Diffusion Limitations Research Articles

Impact of CO hydrogenation diffusion limitation and cluster size on Fischer-Tropsch synthesis mechanism: An integrated experimental and theoretical study

This research examines how varying cobalt particle sizes and support pore sizes affect CO diffusion in Fischer-Tropsch synthesis (FTS). It explores cobalt catalysts deposited on perlite (P) and its modified version (S) with weight loadings ranging from 5% to 20%. N2 adsorption-desorption, BET analysis, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), hydrogen temperature-programmed reduction (H2-TPR), hydrogen temperature-programmed desorption (H2-TPD), transmission electron microscopy (TEM), and Fourier-transform infrared spectroscopy (FT-IR) analyses were utilized for characterization. The results designate that higher cobalt loading on the modified support (S) decreases CH4 selectivity while enhancing selectivity towards C5+ hydrocarbons. Despite larger cobalt particles in 15CP compared to 15CS, 15CP exhibits higher CH4 selectivity and reduced C5+ selectivity because of CO diffusion limitations and smaller pore size. DFT calculations reveal product selectivity depends on FTS mechanism and cobalt cluster size influences reaction pathways, with smaller clusters favoring CO insertion and larger clusters favoring carbide mechanisms.

Two-dimensional model in a laminar flow slit photoreactor for the determination of surface kinetic constants considering the presence of diffusion limitations

Narrow slit reactors are widely used in laboratory testing of photocatalytic materials, as well as for the determination of kinetic coefficients in surface photocatalyzed reactions. In this work, the behavior of a continuous laminar flow slit reactor was described by a two-dimensional model, addressing the advection–diffusion problem in the fluid phase, coupled with a first-order chemical reaction on the reactor walls. The concentration profile of the reactant, in the transverse and longitudinal directions of the reactor, depends on two parameters that characterize the system; the Péclet number and the Damköhler number, in addition to the geometry of the reactor. Once a short distance from the reactor’s entrance has been surpassed, the relationship between the transversally averaged reactant concentration and the concentration existing on the catalytic surface becomes essentially constant, thus defining an (external) effectiveness factor. The simulations carried out for a wide range of values of the parameters characterizing the model showed that the effectiveness factor only depends on the Damköhler number, being independent of the value of the Péclet number and of the relationship between the reactor length and the distance separating the plates. An analytical expression for the effectiveness factor as a function of the Damköhler number was proposed, which allows to determine the value of the kinetic coefficient for the surface reaction (ks), even in the presence of important limitations to mass transfer. The methodology proposed was illustrated performing experiments of photocatalytic oxidation of toluene in air (with Péclet numbers ranging from 20 to 40), employing a paint formulated with carbon-doped TiO2 in a continuous laboratory photoreactor of flat parallel plates. The observed toluene conversions were in the range of 13 to 26 %, consequently, the calculated Damköhler number was Da = 0.0307 and the surface kinetic coefficient thus determined (for 50 % of relative humidity and 42.3 W/m2 of irradiance flux) was ks = 1.262 10-4 m/s. The significance of this work lies in the development of a comprehensive model that can be used to know the surface kinetics in photocatalytic reactions, applicable to scenarios where there are considerable mass transfer limitations or not.

Trade-off between sulfidated zero-valent iron reactivity and air stability: Regulation of iron sulfides by ammonium dihydrogen phosphate

The reactivity and stability of zero-valent iron (ZVI) and sulfidated zero-valent iron (S-ZVI) are inherently contradictory. Iron sulfides (FeSX) on the S-ZVI surface play multiple roles, including electrostatic adsorption and catalyzing reduction. We proposed to balance the reactivity and air stability of S-ZVI by regulating FeSX. Benefiting from the superior coordination and accelerate electron transport capabilities of phosphate, herein, eco-friendly ammonium dihydrogen phosphate (ADP) was employed to synthesize N, P, and S-incorporated ZVI (NPS-ZVI) and regulate the FeSX. Raman, FTIR, XPS, and density functional theory (DFT) calculations were combined to reveal that HPO42- acts as the main P species on the Fe surface. The superior reactivity of NPS-ZVI was quantified by kobs, kSA, and kM of Cr(VI), which were 210.77, 27.44, and 211.17-fold than ZVI, respectively. NPS-ZVI demonstrated excellent reusability, with no risk of secondary pollution. Critically, NPS-ZVI could effectively maintain FeSX stability under the combination of diffusion limitation and surface protection mechanisms of ADP. The superior reactivity of NPS-ZVI was attributed to the fact that ADP maintains FeSX stability and accelerates electron transport. This study provides a novel strategy in balancing the reactivity and air stability of S-ZVI and offers theoretical support for material modification.

Design and evaluation of composite films for in situ synthesis and antibacterial activity of allicin vapour

Although allicin has potent antibiotic properties, its low stability, which is responsible for its persistent biological activity, has posed a significant challenge to its practical application in modern medicine. To harness the healing benefits of this phytochemical, known by humans for thousands of years, we propose a controlled in situ synthesis of allicin vapour near the site of infection. Considering the critical need for novel approaches to prevent pandemic scenarios caused by MDR bacteria, we suggest encapsulating and physically separating allicin precursors (substrate alliin and enzyme alliinase) in alginate-based films and spray-dried chitosan microparticles. The mechanical properties of the hydrogel films of various compositions were evaluated, as well as their ability to protect the encapsulated alliinase against thermal stress and control the overall rate of allicin release upon hydration. Furthermore, the non-contact antibacterial efficacy of free alliin/alliinase reaction mixture (aqueous solution) and three compartmentalised configurations, i.e. film-solution, film-particles, and double-film, were tested against selected bacterial strains, i.e. E. coli, S. epidermidis, and S. aureus. The results indicate that the formation of allicin vapour using the proposed compartmentalised systems addresses allicin’s stability issues and provides better control over the rate of allicin production. The observed antibacterial effect was comparable with directly formed allicin using higher initial amounts of both substances, which is given by diffusion limitations associated with encapsulation. These findings illustrate the potential of compartmentalised systems in developing nature-based wound dressings for infection prevention and promoting healing.

Unveiling Silent Consequences: Impact of Pulmonary Tuberculosis on Lung Health and Functional Wellbeing after Treatment.

Background: Pulmonary tuberculosis (TB) remains a major public health issue in India, with high incidence and mortality. The current literature on post-TB sequelae functional defects focuses heavily on spirometry, with conflicting obstruction vs. restriction data, lacks advanced statistical analysis, and has insufficient data on diffusion limitation and functional impairment. Objective: This study aimed to thoroughly evaluate post-tubercular sequelae after treatment, assessing chest radiology, spirometry, diffusing capacity, and exercise capacity. Methods: A total of 85 patients were studied at a university teaching hospital in Mysuru. The data collected included characteristics, comorbidities, smoking history, and respiratory symptoms. The investigations included spirometry, DLCO, chest X-rays with scoring, and 6MWT. Results: Of the patients, 70% had abnormal X-rays post-treatment, correlating with reduced lung function. Additionally, 70% had impaired spirometry with obstructive/restrictive patterns, and 62.2% had reduced DLCO, with females at higher risk. Smoking increased the risk of sequelae. Conclusions: Most patients had residual radiological/lung function abnormalities post-treatment. Advanced analyses provide insights into obstructive vs. restrictive defects. Ongoing research should explore pathogenetic mechanisms and therapeutic modalities to minimize long-term post-TB disability.

Advancing Plastic Recycling: A Review on the Synthesis and Applications of Hierarchical Zeolites in Waste Plastic Hydrocracking

The extensive use of plastics has led to a significant environmental threat due to the generation of waste plastic, which has shown significant challenges during recycling. The catalytic hydrocracking route, however, is viewed as a key strategy to manage this fossil-fuel-derived waste into plastic-derived fuels with lower carbon emissions. Despite numerous efforts to identify an effective bi-functional catalyst, especially metal-loaded zeolites, the high-performing zeolite for hydrocracking plastics has yet to be synthesized. This is due to the microporous nature of zeolite, which results in the diffusional limitations of bulkier polymer molecules entering the structure and reducing the overall cracking of plastic and catalyst cycle time. These constraints can be overcome by developing hierarchical zeolites that feature shorter diffusion paths and larger pore sizes, facilitating the movement of bulky polymer molecules. However, if the hierarchical modification process of zeolites is not controlled, it can lead to the synthesis of hierarchical zeolites with compromised functionality or structural integrity, resulting in reduced conversion for the hydrocracking of plastics. Therefore, we provide an overview of various methods for synthesizing hierarchical zeolites, emphasizing significant advancements over the past two decades in developing innovative strategies to introduce additional pore systems. However, the objective of this review is to study the various synthesis approaches based on their effectiveness while developing a clear link between the optimized preparation methods and the structure-activity relationship of the resulting hierarchical zeolites used for the hydrocracking of plastics.

A study using a semi-analytical approach to the reaction kinetics that describes the behavior of heterogeneous reaction–diffusion process with a glyoxyl-agarose immobilized enzyme (penicillin G acylase) in a batch reactor

This research article examines the reaction–diffusion process in a batch reactor by means of immobilized penicillin G acylase (PGA) with a glyoxyl-agarose matrix support. This study employs the Akbari-Ganji’s Method (AGM) when the system is under the internal (intra-particle) diffusional restrictions, and the external (film) diffusional restrictions is negligible. The semi-analytical approximations by the AGM technique are employed for computing the dimensionless steady-state solutions for a system of nonlinear differential equations to examine the impact of internal diffusional restrictions (IDR) as a function of catalyst enzyme loading and particle size. The model entails highly nonlinear components that adhere to the standard Michaelis-Menten kinetics. In addition to the proposed model, the dynamics for the mean integrated effectiveness factor (MIEF) of the dimensionless substrate concentration phenylglycine methyl ester (PGME) is presented. In light of this, there is a satisfactory level of agreement on the comparison of the semi-analytical result with the simulated numerical results across the whole concentration range where the dimensionless initial substrate and product concentrations are S1b=30,S2b=15, and P1b=60,P2b=100. The behavior of concentration profiles under the steady/unsteady state conditions for the effect of IDR on the overall reaction rates by immobilized biocatalyst PGA were examined. A sensitivity analysis is carried out to identify the effective parameter which can influence the diffusional restrictions on MIEF. An error analysis is performed on the dimensionless concentration equations to assess the amount of accuracy and the convergence of a solution.

Experimental, theoretical and numerical simulation-based investigations on the fabricated Cu2ZnSn thin-film-based Schottky diodes with enhanced electron transport for solar cell

Copper-zinc-tin Cu2ZnSn (CZT) thin films are promising materials for solar cell applications. This thin film was deposited on a fluorine-doped tin oxide (FTO) using an electrochemical deposition hierarchy. X-ray diffraction of thin-film studies confirms the variation in the structural orientation of CZT on the FTO surface. As the pH of the solution is increased, the nature of the CZT thin-film aggregate changes from a fern-like leaf CZT dendrite crystal to a disk pattern. The FE-SEM surface micrograph shows the dendrite fern leaf and sharp edge disks. The 2-D diffusion limitation aggregation under slippery conditions for ternary thin films was performed for the first time. The simulation showed that by changing the diffusing species, the sticking probability was responsible for the pH-dependent morphological change. Convincingly, diffusion-limited aggregation (DLA) simulations confirm that the initial structure of copper is responsible for the final structure of the CZT thin films. An experimental simulation with pH as a controlled parameter revealed phase transition in CZT thin films. The top and back contact of Ag-CZT thin films based on Schottky behavior give a better electronic mechanism in superstrate and substrate solar cells.

Hierarchical metal–organic framework nanoarchitectures for catalysis

Metal–organic frameworks (MOFs) have garnered significant attention in the field of catalysis due to their unique advantages such as diverse coordination geometry, variable metal nodes, and organic linkers, facilitating precise structural and compositional control for achieving programmable catalytic functionalities. Although their inherent microporous structure could provide excellent shape selectivity during catalysis, it typically impedes the mass transfer process, thereby reducing the use of internal active sites and overall catalytic efficiency. Additionally, employing single MOFs as catalysts presents challenges in achieving complex catalytic reactions that require multifunctional active sites. In recent years, considerable research efforts have focused on designing and constructing hierarchical nanostructured MOFs to alleviate substrate diffusion limitations by introducing secondary nanopores, shortening diffusion distances via the construction of low-dimensional nanoarchitectures, and constructing multifunctional catalysts by integrating distinct MOFs with suitable functions. This review provides a comprehensive overview of the design, synthesis methods, and formation mechanisms of MOF-based hierarchical nanostructures in recent years. Subsequently, it further highlights their applications in thermal catalysis, electrocatalysis, and photocatalysis, along with the relationship between their hierarchical nanostructures and catalytic performances. Finally, it provides an outlook on the challenges and potential development directions of hierarchically structured MOF nanocatalysts.

Asymmetric dual-chamber electrochemical reactor for reducing Fe(Ⅲ)EDTA to remove NOx by enhanced complexing absorption

The complexing absorption technology possesses broad application prospects in the efficient removal of pollutant NO from industrial flue gas. However, the absorbent Fe(Ⅱ)EDTA is readily oxidized to Fe(Ⅲ)EDTA by the inherent O2 in flue gas, which severely impairs denitrification efficiency due to the inability of Fe(Ⅲ)EDTA to absorb NO. Here, a novel asymmetric dual-chamber electrochemical reactor (ADER) was designed to efficiently reduce Fe(Ⅲ)EDTA to Fe(Ⅱ)EDTA. The key to high-efficient reduction was to improve the reaction kinetics utilizing a unique design of larger cathode–anode area ratio. Moreover, cloth served as separator simultaneously satisfied the needs of H+ transfer and diffusion limitation of Fe(Ⅱ)EDTA and Fe(Ⅲ)EDTA, thus safeguarding the reduction efficiency while avoiding membrane contamination. The reduction efficiency and current efficiency of Fe(Ⅲ)EDTA up to 78.05 and 98.19 % in ADER system, respectively, with energy consumption of 0.078 kWh/mol Fe(Ⅲ)EDTA. And the ADER systems coupled with complexing absorption exhibited excellent NO removal efficiencies exceeding 92 % even in the presence of O2. The ADER provided an efficient, convenient and low-cost strategy for the regeneration of absorbent Fe(Ⅱ)EDTA as well as NOx removal.

Electrochemical H2O2 Production Modelling for an Electrochemical Bandage

Hydrogen peroxide (H2O2) is an environmentally friendly oxidizing agent used to treat wound infections. We have developed an electrochemical bandage (e-bandage), which generates H2O2 in situ and shown that it exhibites in vitro and in vivo efficacy. The electrochemical bandage comprises carbon fabric working and counter electrodes, as well as an Ag/AgCl quasi-reference electrode, separated by cotton fabric and the electrolyte is delivered by Xanthan gum with phosphate buffer saline. While the chemistry and electrochemistry of the e-bandage have been experimentally characterized, the system level description could aid in better designing these devices. Here, a model called electrochemical hydrogen peroxide production (EHPP) was used to evaluate factors influencing electrochemical generation of H2O2, including electrode potential, diffusion and reaction rates, temperature, and various geometries. EHPP model parameters estimated based on experimental results indicate that: (i) with diffusion limitations caused by changes in physical conditions (e.g., drying of hydrogel), the rate of H2O2 generation decreases, (ii) higher working electrode overpotentials increase H2O2 generation and higher counter electrode overpotentials do not affect H2O2 generation, (iii) increasing the distance between electrodes by adding more hydrogel reduces H2O2 generation, (iv) net H2O2 generation decreases ∼12% with temperature, and (v) H2O2 production is most effective in the initial 48 h of operation.

Computational modelling of scleral photocrosslinking: from rat to minipig to human.

Selective scleral crosslinking has been proposed as a novel treatment to increase scleral stiffness to counteract biomechanical changes associated with glaucoma and high myopia. Scleral stiffening has been shown by transpupillary peripapillary scleral photocrosslinking in rats, where the photosensitizer, methylene blue (MB), was injected retrobulbarly and red light initiated crosslinking reactions with collagen. Here, we adapted a computational model previously developed to model this treatment in rat eyes to additionally model MB photocrosslinking in minipigs and humans. Increased tissue length and subsequent diffusion and light penetration limitations were found to be barriers to achieving the same extent of crosslinking as in rats. Per cent inspired O2, injected MB concentration and laser fluence were simultaneously varied to overcome these limitations and used to determine optimal combinations of treatment parameters in rats, minipigs and humans. Increasing these three treatment parameters simultaneously resulted in maximum crosslinking, except in rats, where the highest MB concentrations decreased crosslinking. Additionally, the kinetics and diffusion of photocrosslinking reaction intermediates and unproductive side products were modelled across space and time. The model provides a mechanistic understanding of MB photocrosslinking in scleral tissue and a basis for adapting and screening treatment parameters in larger animal models and, eventually, human eyes.

Fast-Charging Carbon Fiber Structural Battery Electrodes Using an Organic Polymer Active Material

Structural batteries require electrodes with integrated energy storage and load-bearing properties. Adoption of structural batteries can lead to mass and volume savings in electrified transportation and aerospace applications by storing energy within the object’s structural elements. However, to date, active materials investigated in structural batteries exhibit poor rate capabilities at higher C-rates and even worse performance at lower temperatures due to diffusion limitations. Organic radical polymers are promising alternatives because they possess fast-charging properties and good cycling stability. In this work, we integrate an organic radical polymer with carbon fiber (CF) fabric, in which the polymer acts as the active cathode material and the CF fabric possesses excellent tensile strength, modulus and electronic conductivity. At 20 °C, the structural cathodes exhibited a reversible capacity of 67 mAh g−1 at 1C-rate and an 88% capacity retention at 25C-rate. Further, these structural electrodes retained more than 50% of their performance at −10 °C (vs 20 °C). These electrodes were further examined in a full cell containing a graphite-based anode, demonstrating a pathway for utilizing redox-active polymer-based active materials in structural and fast-charging organic batteries.

Integrated Approach in Evaluating the Results of Cosmetic Procedures Using Ultrasonography and Magnetic Resonance Tomography (Clinical Study)

Aim. To investigate the capabilities of comprehensive assessment using ultrasound and magnetic resonance imaging (MRI) of the results of cosmetic procedures.Materials and Methods. A study of the soft tissues of the anterior abdominal wall of a 43-year-old woman complaining of skin atony and postpartum stretch marks using highresolution ultrasound and MRI.Results. At ultrasonography in the projection of injection of polylactic acid and calcium hydroxyapatite preparations and radiofrequency treatment, an increase in echogenicity of hypodermis with the presence of acoustic shadow was noted. On MRI at the level of derma and hypoderma of periumbilical region in the projection of radiofrequency treatment, injections of preparations based on polylactic acid on the left, calcium hydroxyapatite on the right in 3 months after subdermal injection, areas of hyperintense MR-signal in T2 weight image (WI) and STIR programs, hypointense in T1 WI, without diffusion limitation on DWI b=1000 were determined. In SWI mode, the focus of hypointense MR signal of rounded shape with clear and smooth contours was visualized on the right side of the image.Conclusion. The clinical study demonstrates the possibilities of an integrated approach in evaluating the results of cosmetic procedures using ultrasonography and MRI.

Sequential diethylaminoethyl dextran-grafting and diethylaminoethyl modification improve the adsorption performance of anion exchangers

Ion exchangers with high adsorption capacity, fast mass transfer, and high salt-tolerance synchronously are highly desired for high-performance protein purification. Here, we propose a sequential diethylaminoethyl dextran-grafting and diethylaminoethyl chloride modification strategy to achieve high-performance anion exchangers. The advantages of the double-modification strategy lie in: (1) the introduction of diethylaminoethyl in the second modification has no diffusion limitation due to the small molecular size, thus a high ionic capacity; (2) the grafting ligands not only provide three-dimensional adsorption space for high adsorption capacitybut alsofacilitate surface diffusion of protein by chain delivery. The maximum adsorption capacity of the obtained anion exchangers for bovine serum albumin reaches 333 mg/mL, the ratio of effective pore diffusivity (De) to free solution diffusivity (D0) reaches 0.69, and the adsorption amount reaches 97 mg/mL even in 100 mmol/L NaCl concentration,. All these results demonstrate the proposed sequential modification strategy are promising for the preparation of high-performance ion exchangers.

Xylooligosaccharides as emerging prebiotics and their sustainable generation from xylan catalysed by endoxylanase immobilized ordered mesoporous silica

The enzyme endo-1,4-β-xylanase has been a prominent candidate for research in recent years due to its ability to catalyse prebiotic production in an environmentally friendly manner. However, the loss of stability and activity when employed in industrial processes has prompted enzyme immobilization. The present study aims to compare the efficiencies of amorphous (such as fumed silica and silicic acid) and ordered mesoporous silica materials (including SBA-15, IITM-41, and MCM-41) as carriers for endoxylanase immobilization, particularly focussing on the novel silicate IITM-41. Recombinant endoxylanase from Bacillus subtilis KCX006 purified by Ni-NTA columns has been used in this study. XRD, TEM, and BET analyses of these nanostructured materials confirmed the mesoporous structure and the nature of the pores. FT-IR spectroscopy confirmed protein binding through characteristic functional group signatures of silica materials and proteins. Zeta potential values of the nanostructure materials became more negative after enzyme binding, and the confirmation of enzyme immobilization was achieved through fluorescent tagging. Although amorphous silica showed a higher yield of immobilization, it resulted in lower recovered activity due to the formation of protein multilayers. Among the ordered mesoporous silica materials, IITM-41 exhibited the highest recovered activity at 74 %, with a maximum yield of xylooligosaccharides reaching 797.61 mg/g. The optimum pH of the immobilized enzymes remained unchanged, but there was a shift in the optimum temperature from 50 to 60 °C, along with a broader optimal range, attributed to increased enzyme rigidity and substrate diffusion limitations. Kinetic parameters Km and Vmax were higher for immobilized enzymes compared to the free enzyme, particularly with higher Vmax values observed for enzymes immobilized on mesoporous materials compared to those on amorphous silica. Generally, mesoporous materials showed superior performance in terms of xylooligosaccharides production, storage stability, and recycling efficiency. Over 10 cycles of reuse, mesoporous materials exhibited a higher yield of total xylooligosaccharides (ranging from X2 to X6) compared to amorphous silica.

Use of Nanobubbles to Improve Mass Transfer in Bioprocesses

Nanobubble technology has emerged as a transformative approach in bioprocessing, significantly enhancing mass-transfer efficiency for effective microbial activity. Characterized by their nanometric size and high internal pressure, nanobubbles possess distinct properties such as prolonged stability and minimal rise velocities, allowing them to remain suspended in liquid media for extended periods. These features are particularly beneficial in bioprocesses involving aerobic strains, where they help overcome common obstacles, such as increased culture viscosity and diffusion limitations, that traditionally impede efficient mass transfer. For instance, in an experimental setup, nanobubble aeration achieved 10% higher soluble chemical oxygen demand (sCOD) removal compared to traditional aeration methods. Additionally, nanobubble-aerated systems demonstrated a 55.03% increase in caproic acid concentration when supplemented with air nanobubble water, reaching up to 15.10 g/L. These results underscore the potential of nanobubble technology for optimizing bioprocess efficiency and sustainability. This review delineates the important role of the mass-transfer coefficient (kL) in evaluating these interactions and underscores the significance of nanobubbles in improving bioprocess efficiency. The integration of nanobubble technology in bioprocessing not only improves gas exchange and substrate utilization but also bolsters microbial growth and metabolic performance. The potential of nanobubble technology to improve the mass-transfer efficiency in biotechnological applications is supported by emerging research. However, to fully leverage these benefits, it is essential to conduct further empirical studies to specifically assess their impacts on bioprocess efficacy and scalability. Such research will provide the necessary data to validate the practical applications of nanobubbles and identify any limitations that need to be addressed in industrial settings.

Dry Reforming of CH4 Using a Microreactor

In the present study, a comparison of the dry reforming of a gas mixture containing methane, carbon dioxide and nitrogen without contaminants to a ruthenium-based Ru/Al2O3 catalyst was carried out in a microreactor for the first time. The influence of the contact time, temperature and composition of the feed on the conversion was exhaustively investigated. The optimal operating conditions were found to be a contact time of 80 milliseconds, a temperature of 700 °C and a CH4:CO2 ratio of 1. The assessment of diffusional limitations reveals that there is no resistance to mass transfer, which reveals the potential benefit of the determination of intrinsic reaction kinetics within a microreactor.

Variation in photosynthetic capacity of Salvia przewalskii along elevational gradients on the eastern Qinghai-Tibetan Plateau, China

Elevational variation in plant growing environment drives diversification of photosynthetic capacity, however, the mechanism behind this reaction is poorly understood. We measured leaf gas exchange, chlorophyll fluorescence, anatomical characteristics, and biochemical traits of Salvia przewalskii at elevations ranging from 2400 m to 3400 m above sea level (a.s.l) on the eastern Qinghai-Tibetan Plateau, China. We found that photosynthetic capacity showed an initial increase and then a decrease with rising elevation, and the best state observed at 2800 m a.s.l. Environmental factors indirectly regulated photosynthetic capacity by affecting stomatal conductance (gs), mesophyll conductance (gm), maximum velocity of carboxylation (Vc max), and maximum capacity for photosynthetic electron transport (Jmax). The average temperature (T) and total precipitation (P) during the growing season had the highest contribution to the variation of photosynthetic capacity of S. przewalskii in subalpine areas, which were 25% and 24%, respectively. Photosynthetic capacity was mainly affected by diffusional limitations (71%–89%), and mesophyll limitation (lm) played a leading role. The variation of gm was attributed to the effects of environmental factors on the volume fraction of intercellular air space (fias), the thickness of cell wall (Tcw), the surface of mesophyll cells and chloroplasts exposed to intercellular airspace (Sm, Sc), and plasma membrane intrinsic protein (PIPs, PIP1, PIP2), independent of carbonic anhydrase (CA). Optimization of leaf tissue structure and adaptive physiological responses enabled plants to efficiently cope with variable climate conditions of high-elevation areas, and the while maintaining high levels of carbon assimilation.

Can membrane permeability of zwitterionic compounds be predicted by the solubility-diffusion model?

Zwitterions contain both positively and negatively charged functional groups, resulting in an overall net neutral charge. Nevertheless, the membrane permeability of the zwitterionic form of a compound is assumed to be much lower than the permeability of the uncharged neutral form. Although a significant proportion of pharmaceuticals are zwitterionic, it has not been clear so far whether their permeability is dominated by the permeation of the zwitterionic or the neutral form, since neutral fractions are often quite low as compared to the zwitterionic fraction. This complicates the in silico prediction of the permeability of zwitterionic compounds. In this work, we re-evaluated existing in vitro permeability data from literature measured with Caco-2/MDCK cell assays, using more strict exclusion criteria for effects like diffusion limitation by the aqueous boundary layers, paracellular transport, active transport and retention. Using this re-evaluated data set, we show that extracted intrinsic permeabilities of the neutral fraction are well predicted by the solubility-diffusion model (RMSE = 1.21; n = 18) if the permeability of the zwitterionic species is assumed negligible. Our work thus suggests that only the neutral species is relevant for the membrane permeability of zwitterionic compounds, and that membrane permeability of zwitterionic compounds is indeed predictable by the solubility-diffusion model.