Central Pore Research Articles

Asymmetric Two-Layer Porous Membrane for Gas Separation.

We present that the porous two-layer membranes of graphene and hexagonal boron nitride (h-BN) are promising for gas mixture separation. For the two-layer membranes, the mechanisms of the gas separation are (i) the different adsorption properties of gases on two membranes inducing a permeation flux difference from one side to the other and (ii) the asymmetric potential energy curves (potential energy of a gas molecule vs distance between the pore center and a gas molecule) of a two-layer membrane leading to a potential energy difference, which can affect gas permeation through the pore. As a concrete example, we explore the gas separation of CO2 and CH4 by the two-layer membrane using molecular dynamics simulations. Finally, on the basis of the distinctive permeation rates in the two directions, a gas separation system with two back-to-back arrayed graphene/h-BN membranes with big pores is designed to realize gas separation.

Permeability determination of hydrate sediments and a new reduction model considering hydrate growth habit

The saturation (SH) and growth habit of hydrates in sediments directly affect the evaluation of permeability. This work was devoted to evaluating the growth habit of hydrates in sediments by permeability measurements. The water effective permeability of hydrate sediments with different SH was obtained, and these permeability data were excellent in stability and accuracy. The change of differential pressure of hydrate sediments with water injection was completely recorded, it was a new found that the differential pressure was unstable in the first few minutes, and it is not recommended to employ short–term data to evaluate the permeability of the hydrate sediments. Based on the hypothesis of wedge tetrahedral filling of sediment particles, the growth habits of hydrates were compered in the models of hydrate coating grain and occupying pore center respectively. By analyzing the experimental data with the reduction models in different hydrate growth habits, it was founded that the hydrates were more likely to form in the pore centers rather than coating grains. Finally, the ratio of the internal surface area of pore space with and without hydrate was simplified to an expression related to SH in a new model of water relative permeability. This model could distinguish the growth habits of hydrate in porous media and provide a more accurate prediction of water relative permeability.

RNF126-Mediated Reubiquitination Is Required for Proteasomal Degradation of p97-Extracted Membrane Proteins

Valosin-containing protein (VCP)/p97 is an AAA-ATPase that extracts polyubiquitinated substrates from multimeric macromolecular complexes and biological membranes for proteasomal degradation. During p97-mediated extraction, the substrate is largely deubiquitinated as it is threaded through the p97 central pore. How p97-extracted substrates are targeted to the proteasome with few or no ubiquitins is unknown. Here, we report that p97-extracted membrane proteins undergo a second round of ubiquitination catalyzed by the cytosolic ubiquitin ligase RNF126. RNF126 interacts with transmembrane-domain-specific chaperone BAG6, which captures p97-liberated substrates. RNF126 depletion in cells diminishes the ubiquitination of extracted membrane proteins, slows down their turnover, and dramatically stabilizes otherwise transient intermediates in the cytosol. We reconstitute the reubiquitination of a p97-extracted, misfolded multispanning membrane protein with purified factors. Our results demonstrate that p97-extracted substrates need to rapidly engage ubiquitin ligase-chaperone pairs that rebuild the ubiquitin signal for proteasome targeting to prevent harmful accumulation of unfolded intermediates.

Revision of the genus Seminemacheilus, with the description of three new species (Teleostei: Nemacheilidae).

The nemacheilid genus Seminemacheilus is revised. Six species are recognised, three of them described herein as new. All species are endemic to Central Anatolia. Seminemacheilus lendlii from the Sakarya River drainage and the endorheic Lake Aksehir and Eber basins, S. ispartensis from Lake Eğirdir basin and S. ahmeti from Sultan Sazlığı are valid species. Although Seminemacheilus lendlii and S. ispartensis have almost identical COI sequences, they are distinguished by the shape of the caudal peduncle and the presence of scales on the caudal peduncle in S. ispartensis (vs. absent in S. lendlii). Seminemacheilus attalicus, new species, from Kırkgöz drainage, is distinguished by having a slightly emarginated caudal fin, a central pore in the supratemporal canal and a marbled flank pattern. Seminemacheilus ekmekciae, new species, from Lake Tuz basin is distinguished by having a roundish caudal fin, a short post-dorsal length and large brown blotches fused into stripes on the flank. Seminemacheilus tubae, new species, from Lake Beyşehir basin, is distinguished by having a truncate caudal fin and 2-5 (6) supraorbital head pores. Seminemacheilus dursunavsari from the Göksu River drainage is not a valid name and this population is identified as S. tubae.

The AAA+ ATPase Msp1 is a processive protein translocase with robust unfoldase activity

Msp1 is a conserved eukaryotic AAA+ ATPase localized to the outer mitochondrial membrane, where it is thought to extract mislocalized tail-anchored proteins. Despite recent in vivo and in vitro studies supporting this function, a mechanistic understanding of how Msp1 extracts its substrates is still lacking. Msp1's ATPase activity depends on its hexameric state, and previous characterizations of the cytosolic AAA+ domain in vitro had proved challenging due to its monomeric nature in the absence of the transmembrane domain. Here, we used a hexamerization scaffold to study the substrate-processing mechanism of the soluble Msp1 motor, the functional homo-hexameric state of which was confirmed by negative-stain electron microscopy. We demonstrate that Msp1 is a robust bidirectional protein translocase that is able to unfold diverse substrates by processive threading through its central pore. This unfoldase activity is inhibited by Pex3, a membrane protein proposed to regulate Msp1 at the peroxisome.

Generalized hydrodynamic analysis of transport through a finite open nanopore for two-component single-file systems

Single-file diffusion (SFD) in finite open nanopores is characterized by nonzero spatially varying tracer diffusion coefficients within a generalized hydrodynamic description. This contrasts with infinite SFD systems where tracer diffusivity vanishes. In standard tracer counterpermeation (TCP) analysis, two reservoirs, each containing a different species, are connected to opposite ends of a finite pore. We implement an extended TCP analysis to allow the two reservoirs to contain slightly different mixtures of the two species. Then, determination of diffusion fluxes through the pore allows extraction of diffusion coefficients for near-constant partial concentrations of the two species. This analysis is applied for a lattice-gas model describing two-component SFD through a finite linear pore represented by a one-dimensional array of cells. Two types of particles, A and B, can hop only to adjacent empty cells with generally different rates, h_{A} and h_{B}. Particles are noninteracting other than exclusion of multiple cell occupancy. Results reveal generalized hydrodynamic tracer diffusion coefficients which adopt small values inversely proportional to pore length in the pore center, but which are strongly enhanced near pore openings.

Measuring Open Porosity of Porous Materials Using THz-TDS and an Index-Matching Medium

The porosity of porous materials is a critical quality attribute of many products ranging from catalysis and separation technologies to porous paper and pharmaceutical tablets. The open porosity in particular, which reflects the pore space accessible from the surface, is crucial for applications where a fluid needs to access the pores in order to fulfil the functionality of the product. This study presents a methodology that uses terahertz time-domain spectroscopy (THz-TDS) coupled with an index-matching medium to measure the open porosity and analyze scattering losses of powder compacts. The open porosity can be evaluated without the knowledge of the refractive index of the fully dense material. This method is demonstrated for pellets compressed of pharmaceutical-grade lactose powder. Powder was compressed at four different pressures and measured by THz-TDS before and after they were soaked in an index-matching medium, i.e., paraffin. Determining the change in refractive index of the dry and soaked samples enabled the calculation of the open porosity. The results reveal that the open porosity is consistently lower than the total porosity and it decreases with increasing compression pressure. The scattering losses reduce significantly for the soaked samples and the scattering centers (particle and/or pore sizes) are of the order of or somewhat smaller than the terahertz wavelength. This new method facilitates the development of a better understanding of the links between material properties (particles size), pellet properties (open porosity) and performance-related properties, e.g., disintegration and dissolution performance of pharmaceutical tablets.

Enhanced pool boiling performance of a porous honeycomb copper surface with radial diameter gradient

Boiling heat transfer (BHT) is widely used in industry and in our daily lives, and it can be enhanced by micro/nano structures. Previously, porous honeycomb copper surfaces have been proven to significantly enhance BHT performance, but how a radial pore diameter gradient affects BHT is still unknown. In this paper, two porous honeycomb copper surfaces with uniform pore diameters of 60 and 120 μm, respectively (named Sample#U1 and Sample#U2), were prepared using the electrochemical deposition method. On this basis, a porous honeycomb surface with a radial diameter gradient was prepared by infusing liquid onto the reaction surface during the deposition process. The range of pore diameters from the center to the edge was 60–120 μm (named Sample#G1). Pool boiling experiment results showed that the sample with the radial diameter gradient had superior heat transfer performance. Sample#G1, with the smallest pores at the center and largest pores at the edge, had a maximum nucleate BHT coefficient hMNB of 16.1 W•cm–2 K–1, which was about 1.4 times that of Sample#U2, 1.5 times that of Sample#U1, and 3.5 times that of a pure copper surface. It was assumed that surfaces with a radial diameter gradient had a superior rewetting ability, and this was confirmed in a capillary wicking experiment. The K/Reff of the sample with the radial diameter gradient was larger than those of the samples with uniform diameter, which indicated that a radial diameter gradient surface could quickly accelerate the replenishment of water in pool boiling. A porous structure provides pores and cavities, which was benefit for activate bubble nucleation under a low superheat. Sampe#G1 had the lowest ΔTONB. The bubble dynamics also proved that the sample with the radial diameter gradient had a faster bubble growth speed than those with a uniform diameter, resulting in faster heat removal.

5-Hydroxymethyl-Furfural and Structurally Related Compounds Block the Ion Conductance in Human Aquaporin-1 Channels and Slow Cancer Cell Migration and Invasion.

Aquaporin-1 (AQP1) dual water and ion channels enhance migration and invasion when upregulated in leading edges of certain classes of cancer cells. Work here identifies structurally related furan compounds as novel inhibitors of AQP1 ion channels. 5-Hydroxymethyl-2-furfural (5HMF), a component of natural medicinal honeys, and three structurally related compounds, 5-nitro-2-furoic acid (5NFA), 5-acetoxymethyl-2-furaldehyde (5AMF), and methyl-5-nitro-2-furoate (M5NF), were analyzed for effects on water and ion channel activities of human AQP1 channels expressed in Xenopus oocytes. Two-electrode voltage clamp showed dose-dependent block of the AQP1 ion current by 5HMF (IC50 0.43 mM), 5NFA (IC50 1.2 mM), and 5AMF (IC50 ∼3 mM) but no inhibition by M5NF. In silico docking predicted the active ligands interacted with glycine 165, located in loop D gating domains surrounding the intracellular vestibule of the tetrameric central pore. Water fluxes through separate intrasubunit pores were unaltered by the furan compounds (at concentrations up to 5 mM). Effects on cell migration, invasion, and cytoskeletal organization in vitro were tested in high-AQP1-expressing cancer lines, colon cancer (HT29) and AQP1-expressing breast cancer (MDA), and low-AQP1-expressing SW480. 5HMF, 5NFA, and 5AMF selectively impaired cell motility in the AQP1-enriched cell lines. In contrast, M5NF immobilized all the cancer lines by disrupting actin cytoskeleton. No reduction in cell viability was observed at doses that were effective in blocking motility. These results define furans as a new class of AQP1 ion channel inhibitors for basic research and potential lead compounds for development of therapeutic agents targeting aquaporin channel activity. SIGNIFICANCE STATEMENT: 5-Hydroxymethyl-2-furfural (5HMF), a component of natural medicinal honeys, blocks the ion conductance but not the water flux through human Aquaporin-1 (AQP1) channels and impairs AQP1-dependent cell migration and invasiveness in cancer cell lines. Analyses of 5HMT and structural analogs demonstrate a structure-activity relationship for furan compounds, supported by in silico docking modeling. This work identifies new low-cost pharmacological antagonists for AQP1 available to researchers internationally. Furans merit consideration as a new class of therapeutic agents for controlling cancer metastasis.

Co-immobilization of a Rh Catalyst and a Keggin Polyoxometalate in the UiO-67 Zr-Based Metal-Organic Framework: In Depth Structural Characterization and Photocatalytic Properties for CO2 Reduction.

The Keggin-type polyoxometalate (POM) PW12O403- and the catalytic complex Cp*Rh(bpydc)Cl2 (bpydc = 2,2'-bipyridine-5,5'-dicarboxylic acid) were coimmobilized in the Zr(IV) based metal organic framework UiO-67. The POM is encapsulated within the cavities of the MOF by in situ synthesis, and then, the Rh catalytic complex is introduced by postsynthetic linker exchange. Infrared and Raman spectroscopies, 31P and 13C MAS NMR, N2 adsorption isotherms, and X-ray diffraction indicate the structural integrity of all components (POM, Rh-complex and MOF) within the composite of interest (PW12,Cp*Rh)@UiO-67. DFT calculations identified two possible locations of the POM in the octahedral cavities of the MOF: one at the center of a UiO-67 pore with the Cp*Rh complex pointing toward an empty pore and one off-centered with the Cp*Rh pointing toward the POM. 31P-1H heteronuclear (HETCOR) experiments ascertained the two environments of the POM, equally distributed, with the POM in interaction either with the Cp* fragment or with the organic linker. In addition, Pair Distribution Function (PDF) data were collected on the POM@MOF composite and provided key evidence of the structural integrity of the POM once immobilized into the MOF. The photocatalytic activity of the (PW12,Cp*Rh)@UiO-67 composite for CO2 reduction into formate and hydrogen were evaluated. The formate production was doubled when compared with that observed with the POM-free Cp*Rh@UiO-67 catalyst and reached TONs as high as 175 when prepared as thin films, showing the beneficial influence of the POM. Finally, the stability of the composite was assessed by means of recyclability tests. The combination of XRD, IR, ICP, and PDF experiments was essential in confirming the integrity of the POM, the catalyst, and the MOF after catalysis.

On the remarkable nesting biology of an Australian bee in the genus Trichocolletes Cockerell (Hymenoptera: Colletidae)

AbstractNests and life‐cycle stages of bees in the endemic Australian bee genus Trichocolletes have been little studied hitherto. Discovery of a nesting aggregation of Trichocolletes orientalis near Perth, Western Australia, provided an excellent opportunity to extend knowledge of the genus, and some surprising facts have emerged from observations made in the field and laboratory. Brood cells were more or less vertically oriented, had cemented earthen walls and were lined internally with thin membranes of cellophane‐like material as is usual for Colletidae. Cell plugs, however, were unusual in that the inner surface of each was thinly coated with white, dry foam and had a central pore. Simple tests indicated that the foam was composed of a wax‐like lipid. Adults were in flight at the nesting site during August and September but absent in November and December when brood cells were excavated. These cells contained mature, pre‐defaecating larvae (not prepupae, as was expected) and no pollen provision or faeces. The larvae and the cell walls around them were heavily coated with oil, evidently pollenkitt derived from pollen in the larval mid‐gut. The possible function and fate of this oil is discussed. Larvae remained dormant through summer, then defaecated, pupated and reached pharate adult stage by late April. Adults eclosed in late May and early June and had very well‐developed fat bodies, so presumably, they overwinter in the nests. Notes on nests of some other Trichocolletes species are included for comparison.

Dermoscopic rosettes as a clue for atypical molluscum contagiosum

Molluscum contagiosum (MC), a frequent viral infection, is generally easy to diagnose because of its characteristic clinical features. However, atypical presentations can be a diagnostic challenge for clinicians. Dermoscopy has helped in this cases by showing a characteristic dermoscopic pattern composed of a central pore or umbilication in conjunction with polylobular white to yellow amorphous structures, surrounded by linear or branched vessels (‘red crown”). However, additional dermoscopic patterns can be found. Herein we present two MC cases where rosettes were seen on dermoscopy

Dendritic Mesoporous Organosilica Nanoparticles: A pH-Triggered Autocatalytic Fenton Reaction System with Self-supplied H2O2 for Generation of High Levels of Reactive Oxygen Species.

Dendritic mesoporous silica nanoparticles represent a new biomedical application platform due to their special central radial pore structure for the loading of drugs and functional modification. Herein, we report functionalized dendritic mesoporous organosilica nanoparticles (DMONs), a pH-triggered Fenton reaction generator (TA/Fe@GOD@DMONs), incorporating natural glucose oxidase (GOD) in the DMONs with tannic acid (TA) grafted using Fe3+ on the surface, that have been designed and constructed for efficient tumor ablation with self-supplied H2O2 and accelerated conversion of Fe3+/Fe2+ by TA. In view of the deficiency of endogenous H2O2, the self-supply through the TA/Fe@GOD@DMONs platform represented a high-yielding source of peroxygen. Furthermore, the production of Fe2+ induced by TA greatly improved the efficiency of the Fenton reaction resulting in significant tumor inhibition. This new design represents as novel paradigm for the development of autocatalytic Fenton nanosystems for effective treatment of tumors.

Structural and molecular insight into the pH-induced low-permeability of the voltage-gated potassium channel Kv1.2 through dewetting of the water cavity.

Understanding the gating mechanism of ion channel proteins is key to understanding the regulation of cell signaling through these channels. Channel opening and closing are regulated by diverse environmental factors that include temperature, electrical voltage across the channel, and proton concentration. Low permeability in voltage-gated potassium ion channels (Kv) is intimately correlated with the prolonged action potential duration observed in many acidosis diseases. The Kv channels consist of voltage-sensing domains (S1-S4 helices) and central pore domains (S5-S6 helices) that include a selectivity filter and water-filled cavity. The voltage-sensing domain is responsible for the voltage-gating of Kv channels. While the low permeability of Kv channels to potassium ion is highly correlated with the cellular proton concentration, it is unclear how an intracellular acidic condition drives their closure, which may indicate an additional pH-dependent gating mechanism of the Kv family. Here, we show that two residues E327 and H418 in the proximity of the water cavity of Kv1.2 play crucial roles as a pH switch. In addition, we present a structural and molecular concept of the pH-dependent gating of Kv1.2 in atomic detail, showing that the protonation of E327 and H418 disrupts the electrostatic balance around the S6 helices, which leads to a straightening transition in the shape of their axes and causes dewetting of the water-filled cavity and closure of the channel. Our work offers a conceptual advancement to the regulation of the pH-dependent gating of various voltage-gated ion channels and their related biological functions.

Influence of Carbon Dioxide on the Adsorption of Methane by Coal Using Low-Field Nuclear Magnetic Resonance

The adsorption capacity of coal with respect to carbon dioxide (CO2) is greater than that of methane (CH4). Studying the CO2 replacement of CH4 in coal is essential to understanding the mechanism behind CO2-enhanced coalbed methane. In this paper, scanning electron microscopy was used to qualitatively study the influence of CO2 injection pressure on the fracture characteristics of coal. The time effect of the CH4/CO2 adsorption and the effect of the CO2 injection on the CH4 adsorption were quantitatively studied using low-field nuclear magnetic resonance. The results suggest three different CH4 states in the coal samples: CH4 adsorbed on the pore surface, free CH4 in the pore center, and free CH4 between the coal particles. Indeed, the greater the CO2 injection pressure, the more developed the fracturing and network, which, in turn, increases connectivity. As the adsorption time of CH4/CO2 increases, the adsorption rate of CO2/CH4 gradually decreases. In essence, CO2 preferentially replaces the CH4 of the minipores. As the CO2 injection pressure increases, the difference between the CO2 equilibrium pressure and the initial injection pressure increases. Moreover, the CH4 production rate increases, the transverse relaxation time (T2) spectrum of the adsorption-state area decreases, the T2 spectrum of the free-state area increases, and the number of adsorption holes gradually decreases, whereas the number of seepage holes increases.

Improving watershed-based pore-network extraction method using maximum inscribed ball pore-body positioning

Pore-scale modeling is a rapidly evolving area of research because modeling multiphase flow directly on 3D pore geometries is of utmost importance in wide variety of research areas, including oil and gas development, hydrology and material sciences. Although there are numerous methods to model flow, only so-called pore-network models are computationally effective enough to perform simulations in large modeling domains, and they are orders of magnitude faster than direct modeling approaches. However, pore-network models require a simplification of the 3D pore geometry to perform simulations, which are usually referred to as pore-network extraction. Such extraction poses a separate problem because it must provide an accurate description of the pore space geometry and topology. Different methods have been proposed in the literature. Recently, watershed-based approaches have been popular due to their effectiveness in working with porous media images of any porosity. A watershed algorithm requires seed placement to segment the space into distinct pores. We propose a hybrid algorithm combining the power of watersheds in finding intersections between pores with the advantages of the maximum inscribed ball technique, which is very effective in finding pore centers. We rigorously verify and test our novel methodology on artificial and X-ray microtomography images of wide variety of porous materials: sphere packings, carbonate, soil, ceramic and sandstone samples. Comparison against a purely watershed-based method and results based solely on the maximum inscribed balls–based method (in terms of pore/throat total number, pore size distributions and connection statistics, and multiphase flow properties including capillary curves and relative permeabilities) revealed the accuracy of our novel technique, consistency with existing classical techniques and great potential in analysing 3D pore images of any complexity. On the other hand, comparison of extracted pore-network topology (as based on Euler number) revealed significant differences between different methodologies, which is rather surprising considering the similarities in two-phase flow properties. While analysing permeability results, we also compared two popular pore-throat partitioning models and advocated in favor of the weight model usually utilized within watershed-based extracted pore networks. Our results illuminate problems in current pore-network models and outline some potential ways to improve their accuracy in future research.

Effects of pore size on water dynamics in mesoporous silica.

Water confined in mesoporous silica plays a central role in its many uses ranging from gas sorption to nanoconfined chemical reactions. Here, the influence of pore diameter (2.5-5.4 nm) on water hydrogen bond (H-bond) dynamics in MCM41 and SBA15 mesoporous silicas is investigated using femtosecond infrared vibrational spectroscopy and molecular dynamics simulations on selenocyanate (SeCN-) anions dissolved in the pores. As shown recently, SeCN- spectral diffusion is a reliable probe of surrounding water H-bond structural motions. Additionally, the long CN stretch vibrational lifetime facilitates measurement of the full range of confined dynamics, which are much slower than in bulk water. The simulations shed light on quantitative details that are inaccessible from the spatially averaged observables. The dependence of SeCN- orientational relaxation and that of spectral diffusion on the distance from the silica interface are quantitatively described with an exponential decay and a smoothed step-function, respectively. The distance-dependence of both quantities is found to be independent of the diameter of the pores, and the spatial distribution of SeCN- is markedly non-uniform, reaching a maximum between the interface and the pore center. The results indicate that the commonly invoked two-state, or core-shell, model is a more appropriate description of spectral diffusion. Using these insights, we model the full time-dependence of the measured dynamics for all pore sizes and extract the "core" and "shell" dynamical correlation functions and SeCN- spatial probability distributions. The results are critically compared to those for water confined in reverse micelles.

Structure of the Native Muscle-type Nicotinic Receptor and Inhibition by Snake Venom Toxins

The nicotinic acetylcholine receptor, a pentameric ligand-gated ion channel, converts the free energy of binding of the neurotransmitter acetylcholine into opening of its central pore. Here we present the first high-resolution structure of the receptor type found in muscle-endplate membrane and in the muscle-derived electric tissues of fish. The native receptor was purified from Torpedo electric tissue and functionally reconstituted in lipids optimal for cryo-electron microscopy. The receptor was stabilized in a closed state by the binding of α-bungarotoxin. The structure reveals the binding of a toxin molecule ateach of two subunit interfaces in a manner that would block the binding of acetylcholine. It also reveals a closed gate in the ion-conducting pore, formed by hydrophobic amino acid side chains, located ∼60Å from the toxin binding sites. The structure provides a framework for understanding gating in ligand-gated channels and how mutations in the acetylcholine receptor cause congenital myasthenic syndromes.

Characterization of Sodium Bicarbonate Transporters NBCe1 and NBCn1 as CO2 channels

The known “gas channels” are members of the AQP and Rh families, and appear to conduct CO2 mainly through the stable hydrophobic central pores of these multimeric proteins. The present work helps to define a new class of gas‐conduction pathway: transporters that may conduct CO2 through cracks that form transiently as the protein cycles through various conformations—much like H2O can leak through certain membrane proteins. To identify CO2 channels, we developed a novel tool—expressing AQP5 in Xenopus oocytes, followed by injection of small amounts of carbonic anhydrase II (CAII), and then exposure of the cell to CO2/HCO3− solutions. Cytosolic CAII promotes the consumption of incoming CO2, thereby maintaining a low [CO2] at the inner surface of the cell membrane, and maximizing the gradient for CO2 influx. A semiquantitative measure of this CO2 influx is the spiking increase in surface pH (DpHS) that occurs as CO2 enters the cell, lowers [CO2] at the outer surface of the cell membrane, causes some HCO3− in the vicinity of the membrane to undergo the reaction HCO3− + H+ ® H2O + CO2, and thereby replenishes some of the lost CO2. The pHS spike is greatest near time‐zero, when the inward CO2 gradient—and thus the reaction HCO3− + H+ ® H2O + CO2—is greatest, and then exponentially wanes as CO2 equilibrates across the cell membrane. Oocytes expressing AQP5 and then injected with Tris buffer (but not CAII) have a much greater DpHS than controls injected with H2O (not cRNA) and Tris (not CAII), confirming that AQP5 is an excellent CO2 channel. Injecting a small amount of CAII (dissolved in Tris) into oocytes not expressing AQP5 produces only a slight increase in DpHS because the rate‐limiting step is membrane CO2 permeability. However, for AQP5‐expressing oocytes, injecting this same small amount of CAII produces a very large increase in DpHS. In other words, the CAII‐dependent change in spike height—D(DpHS)—is much greater in AQP5‐expressing oocytes than H2O‐injected controls. We conclude that the measured D(DpHS) in ±CAII experiments is a powerful tool for assessing CO2 conductance. We applied this method to measuring the CO2 conductance of Na/HCO3 co‐transporters, NBCe1‐A and NBCn1‐G. Preliminary data show that, in the presence of cytosol CAII, D(DpHS) is >1.5‐fold greater in the oocytes expressing NBCe1‐A than the H2O‐injected controls, supporting the hypothesis that NBCe1‐A has a CO2 conductance. Similarly, D(DpHS) is >5‐fold greater in the oocytes expressing NBCn1‐G than the H2O‐injected controls, supporting the hypothesis that NBCn1‐G has a CO2 conductance.Support or Funding InformationSupported by grants GM111251 (the U grant) and N000141612535 (the MURI grant)

Mechanical mechanism for the translocation of hexameric and nonstructural helicases: Dependence on physical parameters.

Three recently observed facts of the translocation of actual hexameric and nonstructural (NS) helicases are related to the various physical quantities and are in accordance with the recently proposed mechanical mechanism: a) the translocation of hexameric helicases might be led by either the N-terminal domain (NTD) or C-terminal domain (CTD) depending on which domain has a smaller central pore, b) the translocation speed (vt) of the ring-shaped helicases and NS helicases decreased with decreasing applied tension, and c) a large difference in the vt of the NS helicase was observed for the helicase translocating on DNA and RNA. These findings are the effects of the physical quantities of the helicase/nuclei acid strands on the translocation of helicases and are difficult to explain with biochemical models. We predict that a similar behavior as described in b) and c) is also shown by hexameric helicases. The validity of the mechanical mechanism is demonstrated in simulation experiments.