposition.     

Kinetics study of the substitution reaction of fac-[Fe(CN)2(CO)3I] with PPh3

Substitution reaction of fac-[Fe^II(CN)₂(CO)₃I]⁻ with triphenylphosphine (PPh₃) produced mono phosphine substituted complex cis-cis-[Fe^II(CN)₂(CO)₂(PPh₃)I]⁻. Crystal structure of the product showed that carbonyl positioned trans- to iodide was replaced by PPh₃. The substitution reaction was monitored by quantitative infrared spectroscopic method, and the rate law for the substitution reaction was determined to be rate = k[[Fe^II(CN)₂(CO)₂(PPh₃)I]⁻][PPh₃]. Transition state enthalpy and entropy changes were obtained from Eyring equation k = (k_BT/h)exp(−ΔH^≠/RT + ΔS^≠/R) with ΔH^≠ = 119(4) kJ mol⁻¹ and ΔS^≠ = 102(10) J mol⁻¹ K⁻¹. Positive transition state entropy change suggests that the substitution reaction went through a dissociative pathway.     

Effects of flooding on photosynthesis and root respiration in saltcedar (Tamarix ramosissima), an invasive riparian shrub

The introduced shrub Tamarix ramosissima invades riparian zones, but loses competitiveness under flooding. Metabolic effects of flooding could be important for T. ramosissima, but have not been previously investigated. Photosynthesis rates, stomatal conductance, internal (intercellular) CO₂, transpiration, and root alcohol dehydrogenase (ADH) activity were compared in T. ramosissima across soil types and under drained and flooded conditions in a greenhouse. Photosynthesis at 1500 μmol quanta m⁻² s⁻¹ (A₁₅₀₀) in flooded plants ranged from 2.3 to 6.2 μmol CO₂ m⁻² s⁻¹ during the first week, but A₁₅₀₀ increased to 6.4–12.7 μmol CO₂ m⁻² s⁻¹ by the third week of flooding. Stomatal conductance (g_s) at 1500 μmol quanta m⁻² s⁻¹ also decreased initially during flooding, where g_s was 0.018 to 0.099 mol H₂O m⁻² s⁻¹ during the first week, but g_s increased to 0.113–0.248 mol H₂O m⁻² s⁻¹ by the third week of flooding. However, photosynthesis in flooded plants was reduced by non-stomatal limitations, and subsequent increases indicate metabolic acclimation to flooding. Root ADH activities were higher in flooded plants compared to drained plants, indicating oxygen stress. Lower photosynthesis and greater oxygen stress could account for the susceptibility of T. ramosissima at the onset of flooding. Soil type had no effect on photosynthesis or on root ADH activity. In the field, stomatal conductance, leaf water potential, transpiration, and leaf δ¹³C were compared between T. ramosissima and other flooded species. T. ramosissima had lower stomatal conductance and water potential compared to Populus deltoides and Phragmites australis. Differences in physiological responses for T. ramosissima could become important for ecological concerns.     

Pilot-scale production of 1,3-propanediol using Klebsiella pneumoniae

The conversion of glycerol to 1,3-propanediol (PDO) using Klebsiella pneumoniae M5al under anaerobic condition was scaled up from scale 5 to 5000 l in series. A simple strategy for scale-up was to transfer the optimized conditions of a lab scale bioreactor to pilot-scale fermentation. Multistage inocula were developed and their fermentation abilities were assessed in a small-scale fermenter. The experimental results showed that inoculum development in the early steps of a scale-up process could influence the outcomes of a large scale fermentation. Through three-stage liquid inoculum development and a pulse addition of (NH₄)₂SO₄ and yeast extract at 30 h of fermentation, the best results in a 5000 l fermentation were achieved leading to 58.8 g l⁻¹ 1,3-propanediol with a yield of 0.53 mol mol⁻¹ glycerol and productivity of 0.92 g l⁻¹ h⁻¹. This is the first report on pilot-scale 1,3-propanediol production using K. pneumoniae.     

Exchange rates of pyridine on ferro- and ferriprotoporphyrin (IX) dimethylester in chloroform.

Nuclear magnetic resonance line-widths data have been used to determine the rate of solvent exchange from the first coordination sphere of ferro-and ferriprotoporphyrin(IX) dimethylester (Fe-PPD) in pyridine/chloroform. The average values of kinetic parameters for pyridine (PY) exchange indicate an SN2 mechanism tor Fe(III)-PPD(ΔH^&;#; = 36 kJ · mol⁻¹ ; ΔS^&;#; = −53 J·mol⁻¹K⁻¹; T_M(298 K) = 0.07 msec) and an SNI mechanism for Fe(II)-PPD (ΔH^&;#; = 67 kJ·mol⁻¹; ΔS^&;#; = 42 J · mol⁻¹K⁻¹; T_M(298 K) = 0.06 msec). Parallel to the accelerated ligand exchange rate at rising temperatures a redistribution of the electrons causing a transition of the metal porphyrin from the low-spin state to the high-spin state is observed. Enthalpy and entropy of the thermodynamic equilibrium between low- and high-spin Fe-PPD have been determined from experimental values of the average magnetic moment. A mean lifetime of low-spin Fe(III)-PPD was estimated from line. widths changes (T_L→H(298 K)≈ 20 msec) and the corresponding activation parameters have been obtained (ΔH^&;#;_L→H(298 K) = 26 kJ · mol⁻¹; ΔS^&;#;_L→H(298K) = −125 J · mol⁻¹K⁻¹).     

Biosorption of food dyes onto Spirulina platensis nanoparticles: equilibrium isotherm and thermodynamic analysis

The biosorption of food dyes FD&C red no. 40 and acid blue 9 onto Spirulina platensis nanoparticles was studied at different conditions of pH and temperature. Four isotherm models were used to evaluate the biosorption equilibrium and the thermodynamic parameters were estimated. Infra red analysis (FT-IR) and energy dispersive X-ray spectroscopy (EDS) were used to verify the biosorption behavior. The maximum biosorption capacities of FD&C red no. 40 and acid blue 9 were found at pH 4 and 298 K, and the values were 468.7 mg g⁻¹ and 1619.4 mg g⁻¹, respectively. The Sips model was more adequate to fit the equilibrium experimental data (R² > 0.99 and ARE < 5%). Thermodynamic study showed that the biosorption was exothermic, spontaneous and favorable. FT-IR and EDS analysis suggested that at pH 4 and 298 K, the biosorption of both dyes onto nanoparticles occurred by chemisorption.     

Pyruvate producing biocatalyst with constitutive NAD-independent lactate dehydrogenases

Production of pyruvate from lactate through biocatalysis is a valuable process for its simple composition of reaction system and convenience of recovery. Biocatalyst with lactate-induced NAD-independent lactate dehydrogenases (iLDHs) can effectively catalyze lactate into pyruvate. To reduce the cost of biocatalyst preparation caused by indispensable lactate addition, the mutants with constitutive iLDH of Pseudomonas sp. XP-M2 were screened. Mutant XP-LM exhibited high iLDHs activities in minimal salt medium with cheap substrate glucose as the carbon source. The biocatalyst (8.2 g dry cell weight l⁻¹) containing 169.8 U l⁻¹ l-iLDH was prepared with 20 g 1⁻¹ glucose. The cost-effective biocatalyst prepared from the mutant XP-LM could efficiently catalyze lactate into pyruvate with high yield (0.961 mol mol⁻¹). Based on the different thermostability of d-iLDH and l-iLDH in the biocatalyst, whole cells of the strain might also have the potential in production of pyruvate and d-lactate from racemic lactate.     

The effects of NH4+ and NO3− on growth,resource allocation and nitrogen uptake kinetics of Phragmites australis and Glyceria maxima

The effects of NH₄⁺ or NO₃⁻ on growth, resource allocation and nitrogen (N) uptake kinetics of two common helophytes Phragmites australis (Cav.) Trin. ex Steudel and Glyceria maxima (Hartm.) Holmb. were studied in semi steady-state hydroponic cultures. At a steady-state nitrogen availability of 34 μM the growth rate of Phragmites was not affected by the N form (mean RGR = 35.4 mg g⁻¹ d⁻¹), whereas the growth rate of Glyceria was 16% higher in NH₄⁺-N cultures than in NO₃⁻-N cultures (mean = 66.7 and 57.4 mg g⁻¹ d⁻¹ of NH₄⁺ and NO₃⁻ treated plants, respectively). Phragmites and Glyceria had higher S/R ratio in NH₄⁺ cultures than in NO₃⁻ cultures, 123.5 and 129.7%, respectively.Species differed in the nitrogen utilisation. In Glyceria, the relative tissue N content was higher than in Phragmites and was increased in NH₄⁺ treated plants by 16%. The tissue NH₄⁺ concentration (mean = 1.6 μmol g fresh wt⁻¹) was not affected by N treatment, whereas NO₃⁻ contents were higher in NO₃⁻ (mean = 1.5 μmol g fresh wt⁻¹) than in NH₄⁺ (mean = 0.4 μmol g fresh wt⁻¹) treated plants. In Phragmites, NH₄⁺ (mean = 1.6 μmol g fresh wt⁻¹) and NO₃⁻ (mean = 0.2 μmol g fresh wt⁻¹) contents were not affected by the N regime. Species did not differ in NH₄⁺ (mean = 56.5 μmol g⁻¹ root dry wt h⁻¹) and NO₃⁻ (mean = 34.5 μmol g⁻¹ root dry wt h⁻¹) maximum uptake rates (V_max), and V_max for NH₄⁺ uptake was not affected by N treatment. The uptake rate of NO₃⁻ was low in NH₄⁺ treated plants, and an induction phase for NO₃⁻ was observed in NH₄⁺ treated Phragmites but not in Glyceria. Phragmites had low K_m (mean = 4.5 μM) and high affinity (10.3 l g⁻¹ root dry wt h⁻¹) for both ions compared to Glyceria (K_m = 6.3 μM, affinity = 8.0 l g⁻¹ root dry wt h⁻¹). The results showed different plasticity of Phragmites and Glyceria toward N source. The positive response to NH₄⁺-N source may participates in the observed success of Glyceria at NH₄⁺ rich sites, although other factors have to be considered. Higher plasticity of Phragmites toward low nutrient availability may favour this species at oligotrophic sites.     

Accumulation of phycocyanin in heterotrophic and mixotrophic cultures of the acidophilic red alga Galdieria sulphuraria

The relationship between light intensity, nitrogen availability and pigmentation was investigated in mixotrophic and heterotrophic cultures of the unicellular red alga Galdieria sulphuraria 074G, a potential host for production of the blue pigment, phycocyanin (PC). During the exponential growth phase of batch cultures, G. sulphuraria 074G contained 2–4 mg phycocyanin per g dry weight. In carbon-limited and nitrogen-sufficient batch cultures grown in darkness, this value increased to 8–12 mg g⁻¹ dry weight during the stationary phase, whereas the phycocyanin content in nitrogen-deficient cells decreased to values below 1 mg g⁻¹ dry weight during stationary phase. Light intensities between 0 and 100 μmol photons m⁻² s⁻¹ had no influence on phycocyanin accumulation in mixotrophic cultures grown on glucose or fructose, while light stimulated phycocyanin synthesis in cultures grown on glycerol, in which the phycocyanin content in stationary phase was increased from 10 mg g⁻¹ dry weight in darkness to 20 mg g⁻¹ dry weight at a light intensity of 80 μmol photons m⁻² s⁻¹. At higher light intensities, less phycocyanin accumulated than at lower intensities, irrespective of the carbon substrate used. In carbon-limited continuous flow cultures grown on glucose or glycerol at a dilution rate of 0.63 day⁻¹, corresponding to 50% of the maximum specific growth rate, the highest steady-state phycocyanin content of 15–28 mg g⁻¹ dry weight was found at 65 μmol photons m⁻² s⁻¹. In contrast to the apparent glucose repression of light-induced PC synthesis observed in batch cultures, no glucose repression of the light stimulation was observed in continuous flow cultures because the glucose concentration in the culture supernatant always remained at limiting levels. Despite the fact that G. sulphuraria 074G contains less phycocyanin than some other microalgae and cyanobacteria, the ability of G. sulphuraria 074G to grow and synthesize phycocyanin in heterotrophic or mixotrophic cultures makes it an interesting alternative to the cyanobacterium, Spirulina platensis presently used for synthesis of phycocyanin.     

Lipase-catalyzed interfacial polymerization of ω-pentadecalactone in aqueous biphasic medium: A mechanistic study

The synthetic activity of lipases in biphasic o/w systems was investigated with respect to their use in the synthesis of polyester chains via transesterification reactions. Lipase-catalyzed ring-opening polymerization (ROP) of pentadecalactone (ω-PDL) dispersed in water was used as a model reaction to understand the synthetic activity of lipases in biphasic o/w system. We conducted a systematic investigation of the influence of reaction conditions on the macromolecular characteristics of oligo(ω-PDL) encompassing chemical, thermophysical and colloidal properties of the reaction medium. A model was proposed assuming Michaelis–Menten interfacial kinetics followed by chain extension via lipase-catalyzed linear polycondensation. The solidification of oligo(ω-PDL) chains with a degree of polymerization of approximately three was identified as a major factor limiting the molecular weight of obtained oligomers to ∼870 g mol⁻¹, despite the fast reaction rate and complete conversion of ω-PDL. The addition of toluene into the dispersed phase at a volumetric ratio of 0.3–0.5 of toluene to ω-PDL allowed us to circumvent this problem and increase the molecular weight of obtained oligomers up to 1460 g mol⁻¹. The molecular weight of polymer product according to this model was thus inversely related to the weight ratio percentage of interfacial lipase PS to ω-PDL per droplet and correspondingly correlated with the activity of lipase. Taking into account all these parameters allowed increasing the molar mass of oligo(ω-PDL) from 870 g mol⁻¹ to 3507 g mol⁻¹.     

Supramolecular interaction of 6-shogaol,a therapeutic agent of Zingiber officinale with human serum albumin as elucidated by spectroscopic,calorimetric and molecular docking methods

Background6-Shogaol, one of the main bioactive constituents of Zingiber officinale has been shown to possess various therapeutic properties. Interaction of a therapeutic compound with plasma proteins greatly affects its pharmacokinetic and pharmacodynamic properties.PurposeThe present investigation was undertaken to characterize the interaction between 6-shogaol and the main in vivo transporter, human serum albumin (HSA).MethodsVarious binding characteristics of 6-shogaol–HSA interaction were studied using fluorescence spectroscopy. Thermal stability of 6-shogaol–HSA system was determined by circular dichroism (CD) and differential scanning calorimetric (DSC) techniques. Identification of the 6-shogaol binding site on HSA was made by competitive drug displacement and molecular docking experiments.ResultsFluorescence quench titration results revealed the association constant, K_a of 6-shogaol–HSA interaction as 6.29 ± 0.33 × 10⁴ M⁻¹ at 25 ºC. Values of the enthalpy change (−11.76 kJ mol⁻¹) and the entropy change (52.52 J mol⁻¹ K⁻¹), obtained for the binding reaction suggested involvement of hydrophobic and van der Waals forces along with hydrogen bonds in the complex formation. Higher thermal stability of HSA was noticed in the presence of 6-shogaol, as revealed by DSC and thermal denaturation profiles. Competitive ligand displacement experiments along with molecular docking results suggested the binding preference of 6-shogaol for Sudlow's site I of HSA.ConclusionAll these results suggest that 6-shogaol binds to Sudlow's site I of HSA through moderate binding affinity and involves hydrophobic and van der Waals forces along with hydrogen bonds.