pH-driven Processes Research Articles

Improving the properties of whey protein isolate-zein nanogels with novel acidifiers: Re-dispersity, stability and quercetin bioavailability

Low bioavailability of quercetin (Que) reduces its preclinical and clinical benefits. In order to improve Que bioavailability, a novel whey protein isolate (WPI)-zein nanogel was prepared by pH-driven self-assembly and heat-induced gelatinization. The results showed that hydrochloric acid can be substituted by both acetic acid and citric acid during the pH-driven process. After encapsulation, the bioavailability of Que in nanogels (composed of 70 % WPI) induced by different acidifiers increased to 19.89 % (citric acid), 21.65 % (hydrochloric acid) and 24.34 % (acetic acid), respectively. Comparatively, nanogels induced by acetic acid showed higher stability (pH and storage stability), re-dispersibility (75.62 %), Que bioavailability (24.34 %), and antioxidant capacity (36.78 % for DPPH scavenging rates). s improved performance of nanogels. In mechanism, acetic acid significantly balanced different intermolecular forces by weakening “acid-induced denaturation” effect. Moreover, the faster binding of Que and protein as well as higher protein molecular flexibility and randomness (higher ratio of random coil) was also observed in nanogels induced by acetic acid. All of these changes contributed to improve nanogels performances. Overall, WPI-zein nanogels induced by acetic acid might be a safe, efficiency and stable delivery system to improve the bioavailability of hydrophobic active ingredients.

PH-driven-assembled soy peptide nanoparticles as particulate emulsifier for oil-in-water Pickering emulsion and their potential for encapsulation of vitamin D3

Pickering emulsions prepared by food-grade particles have gained growing attention due to their promising application in functional food and pharmaceutical industries. In this study, we successfully fabricated soy peptide-based nanoparticles (SPN) through pH-driven process. Obtained particles with small particle size were surface active and shared intermediate wettability, and they could be well applied as an efficient particulate emulsifier for stabilizing oil-in-water Pickering emulsions at SPN concentration above 0.25 wt%. Furthermore, formed emulsions stabilized with SPN exhibited good protection towards Vitamin D3 against UV irradiation and oxidative deterioration, where controlled release of Vitamin D3in vitro could also be well achieved by modulating particle concentration. The whole process can contribute to a sustainable development of low-value peptide byproducts as functional food ingredients.

Ferric iron triggers greenalite formation in simulated Archean seawater

AbstractSedimentary rock deposits provide the best records of (bio)geochemical cycles in the ancient ocean. Studies of these sedimentary archives show that greenalite, an Fe(II) silicate with low levels of Fe(III), was an early chemical precipitate from the Archean ocean. To better understand the formation of greenalite, we explored controls on iron silicate precipitation through experiments in simulated Archean seawater under exclusively ferrous conditions or supplemented with low Fe(III). Our results confirm a pH-driven process promoting the precipitation of iron-rich silicate phases, and they also reveal an important mechanism in which minor concentrations of Fe(III) promote the precipitation of well-ordered greenalite among other phases. This discovery of an Fe(III)-triggering iron silicate formation process suggests that Archean greenalite could represent signals of iron oxidation reactions, potentially mediated by life, in circumneutral ancient seawater.

Fabrication, characterization, stability and re-dispersibility of curcumin-loaded gliadin-rhamnolipid composite nanoparticles using pH-driven method

The aim of the present work was to investigate the fabrication, characterization, physical and chemical stability, and re-dispersibility of curcumin-loaded gliadin-rhamnolipid composite nanoparticles, which were prepared by pH-driven method with the novel acidifying agent (phytic acid). During the pH-driven process, the gliadin and rhamnolipid were self-assembly to form composite nanoparticles. Gliadin-rhamnolipid composite nanoparticles showed a higher encapsulation efficiency (98.70%) of curcumin than that (64.70%) of gliadin nanoparticles. Fourier transform infrared spectroscopy and potentiometric analysis revealed that hydrogen bonding, hydrophobic and electrostatic interactions were mainly involved in the formation of curcumin-loaded gliadin-rhamnolipid composite nanoparticles. Compared to the impact of the novel acidifying agent (phytic acid) and the conventional acidifying agent (hydrochloric acid) on the attribute of curcumin-loaded gliadin-rhamnolipid nanoparticles, the nanoparticles prepared with phytic acid had better thermal stability, photostability, physical stability and re-dispersibility than those prepared with hydrochloric acid. The physicochemical properties of the composite nanoparticles also depended on the mass ratio of gliadin to rhamnolipid. Furthermore, rhamnolipid exhibited an obviously positive effect on enhancing the stability of curcumin and the re-dispersibility of gliadin-based nanoparticles. Consequently, by using the pH-driven method with phytic acid, the fabricated gliadin-rhamnolipid composite nanoparticles have potential as a better nano-delivery system for curcumin in functional foods.

Exploring Coupled Redox and pH Processes with a Force-Field-Based Approach: Applications to Five Different Systems.

Coupled redox and pH-driven processes are at the core of many important biological mechanisms. As the distribution of protonation and redox states in a system is associated with the pH and redox potential of the solution, having efficient computational tools that can simulate under these conditions becomes very important. Such tools have the potential to provide information that complement and drive experiments. In previous publications we have presented the implementation of the constant pH and redox potential molecular dynamics (C(pH,E)MD) method in AMBER and we have shown how multidimensional replica exchange can be used to significantly enhance the convergence efficiency of our simulations. In the current work, after an improvement in our C(pH,E)MD approach that allows a given residue to be simultaneously pH- and redox-active, we have employed our methodologies to study five different systems of interest in the literature. We present results for capped tyrosine dipeptide, two maquette systems containing one pH- and redox-active tyrosine (α3Y and peptide A), and two proteins that contain multiple heme groups (diheme cytochrome c from Rhodobacter sphaeroides and Desulfovibrio vulgaris Hildenborough cytochrome c3). We show that our results can provide new insights into previous theoretical and experimental findings by using a fully force-field-based and GPU-accelerated approach, which allows the simulations to be executed with high computational performance.

Proteins as pH Sensors and Switches

Ionizable residues buried in dry or hydrophobic environments in proteins can have extremely anomalous pKa values (pKa > 10 for Asp and Glu, pKa 5 for Lys). These buried ionizable groups are involved in many forms of biological energy transduction. Their anomalous pKa values are usually essential for function in many pH-driven processes. Anomalous pKa values impart unusual pH sensitivity to the stability of the folded state. Previously we examined energetic consequences of burial of Lys, His, Arg, Asp and Glu in hydrophobic environments systematically, using staphylococcal nuclease (SNase) as a model system. With NMR spectroscopy and X-ray crystallography we have now characterized a variety of different types of conformational reorganization coupled to the ionization of these buried groups. Not only are conformational transitions driven by the binding or release of an H+ the basis of many H+-coupled energy transduction processes (e.g. ATPase), the detailed structural characterization of locally, partially or subglobally unfolded forms of SNase has also contributed a unique perspective on its folding landscape. When they occur, these structural transitions determine the magnitude of the shift in the pKa of internal groups. We have now also demonstrated that the unusual properties of buried ionizable groups can be harnessed to engineer proteins capable of acting as pH sensors and pH-driven switches. These switch proteins are sensitive to changes in pH in the physiological range despite using Asp, Glu or Lys residues for purposes of pH sensing. I will describe specific examples illustrating general and transferable engineering principles that may be used to turn any protein into a pH-sensitive protein with structure or function that can be modulated by small changes in pH in the physiological range.

Development of “all natural” layer-by-layer redispersible solid lipid nanoparticles by nano spray drying technology

Solid lipid nanoparticles (SLNs) have gained tremendous attraction as carriers for controlled drug delivery. Despite numerous advances in the field, one long-standing historical challenge for their practical applications remains unmet: redispersibility after drying. In this work, a novel design of SLNs using a layer-by-layer (LbL) technique was developed and the formulations were optimized by surface response methodology (Box-Behnken design). To the best of our knowledge, this is the first study reporting the fabrication of SLNs from all natural ingredients in the absence of any synthetic surfactants or coatings. The SLNs were prepared by a combined solvent-diffusion and hot homogenization method, with soy lecithin as natural emulsifier (first layer), followed by the subsequent coating with sodium caseinate (second layer) and pectin (third layer), both of which are natural food biopolymers. The adsorption of pectin coating onto caseinate was reinforced by hydrophobic and electrostatic interactions induced by a pH-driven process along with thermal treatment. The innovative nano spray drying technology was further explored to obtain ultra-fine powders of SLNs. Compared to uncoated or single-layer coated SLNs powders, which showed severe aggregation after spray drying, the well-separated particles with spherical shape and smooth surface were obtained for layer-by-layer (LbL) SLNs, which were redispersible into water without variation of dimension, shape and morphology. The SLNs were characterized by Fourier transform infrared and high-performance differential scanning calorimetry for their physical properties. The LbL-coated SLNs based on all natural ingredients have promising features for future applications as drug delivery systems, overcoming the major obstacles in conventional spray drying and redispersing SLNs-based formulations.

Rate-specific responses of prokaryotic diversity and structure to nitrogen deposition in the Leymus chinensis steppe

Serious nitrogen (N) deposition in terrestrial ecosystems causes soil acidification and changes the structure and function of the microbial community. However, it is unclear how these changes are dependent on N deposition rates, other factors induced by N (e.g., pH), and their interactions. In this study, we investigated the responses of soil prokaryotic community structure and stability after a 13-year N addition in the semi-arid Leymus chinensis steppe in Inner Mongolia, China. Our results demonstrated that the prokaryotic community structure changed at the low N addition rate of 1.75 g N m−2 yr−1; however, dramatic changes in microbial abundance, respiratory quotient, and prokaryotic diversity occurred at N addition rates of more than 5.25 g N m−2 yr−1 when the soil pH dropped below 6.0. The two patterns indicated the difference in driving forces for different microbial properties. The N-driven and pH-driven processes are likely the most important mechanisms determining the responses of bacterial community to N. Some copiotrophic/oligotrophic bacteria, e.g., Proteobacteria and Acidobacteria, changed their relative abundances with the N addition continuously even at a low rate, indicating that they were more sensitive to N directly. Some bacterial groups significantly changed their relative abundance at a high N addition rate when pH dropped below 6.0, e.g., Verrucomicrobia and Armatimonadetes, indicating that they were more sensitive to pH below 6.0. N addition altered the prokaryotic community structure through enrichment of copiotrophic bacteria (species adjustment) at low N addition rates and through enrichment of nitrophilous taxa and significant loss of diversity at high N rates. The results also demonstrated that a high N addition diminished the stability of the prokaryotic community structure and activity through reduction in species diversity and bacterial interaction. Overall, this study supported the hypothesis that the responses of prokaryota to N were dependent on deposition rates, and N-driven and pH-driven processes were the important mechanisms to control the shift of the prokaryotic community.

A laboratory study of the initial effects of dissolved carbon dioxide (CO2) on metal release from shallow sediments

Sequestration of carbon dioxide (CO2) into deep geologic reservoirs is a potential approach for controlling the rise of CO2 concentrations in the atmosphere. Proper characterization and permitting of storage sites is expected to include an assessment of the potential impacts of CO2 intrusion into overlying groundwater formations. In most natural settings, the dissolution of CO2 into groundwater will decrease the pH and can also release carbonate ligands into solution. These effects can lead to metals being mobilized from sediments through mechanisms such as ion exchange, desorption, and mineral dissolution. This laboratory-based study evaluates the extent of metal release from a groundwater system in the presence of elevated CO2 concentrations. In particular, the research investigates the geochemical mechanisms involved in metal release, with a focus on distinguishing between pH-driven processes (e.g. carbonate dissolution and ion exchange) and carbonate-driven processes (e.g. reactions enhanced by the formation of metal–carbonate complexes). Measurements from lab-scale leaching experiments and sediment characterizations are compared to data from a concurrent controlled-release field test, where CO2 was injected into a poorly buffered sandy groundwater formation at ∼50m depth. Results show that the immediate effect of the introduction of CO2 is a pH drop of ∼3 units, which leads to the quick release of some elements (e.g. Ca, Mg, Ba, Sr) by primarily pH-driven processes. The extent of metal release was different across geochemically distinct sediment types from the field site. The results suggest that it would be useful to group constituents by their pH-release trends, and to include pH and concentrations of certain indicator cations (Li, K, Na, Ba, Ca, Mg, Sr, Mn and Si) as parameters in site monitoring plans.

Comparison of the pH-induced conformational change of different clostridial neurotoxins*1

Clostridial neurotoxins are internalized inside acidic compartments, wherefrom the catalytic chain translocates across the membrane into the cytosol in a low pH-driven process, reaching its proteolytic substrates. The pH range in which the structural rearrangement of clostridial neurotoxins takes place was determined by 8-anilinonaphthalene-1-sulfonate and tryptophan fluorescence measurements. Half conformational change was attained at pH 4.55, 4.50, 4.40, 4.60, 4.40, and 4.40 for tetanus neurotoxin and botulinum neurotoxin serotypes /A, /B, /C, /E, and /F, respectively. This similarity indicates the key residues for the conformation transition are strongly conserved. Acidic liposomes support the conformational rearrangement shifting the effect versus higher pH values, whereas zwitterionic liposomes do not. The disulfide bridge linking the light and the heavy chains together needs to be oxidized to allow toxin membrane insertion, indicating that in vivo its reduction follows exposure to the cytosol after penetration of the endosomal membrane.

Comparison of the pH-induced conformational change of different clostridial neurotoxins

Clostridial neurotoxins are internalized inside acidic compartments, wherefrom the catalytic chain translocates across the membrane into the cytosol in a low pH-driven process, reaching its proteolytic substrates. The pH range in which the structural rearrangement of clostridial neurotoxins takes place was determined by 8-anilinonaphthalene-1-sulfonate and tryptophan fluorescence measurements. Half conformational change was attained at pH 4.55, 4.50, 4.40, 4.60, 4.40, and 4.40 for tetanus neurotoxin and botulinum neurotoxin serotypes /A, /B, /C, /E, and /F, respectively. This similarity indicates the key residues for the conformation transition are strongly conserved. Acidic liposomes support the conformational rearrangement shifting the effect versus higher pH values, whereas zwitterionic liposomes do not. The disulfide bridge linking the light and the heavy chains together needs to be oxidized to allow toxin membrane insertion, indicating that in vivo its reduction follows exposure to the cytosol after penetration of the endosomal membrane.

The weathering of mineral soil by natural soil solutions

Chemical weathering is an important neutralisation process and sourceof cations in forest soil. The presence of dissolved organic matter in the soil solution can have a considerable influence on weathering release. The aim of this study is to compare the weathering potentialof natural soil solutions, collected from Norway spruce, Scots pine and birch sites, to release Al, Ca, Mg, K, Na, and Si from the fine fraction in the C horizon of a podzol. Residual organic matter in the mineral soil was removed with H2O2. The <0.06 mm fraction of the mineral soil was suspended in soil solution, collected from the three sites, for 11 days with continuous agitation. Ultrapure water was used as a control. The pH of the suspensions was maintained at 5.4 by bubbling with CO2. The initial mean DOC concentrations in the soil solutions were 65, 56 and 40 mg L-1 for the spruce, pine and birch sites, respectively. The presence of DOM in the soil solution did not significantly enhance the capacity to weather mineral soil material, and no systematic differences were found between the three sites. However, Al release from the mineral soil was slightly higher in the soil solutions containing DOM compared to the control solution with no DOM. The proportions of DOM fractions capable of enhancing weathering were comparable with those reported in earlier studies. The weathering of metals was found to be primarily due to pH-driven processes. The lack of considerable weathering enhancement by DOM could be due to the fact that the cation-binding sites of the organic ligands were already saturated by e.g. Al and Fe in the soil solution derived from these podzolic, Al- and Fe-rich soils.

Molecular machines based on metal ion translocation.

Transition metal ions can be moved reversibly between the two coordinatively unequivalent compartments A and B of a ditopic ligand, using as an input the variation of a bulk solution parameter, either pH or redox potential. In a redox-driven translocation, the metal moves reversibly from A to B on cycling between two consecutive oxidation states (e.g., Cu(II)/Cu(I); Fe(III)/Fe(II)) by means of auxiliary oxidation and reduction reactions. In a pH-driven process, one compartment displays also acid-base properties (AH(n)() left arrow over right arrow A(n)(-) + nH(+)), and the M(n)(+) ion is translocated between B and A(n)(-) through consecutive addition of base and acid.

Enhanced lysosomal acidification leads to increased chloroquine accumulation in CHO cells expressing the pfmdr1 gene

Expression of the pfmdr1-encoded Pgh1 protein of Plasmodium falciparum in CHO cells confers a phenotype of increased sensitivity to chloroquine due to an increased Pgh1-mediated accumulation of this antimalarial. Pgh1 carrying amino acid substitutions associated with chloroquine resistance in P. falciparum does not confer this phenotype. Here, we present studies on the underlying mechanism of Pgh1 mediated chloroquine influx into CHO cells. First, we measured intralysosomal pH using FITC-labelled dextran and found the intralysosomal pH in Pgh1 expressing cells to be decreased. A decreased lysosomal pH was not observed in cells expressing Pgh1 carrying the S1034C and N1042D double substitution found in some chloroquine-resistant P. falciparum parasites. Secondly, Pgh1-mediated uptake of chloroquine was abolished in the presence of bafilomycin A1, a specific inhibitor of vacuolar [H +]ATPases and was nearly abrogated in the presence of NH 4Cl. Finally, cells expressing wild-type Pgh1 showed increased uptake of both (+)- and (−)[ 3H]chloroquine enantiomers, indicating that Pgh1-mediated uptake of chloroquine is not enantioselective and in agreement with a pH-driven process. We conclude from these studies that Pgh1 does not transport chloroquine, but instead influences chloroquine accumulation by modulating the pH of acidic organelles. This function is abolished in Pgh1 carrying amino acid substitutions S1034C and N1042D. We speculate that the pfmdr1 gene encodes a vacuolar chloride channel.