Stoichiometrically Research Articles

Self-Assembly of Phycoerythrin with Oligochitosan by Electrostatic Interaction for Stabilization of Phycoerythrin.

Phycoerythrin (PE) is a natural water-soluble pigment protein with characteristic phycobilins and is sensitive to thermal and light environmental changes. In this study, PE was extracted from Porphyra haitanensis and PE-oligochitosan complexes (POC) were fabricated by a self-assembly approach. The effects of cationic oligochitosan on the binding interaction, structure, size distribution, and color stability of PE were evaluated. The stoichiometric number n was calculated to be 21.67 ± 2.65 (oligochitosan/PE) and the binding constant K was (6.47 ± 0.48) × 105 M-1. Cationic oligochitosan could electrostatically interact with PE and affect the PE structure by increasing the α-helix content. In addition, high concentrations of oligochitosan led to the formation of dense phycoerythrin protein granules. Moreover, at a reaction ratio of 20.0:1 (oligochitosan/PE), being approximately the predicted stoichiometric number n, the thermal stability (40-80 °C), natural light stability, and ultraviolet light irradiation (254 nm) stability of the POC were improved. This study provides an approach to reduce the susceptibility of PE upon environmental changes by forming a stable self-assembly complex, which will promote the application of PE as a natural pigment protein in food and chemical applications.

An experimental study of sepiolite dissolution and growth rates as function of the aqueous solution saturation state at 60 °C

Clay mineral growth can directly affect the chemistry and permeability of many natural systems, including soils, marine sediments, Earth surface waters, geothermal powerplants and carbon dioxide storage sites. Notably, the sluggish precipitation of clay minerals has been hypothesised to hinder Earth surface weathering rates. To date, there are limited data on the rate of clay mineral growth and on the effect of the aqueous solution saturation states on these rates. In this study we quantify the growth and dissolution rates of sepiolite, a 2:1 layered Mg-phyllosilicate (Mg4Si6O15(OH)2 * 6H2O) as a function of aqueous solution saturation state in a series of mixed flow experiments. Results of the both the dissolution and growth experiments are consistent with r=-10-14.801-expΔGrσRT, where r refers to the surface area normalized growth rate of sepiolite in units of mol/cm2/s, ΔGr denotes Gibbs Free energy of the sepiolite dissolution reaction, which is <0 for undersaturated solutions, 0 at equilibrium and >0 for supersaturated solutions, σ refers to Temkin’s stoichiometric number, R stands for the gas constant and T symbolizes absolute temperature. This rate equation suggests that sepiolite dissolution and growth are consistent with transition state theory and the concept of micro-reversibility. The relative decrease in aqueous Mg2+ and Si concentrations in the outlet aqueous solutions of the experiments, and the X-Ray diffraction patterns of the precipitates collected from the experiments, confirm the growth of crystalline sepiolite. The results of longer-term experiments suggest that sepiolite growth rates decrease over time. Such a decrease in the growth rate has been associated with poisoning or destruction of the sepiolite reactive surface sites. Calculations of sepiolite growth coupled to either forsterite or enstatite dissolution, based on laboratory measured rates suggest that the dissolution of the primary mineral is the rate-limiting step during the weathering of most natural systems. These results, however, contradict the saturation states of natural fluids such as observed in Icelandic waters, where clays are strongly supersaturated, whilst primary phases are relatively close to equilibrium. This discrepancy may be due to the consumption of reactive sites over time such that clay mineral precipitation rates in most natural systems are controlled by the relative slow nucleation rates of new reactive sites on the clay mineral surface.

Derivative‐Free Optimization of a Fixed‐Bed Methanol Synthesis Reactor

AbstractA one‐dimensional heterogeneous fixed‐bed methanol synthesis reactor model was developed for the optimization of carbon conversion and rate of methanol production. The developed model was validated both at pellet and reactor scales. The reactor inlet syngas composition and cooling water temperature, i.e., the design variables, providing the highest carbon conversion and rate of methanol production were determined by a constrained derivative‐free optimization algorithm. The results revealed that after optimization the carbon conversion was increased by 12 % and the rate of methanol production by 48 %. Under the optimized conditions, the stoichiometric number was in the range of industrial values.

Methanol production via integrated methane reforming and chemical looping combustion: Process simulation and techno-economic assessment

In this research, a novel technology is proposed that combines chemical looping combustion and methane reforming in order to convert CO2 to value added chemicals. The stoichiometric number is improved via adding assisted steam and utilizing pressure swing adsorption unit. The process simulation is conducted utilizing Aspen plus-V10 simulation software. The new built-in model featured by AspenTech, dubbed Fluidbed, in Aspen Plus is employed to simulate the CLC section. In Aspen Plus flowsheet environment, a hierarchal approach is considered in that the main flowsheet is divided into 5 hierarchy models or sub flowsheets. The process simulation results display that the overall conversions of CH4 and CO2 via this process are 97 % and 87 %, respectively. The heat integration results depict that 138.7 MW heat energy can be saved in the process. The CO2 emission of the whole process per methanol production is 0.1 tCO2/tMe. The estimated capital expenditure, and operating expenditure are US$ 509 million and US$ 159 million/yr, respectively. A discounted cash flow model combined with sensitivity analyses showed that a breakeven point could be reached with a methanol price of US$ 242/tonne. Net present value, internal rate of return, and pay out time are evaluated based on DCF results.

Design of ECU (Electronic Control Unit) as Biogas Pressure Control to Micro Genset

Biogas fueled generator has a unique character because the converter kit on the generator has the characteristic of absorbing the biogas supply. Therefore, in this study, biogas is accommodated in a plastic container equipped with a valve that can adjust the amount of biogas supply pressure. Combustion in the Genset combustion chamber will occur if the ratio of air supply and supply of biogas (AFR = Air to Fuel Ratio) meets the stoichiometric number. So a ratio control is needed on air supply and biogas supply, which can follow changes in biogas conditions in plastic bags and generator loads. For this reason, this paper presents an ECU (Electronic Control Unit) that can regulate the ratio of air supply to biogas supply by using a servo motor valve, which is commanded by a control signal from the ECU. The resulting ECU prototype is equipped with a multi-input connector (to receive the signal output of CH 4 methane gas sensor and the biogas supply pressure sensor), multi-output (control signal to the servo motor valve), and multi regulators to adjust the Set Point value for the methane gas content CH4, the value of the set point of the biogas supply pressure and to tune the value of the proportional gain control (KP). The results of the ECU performance test by trial and error by giving a dummy input signal (analogy to the signal output of methane gas sensor and the analog of the pressure sensor output signal in the form of an electric voltage generated from the signal generator) can drive the servo motor, valve air supply, and servo motor, valve supply biogas. Accordance with the gain control input from the regulator.

Process design and simulation study: CO2 utilization through mixed reforming of methane for methanol synthesis

A case study is performed using Aspen HYSYS to investigate CO2 utilization for methanol synthesis. The base case is a conventional steam methane reforming process for methanol production. Two modified cases (mixed reforming with/without CO2 capture after the reformer) are developed with the aim to increase CO2 utilization. Simulation results revealed that both stoichiometric number and CO/CO2 ratio played an important role on syngas reactivity, which led to an overall increase in methanol yield. As a result, the volume of unreacted gas in the synthesis loop was reduced and allowed for a smaller reactor design. In terms of overall CO2 intensity, no significant improvements can be observed since the base case had a large volume of H2 rich purged gas sent to the reformer furnace. When the purged gas was not used as a reformer fuel, the CO2 intensity of the other cases became 50% less than the base case.

Conceptual Design Development of Coal-to-Methanol Process with Carbon Capture and Utilization

Methanol is a clean fuel and an important feedstock for the petrochemical industry. Conventionally, the coal-to-methanol process generates a substantial amount of CO2 emissions with a low yield of methanol. In this study, we propose the conceptual design development of coal-to-methanol process using captured CO2 from the gasification plant by implying process intensification. The base case and three alternative designs have been developed using the Aspen Plus to analyze the process performance. The four designs have been compared in terms of their energy consumption, economics, methanol production rate, and carbon emissions while maintaining the gasifier operation conditions, sulfur content in the syngas, and stoichiometric number at the methanol synthesis reactor. Among the developed designs, the most feasible design requires an energy requirement and product cost of 21.9 GJ and $142.5 per ton of methanol product. The study also showed that the coal-to-methanol process assisted with external hydrogen from electrolysis plant for achieving higher production rates and low CO2 emissions is currently not economical due to the high H2 cost.

Methanol Production from Pyrolysis Oil Gasification—Model Development and Impacts of Operating Conditions

A novel process model simulating methanol production through pyrolysis oil gasification was developed, validated, then used to predict the effect of operating conditions on methanol production yield. The model comprised gasification, syngas post-treatment, and methanol synthesis units. The model was validated using experimental data from the literature, and the results obtained by the model were consistent with reference data. The simulation results revealed that gasification temperature has a significant impact on syngas composition. Indeed, rising temperature from 400 °C to 600 °C leads to higher syngas stoichiometric number (SN) value. Conversely, SN value decreases when the gasifier temperature is above 1000 °C. Moisture content in pyrolysis oil also affects both syngas composition and SN value; an increase in the first (from 10 to 30%) leads to an increase in SN value. The Rectisol unit deeply influences the syngas SN value and methanol yield, the best results being obtained with operating conditions of −20 °C and 40 bar. Increasing the operating temperature of the methanol synthesis unit from 150 °C to 250 °C leads to an increase in the yield of methanol production; the yield decreases beyond 250 °C. Although high pressures favor the methanol production yield, the operating pressure in the synthesis unit is limited at 50 bar for practical considerations (e.g., equipment price, equipment requirements, or operational risks).

Evaluation of CO2 utilization for methanol production via tri-reforming of methane

Abstract This work evaluates the performance of a plant to produce 2000 tonnes per day of methanol using CH4 tri-reforming for syngas production. A sensitivity analysis is performed on the feed composition of a tri-reformer operating at 3000 kPa and 1000 °C to maximize its CO2 conversion and syngas stoichiometric number (SN). The optimum feed composition is determined as CO2: 0.20, H2O: 0.35, and O2: 0.48 for each mole of CH4, resulting in a CO2 conversion of 50% and an SN of 1.57. Oxygen is injected at four locations along the length of the tri-reformer to manage the temperature profile of the reactor. A pressure swing adsorption unit with four columns and eight steps recovers hydrogen from unreacted gases to increase the syngas SN to 2.04, which is suitable for methanol synthesis. The net present value of the plant is $161 million for a 15-year economic life based on a methanol sale price of $390 per tonne. Monte-Carlo analysis of the plant economics indicates that there is an 84% chance for the plant to be viable considering the simultaneous uncertainties in influential economic parameters. The plant has net CO2 emissions of 0.91 kg CO2/kg methanol, which is 50% less than the conventional methanol plants based on the steam reforming of methane and 35% lower than other methanol plants based on CH4 tri-reforming.

Effect of submerged combustion evaporation on Cd complexation potential of organic matter in municipal solid waste landfill leachate

Despite the strong ability for complexation of heavy metals, a high amount of humic substances (HS) is always contained in membrane retentate from municipal solid waste landfill leachates treatment processes. Submerged Combustion Evaporation (SCE) can be used to effectively concentrate the membrane retentate. However, the impact of the SCE treatment on HS complexation capacity is still unclear. Since cadmium (Cd) plays a vital role in the pollution of soil and food, this study investigated the influence of the SCE treatment on the Cd complexation potential of leachate HS. Specific effects and complexation properties on Cd complexation of leachate HS before and after the SCE treatment were demonstrated using the Non-Ideal Competitive Adsorption model. The results showed that the Cd complexation capacity of carboxyl sites increased from 3.76 to 4.65 mol/kg-Total Organic Carbon (TOC) after the SCE treatment, which agreed with the stoichiometric number of proton binding sites. Moreover, characterization results indicated that SCE increased the affinity of HS for Cd by enhancing the overall aromaticity of HS (E2:E4 from 9.8 to 9.3), dominantly due to the humic acid contribution. By modeling the practical Cd remediation scenarios, the enhanced Cd complexation performance of HS after SCE treatment was observed even at low pH values (pH = 5) or insufficient TOC content (TOC = 50 mg/L).

Thermodynamically consistent forward and reverse degrees of rate control in reversible reactions

The net rate of a composite reaction is the difference between the forward and reverse reaction rates, which are kinetically distinct despite sharing elementary reaction steps and therefore have different rate-controlling transition states and species. Thus, degrees of rate control defined to identify rate-controlling transition states and species for the net rate confound contributions from the forward and reverse reactions. Herein, the forward and reverse degrees of rate control are defined to independently quantify the extent that species and transition states control the forward and reverse rates in reversible reactions. These degrees of rate control are defined as the relative change in the forward and reverse reaction rates per kBT decrease in the standard-state molecular free energies of transition states and species, and they are related to experimentally measurable quantities such as steady-state fractional coverages, reaction orders, and energies and entropies of activation of the forward and reverse reaction rates. The forward/reverse degrees of rate control represent stoichiometric coefficients for species and transition states in an equilibrium between the initial states and transition states of the apparent rate-controlling steps of forward and reverse overall reactions. At equilibrium, the apparent transition states for the forward reaction and reverse reaction converge, and thus the forward and reverse rate-controlling steps combine to form a single apparent rate-controlling step. This apparent rate-controlling step is comprised of an apparent initial state, transition state, and final state, where the apparent final state of the forward reaction is the apparent initial state of the reverse reaction. The apparent rate-controlling step behaves identically to an elementary step reaction at equilibrium with a pseudo-mass-action rate function given by the transition-state-theory (TST) form rate function (Foley and Bhan, 2020) with a stoichiometric number equal to the affinity-averaged stoichiometric number, σ¯. The ratio of the forward and reverse TST-form rate functions is identical to the overall thermodynamic equilibrium relation, consistent with the principles of microscopic reversibility and detailed balance at equilibrium.

Kinetic modelling of methanol synthesis over commercial catalysts: A critical assessment

Kinetic modelling of methanol synthesis over commercial catalysts is of high importance for reactor and process design. Literature kinetic models were implemented and systematically discussed against a newly developed kinetic model based on published kinetic data. Deviations in the sensitivities of the kinetic models were explained by means of the experimentally covered parameter range. The simulation results proved that an extrapolation of the working range of the kinetic models can lead towards significant simulation errors especially with regard to pressure, stoichiometric number and CO/CO2-ratio considerably limiting the applicability of kinetic models frequently applied in scientific literature. Therefore, the validated data range for kinetic models should be considered when detailed reactor simulations are carried out. With regard to Power-to-Methanol processes special attention should be drawn towards the rate limiting effect of water at high CO2 contents in the syngas. Moreover, it was shown that kinetic models based on data measured over outdated catalysts show significantly lower activity than those derived from state-of-the-art catalysts and should therefore be applied with caution for reactor and process simulations. The plausible behavior of the herein proposed kinetic model was demonstrated by a systematic comparison towards established kinetic approaches within both, an ideal kinetic reactor and an industrial steam cooled tubular reactor. Relative to the state-of-the-art kinetic models it was proven that the herein proposed kinetic model can be applied over the complete industrially relevant working range for methanol synthesis.

Development of a CT number calibration audit phantom in photon radiation therapy: A pilot study.

PurposeIn photon radiation therapy, computed tomography (CT) numbers are converted into values for mass density (MD) or relative electron density to water (RED). CT‐MD or CT‐RED calibration tables are relevant for human body dose calculation in an inhomogeneous medium. CT‐MD or CT‐RED calibration tables are influenced by patient imaging (CT scanner manufacturer, scanning parameters, and patient size), the calibration process (tissue‐equivalent phantom manufacturer, and selection of tissue‐equivalent material), differences between tissue‐equivalent materials and standard tissues, and the dose calculation algorithm applied; however, a CT number calibration audit has not been established. The purposes of this study were to develop a postal audit phantom, and to establish a CT number calibration audit process.MethodsA conventional stoichiometric calibration conducts a least square fit of the relationships between the MD, material weight, and measured CT number, using two parameters. In this study, a new stoichiometric CT number calibration scheme has been empirically established, using three parameters to harmonize the calculated CT number with the measured CT number for air and lung tissue. In addition, the suitable material set and the minimal number of materials required for stoichiometric CT number calibration were determined. The MDs and elemental weights from the International Commission on Radiological Protection Publication 110 were used as standard tissue data, to generate the CT‐MD and CT‐RED calibration tables. A small‐sized, CT number calibration phantom was developed for a postal audit, and stoichiometric CT number calibration with the phantom was compared to the CT number calibration tables registered in the radiotherapy treatment planning systems (RTPSs) associated with five radiotherapy institutions.ResultsWhen a least square fit was performed for the stoichiometric CT number calibration with the three parameters, the calculated CT number showed better agreement with the measured CT number. We established stoichiometric CT number calibration using only two materials because the accuracy of the process was determined not by the number of used materials but by the number of elements contained. The stoichiometric CT number calibration was comparable to the tissue‐substitute calibration, with a dose difference less than 1%. An outline of the CT number calibration audit was demonstrated through a multi‐institutional study.ConclusionsWe established a new stoichiometric CT number calibration method for validating the CT number calibration tables registered in RTPSs. We also developed a CT number calibration phantom for a postal audit, which was verified by the performances of multiple CT scanners located at several institutions. The new stoichiometric CT number calibration has the advantages of being performed using only two materials, and decreasing the difference between the calculated and measured CT numbers for air and lung tissue. In the future, a postal CT number calibration audit might be achievable using a smaller phantom.

Nanocellulose‐Mediated Transition of Lithium‐Rich Pseudo‐Quaternary Metal Oxide Nanoparticles into Lithium Nickel Cobalt Manganese Oxide (NCM) Nanostructures

AbstractWe report the syntheses of various compounds within the pseudo‐quaternary system of the type LiwNixCoyMnzOδ (δ≤1) (pre‐NCMs). Four different compositions of this compound were realized as ultrasmall crystalline nanoparticles of 1–4 nm diameter using low‐temperature solvothermal reaction conditions in tert‐butanol at only 170 °C. All of the pre‐NCMs crystallize in the rock‐salt structure and their lithium content is between 20% and 30% with respect to the complete metal content. By adjusting the lithium content to 105% stoichiometry in the solvothermal reaction, the pre‐NCMs can easily react to the respective Li(NixCoyMnz)O2 (NCM) nanoparticles. Furthermore, nanosized desert‐rose structured NCMs were obtained after addition of nanocellulose during the synthesis. By using the mixed metal monoxides as precursor for the NCMs, cation mixing between lithium and nickel is favored and gets more pronounced with increasing nickel content. The cation mixing effect compromises good electrochemical capacity retention, but the desert‐rose structure nevertheless enables enhanced stability at high power conditions, especially for NCM333.

Methanol production from water electrolysis and tri-reforming: Process design and technical-economic analysis

CO2 utilization via methanol synthesis can be an effective approach to mitigate the issue of global warming. This study developed an innovative process to produce methanol by combining water electrolysis with tri-reforming of methane (TRM). The proposed design utilized carbon-free electricity to split water into O2 and H2; O2 is collected for partial oxidation reaction in the TRM and H2 is collected for stoichiometric number (SN) optimization. This process configuration eliminates the typical problems of H2 surplus or deficit associated with methanol synthesis and allows a substantial amount of CO2 to be converted. The main process flowsheet was developed with Aspen HYSYS process simulator and then the feasibility of this project was evaluated based on its technical, environmental, and economic performances. The estimated capital expenditure (CAPEX), operating expenditure (OPEX) and GHG emissions of the baseline plant are US$774 million, US$263 million/yr. and −0.14 kgCO2eq/kgMeOH, respectively. In particular, water electrolysis process accounted for 34 % of CAPEX and 51 % of OPEX. A discounted cash flow (DCF) model combined with sensitivity analyses showed that a breakeven point could be reached with a methanol price of US$491/ton. The results demonstrated that combining water electrolysis with TRM could achieve a sustainable carbon-sink process for methanol production.

Parametric study on dynamic heat and mass transfer response in polymer electrolyte membrane fuel cell for automotive applications

Numerical study on dynamic responses of a polymer electrolyte fuel cell vehicle was performed to understand the effect of operating parameters on heat and mass transfer. We developed a system-level model to analyze dynamic behavior of fuel cell car applied with a variable pressure system consisting blower and back pressure control valve at cathode. Increased hydrogen mass flow rate increased both the anode pressure and relative humidity; when the stoichiometric number increased from 1.5 to 2.5, the pressure increased from 1.6 bar to 3.4 bar and the relative humidity increased from 32% to 62%, respectively. Although pressure increased with air mass flow rate from the blower, relative humidity decreased because blower outlet temperature increased; the relative humidity is 90% at 1.9 bar, but 30% at 1.4 bar. For back pressure control valve, pressure changed without altering mass flow rate or temperature. Therefore, relative humidity is proportional to pressure; the relative humidity is 70% at 1.6 bar, but 98% at 1.9 bar. The operating temperature was changed by controlling the fan speed. Relative humidity was inversely proportional while vehicle performance was proportional to temperature; the relative humidity is 95% at 55 °C, but 58% at 70 °C. Under drastic load variations, changes in pressure by blower and pressure regulator affected dynamic behavior differently; changing the pressure by controlling blower resulted in over/under-shoot, but that by controlling pressure regulator resulted in smooth variation. This analysis can help to understand the dynamic behavior of fuel cell vehicles at the system level and provide the proper direction for the operating strategy of fuel cell vehicles.

Simulation of Batch Slow Pyrolysis of Biomass Materials Using the Process-Flow-Diagram COCO Simulator

The commercial COCO simulation program was used to mimic the experimental slow pyrolysis process of five different biomasses based on thermodynamic consideration. The program generated the optimum set of reaction kinetic parameters and reaction stoichiometric numbers that best described the experimental yields of solid, liquid and gas products. It was found that the simulation scheme could predict the product yields over the temperature range from 300 to 800 °C with reasonable accuracy of less than 10% average error. An attempt was made to generalize the biomass pyrolysis behavior by dividing the five biomasses into two groups based on the single-peak and two-peak characteristics of the DTG (derivative thermogravimetry) curves. It was found that this approximate approach was able to predict the product yields reasonably well. The proposed simulation method was extended to the analysis of slow pyrolysis results derived from previous investigations. The results obtained showed that the prediction errors of product yields were relatively large, being 12.3%, 10.6%, and 27.5% for the solid, liquid, and gas products, respectively, possibly caused by differing pyrolysis conditions from those used in the simulation. The prediction of gas product compositions by the simulation program was reasonably satisfactory, but was less accurate for predicting the compositions of liquid products analyzed in forms of hydrocarbons, aromatics and oxygenated fractions. In addition, information on the kinetics of thermal decomposition of biomass in terms of the variation of fractional conversion with time was also derived as a function of temperature and biomass type.

Investigation on electrochemical capture of CO2 in p-Benzoquinone solutions by in situ FT-IR spectroelectrochemistry

The electrochemical capture of carbon dioxide in p-Benzoquinone (BQ) - acetonitrile and BQ - aqueous solutions have been investigated by cyclic voltammetry (CV). Two well-defined couples of anodic and cathodic peaks in CV curve of BQ in acetonitrile solvent are substituted by one redox couple peaks with E1/2 = −0.30 V once CO2 is added. The absorption peak at 2348 cm−1 of in situ FT-IR spectroelectrochemistry indicates that CO2 is involved in the electrochemical reactions. Cyclic voltabsorptometry (CVA) and derivative cyclic voltabsorptometry (DCVA) of BQ - acetonitrile with different concentration of CO2 are investigated to demonstrate the reaction stoichiometry. The absorbance value of BQ at 2348 cm−1 is a constant during reduction process when CO2 concentration is lower than 50%. However, there is a decrease at −1.3 V potential due to the CO2 reduction when it is higher than 50%. This indicates the stoichiometry of BQ•− to CO2 is 1:1 during electrochemical capture of CO2 in CH3CN solution. Similar reaction between CO2 and BQ is observed in aqueous solution. However, the stoichiometric number of BQ•− to CO2 is 1:2, forming [BQ-2CO2]•−, which is finally reduced to [BQ-2CO2]2-. The mechanism proposed is consistent with theoretical calculation since the activation energy of [BQ-CO2]•− formation is much lower than that of BQ•− reduction. The stable structures of CO2 adduct of the reactions in aprotic and aqueous solvents are also proposed according to density functional theoretical (DFT) calculations.

Thermodynamic Study of Ion-Driven Aggregation of Cellulose Nanocrystals

The thermodynamics of interactions between cations of the second group of the periodic table and differently negatively charged cellulose nanocrystals was investigated using isothermal titration calorimetry (ITC). The interaction of cations with the negatively charged CNCs was found to be endothermic and driven by an increase in entropy upon adsorption of the ions, due to an increase in degrees of freedom gained by the surface bound water upon ion adsorption. The effect was pH-dependent, showing an increase in enthalpy for cellulose suspensions at near-neutral pH (6.5) when compared to acidic pH (2). Sulfated cellulose nanoparticles were found to readily interact with divalent ions at both pH levels. The adsorption on carboxylate nanocrystals was found to be pH dependent, showing that the carboxylic group needs to be in the deprotonated form to interact with divalent ions. For the combined system (sulfate and carboxylate present at the same time), at neutral pH, the adsorption enthalpy was higher than the value obtained from cellulose nanocrystals containing a single functional group, while the association constant was higher due to an increased favorable entropic contribution. The higher entropic contribution indicates a more restricted surface-bound water layer when multiple functionalities are present. The stoichiometric number n was nearly constant for all systems, showing that the adsorption depends almost completely on the ion valency and on the amount of ionic groups on the CNC surface, independent of the type of functional group on the CNC surface as long as it is deprotonated. In addition, we showed that the reduction in Gibbs free energy drives the ionotropic gelation of nanocellulose suspensions, and we show that ITC is able to detect gel formation at the same time as determining the critical association concentration.

Molecular docking and muiltple spectroscopy investigation on the binding characteristics of aloe-emodin to pepsin

In this study, the binding characteristics of aloe-emodin (AE) and pepsin were investigated via multiple spectroscopic and molecular docking methods. The Stern-Volmer quenching constant (Ksv) suggested that AE quenched the fluorescence intensity of pepsin via static quenching. The value of the stoichiometric binding number (n) suggested only a single binding site on pepsin for AE. The value of Gibbs’ free energy change (ΔG) < 0 indicated a spontaneous binding mechanism. The negative values of enthalpy change (ΔH) and entropy change (ΔS) indicated that hydrogen bonds and van der Waals forces were the main forces mediating the binding process. The conformational changes in the structure of pepsin were confirmed by synchronous, three-dimensional fluorescence and Fourier-transform infrared (FT-IR) spectroscopy. The α-helix and β-sheet content of pepsin decreased, whereas the random coil and β-turn content increased. Molecular docking results further validated the findings and displayed the specific mode of interaction. The catalytic activity of pepsin and the antioxidant effect of AE were decreased upon binding of the two molecules. This study provides a theoretical basis for protecting the antioxidant activity of AE and suggests strategies to improve the safety of foods and regulate the multiple biologically relevant processes.