Insights into the mechanisms of the selectivity filter of Escherichia coli aquaporin Z

Aquaporin Z (AQPZ) is a tetrameric protein that forms water channels in the cell membrane of Escherichia coli. The histidine residue (residue 174) in the selectivity filter (SF) region plays an important role in the transport of water across the membrane. In this work, we perform equilibrium molecular dynamics (MD) simulations to illustrate the gating mechanism of the SF and the influences of residue 174 in two different protonation states: Hsd174 with the proton at Nδ, and Hse174 with the proton at Nε. We calculate the pore radii in the SF region versus the simulation time. We perform steered MD to compute the free-energy profile, i.e., the potential of mean force (PMF) of a water molecule through the SF region. We conduct a quantum mechanics calculation of the binding energy of one water molecule with the residues in the SF region. The hydrogen bonds formed between the side chain of Hsd174 and the side chain of residue 189 (Arg189) play important roles in the selectivity mechanism of AQPZ. The radii of the pores, the hydrogen-bond analysis, and the free energies show that it is easier for water molecules to permeate through the SF region of AQPZ with residue 174 in the Hse state than in the Hsd state.     

Concerted action of two cation filters in the aquaporin water channel

Aquaporin (AQP) facilitated water transport is common to virtually all cell membranes and is marked by almost perfect specificity and high flux rates. Simultaneously, protons and cations are strictly excluded to maintain ionic transmembrane gradients. Yet, the AQP cation filters have not been identified experimentally. We report that three point mutations turned the water-specific AQP1 into a proton/alkali cation channel with reduced water permeability and the permeability sequence: H⁺ ≫K⁺ >Rb⁺ >Na⁺ >Cs⁺ >Li⁺. Contrary to theoretical models, we found that electrostatic repulsion at the central asn-pro-ala (NPA) region does not suffice to exclude protons. Full proton exclusion is reached only in conjunction with the aromatic/arginine (ar/R) constriction at the pore mouth. In contrast, alkali cations are blocked by the NPA region but leak through the ar/R constriction. Expression of alkali-leaking AQPs depolarized membrane potentials and compromised cell survival. Our results hint at the alkali-tight but solute-unselective NPA region as a feature of primordial channels and the proton-tight and solute-selective ar/R constriction variants as later adaptations within the AQP superfamily.     

Molecular Mechanisms of How Mercury Inhibits Water Permeation through Aquaporin-1: Understanding by Molecular Dynamics Simulation

Aquaporin (AQP) functions as a water-conducting pore. Mercury inhibits the water permeation through AQP. Although site-directed mutagenesis has shown that mercury binds to Cys¹⁸⁹ during the inhibition process, it is not fully understood how this inhibits the water permeation through AQP1. We carried out 40 ns molecular dynamics simulations of bovine AQP1 tetramer with mercury (Hg-AQP1) or without mercury (Free AQP1). In Hg-AQP1, Cys¹⁹¹ (Cys¹⁸⁹ in human AQP1) is converted to Cys-SHg⁺ in each monomer. During each last 10 ns, we observed water permeation events occurred 23 times in Free AQP1 and never in Hg-AQP1. Mercury binding did not influence the whole structure, but did induce a collapse in the orientation of several residues at the ar/R region. In Free AQP1, backbone oxygen atoms of Gly¹⁹⁰, Cys¹⁹¹, and Gly¹⁹² lined, and were oriented to, the surface of the water pore channel. In Hg-AQP1, however, the SHg⁺ of Cys191-SHg⁺ was oriented toward the outside of the water pore. As a result, the backbone oxygen atoms of Gly¹⁹⁰, Cys¹⁹¹, and Gly¹⁹² became disorganized and the ar/R region collapsed, thereby obstructing the permeation of water. We suggest that mercury disrupts the water pore of AQP1 through local conformational changes in the ar/R region.     

High resolution nuclear magnetic resonance studies of the active site of chymotrypsin. I. The hydrogen bonded protons of the "charge relay" system

High resolution proton nuclear magnetic resonance has been used to observe protons at the active site of chymotrypsin A_δ and at the same region of chymotrypsinogen A. A single resonance with the intensity of one proton is located in the low field region of the nuclear magnetic resonance spectrum. This resonance is observed in H₂O solutions but not in ²H₂O. On going from low to high pH the resonance titrates upfield 3 parts per million in both proteins and has a pK of 7.5. The titration can be prevented by alkylating His57 with either of two active site directed chloromethyl ketones. Using these data the proton resonance has been assigned to a proton in a hydrogen bond between His57 and Asp102. Further confirmation of this assignment lies in the observation of a similar resonance in this same low field region of the nuclear magnetic resonance spectrum of trypsin, trypsinogen, subtilisin BPN′ and α-lytic protease all of which have the Asp-His-Ser triad at their active sites.This proton resonance in chymotrypsin A_δ was used as a probe to monitor the charge state of the active site upon formation of a stable acyl-enzyme analogue N²(N-acetylalanyl)-N¹benzoylcarbazoyl-chymotrypsin A_δ. In this derivative the His-Asp proton resonance titrates from the same low pH end point as in the native enzyme, ?18 parts per million, to a new high pH end point of ?14.4 parts per million (versus ?15.0 parts per million in the native enzyme). The difference of 0.6 parts per million in the high pH end points between the native and acyl enzyme is interpreted as supporting the suggestion that a hydrogen bond exists between Ser195 and His57 in the native enzyme and zymogen.We conclude from these studies that the charge relay system from Asp102 across His57 to Ser195 is intact in chymotrypsin A_δ and chymotrypsinogen A, and that, in the native enzyme, it slightly polarizes Ser195.     

Lactones in the Flavor of Heated Pork Fat

Pork fat was heated at 160~170°C for 3 hr under bubbling with air, and the volatile compounds were collected in the cold trap. After the acidic compounds were removed from the volatile compounds by extraction with 3% aqueous sodium carbonate solution, lactones were obtained from the nonacidic compounds by saponification. Gas chromatographic analyses of lactones were carried out on the PEG-20M and Apiezon L packed columns, and then each lactone was fractionated by repeated gas chromatography. Each isolated lactone was identified by infrared spectrometry, and also three major lactones were identified by mass spectrometry. Consequently, γ-C₅—C₁₂ and δ-C₉, δ-C₁₀, δ-C₁₂ and δ-C₁₄ lactones were found in the flavor of heated pork fat. Gamma-lactones, especially γ-C₇, γ-C₈ and γ-C₉, were predominant in the flavor, and unsaturated lactones were not detected. Mechanisms for the formation of the lactones were discussed.     

High resolution nuclear magnetic resonance studies of the active site of chymotrypsin. II. Polarization of histidine 57 by substrate analogues and competitive inhibitors

The proton nuclear magnetic resonance signal of the His57-Asp102 hydrogen bonded proton in the charge relay system of chymotrypsinogen A and chymotrypsin A_δ has been monitored to determine the influence of substrate analogues and competitive inhibitors on the electronic state of the active site regions. Borate ion, benzene boronic acid and 2-phenylethylboronic acid, when bound to chymotrypsin at pH 9.5 shift the resonance position of the His-Asp hydrogen bonded proton to ?15.9, ?16.3 and ?17.2 parts per million, respectively. These positions are intermediate between the low pH position in the free enzyme of ?18.0 parts per million and the high pH position of ?14.9 parts per million. The presence of these analogues prevents the His-Asp proton resonance from titrating in the region of pH 6 to 9.5. Similar low field shifts are observed for the hydrogen bonded proton resonance of subtilisin BPN′ when complexed with these boronic acids. The results support the chemical and crystallographic data which show that negatively charged tetrahedral adducts of the boronic acid substrate analogues are formed at the active sites of these enzymes. When combined with similar nuclear magnetic resonance data for the binding of N-acetyl-l-tryptophan to chymotrypsin A_δ, they suggest that a direct interaction occurs between the active site histidine and the atom occupying the leaving group position of the substrate, presumably a hydrogen bond.The His-Asp proton resonance was also monitored in complexes of chymotrypsin A_δ with bovine pancreatic trypsin inhibitor over the pH range 4 to 9. In the complex the low field proton resonance had a field position of ?14.9 parts per million over the pH range 4 to 9 indicating that His57 is in the neutral form, similar to the active enzyme at high pH.     

A competing salt-bridge suppresses helix formation by the isolated C-peptide carboxylate of ribonuclease A

The C-peptide of ribonuclease A (residues 1 to 13) is obtained by cyanogen bromide cleavage at Met13, which converts methionine to a mixture of homoserine lactone (giving C-peptide lactone) and homoserine carboxylate (giving C-peptide carboxylate). The helix-forming properties of C-peptide lactone have been reported. The helix is formed intramolecularly in aqueous solution, is stabilized at low temperatures (0 to 20 °C) and also by a pH-dependent interaction between sidechains. The C-peptide lactone helix is about 1000-fold more stable than expected from “host-guest” data for helix formation in synthetic polypeptides.Here we report the failure of C-peptide carboxylate to form an α-helix in comparable conditions. Formation of a salt-bridge between the α-COO^? group and the imidazolium ring of His12⁺ appears to be responsible for the suppression of helix formation. The presence of the Hse13-COO^? … His12⁺ salt-bridge in C-peptide carboxylate is shown by ¹H nuclear magnetic resonance titration of the amide proton resonances of His12 and Hse13, and is expected from model peptide studies. The most probable reason why C-peptide carboxylate does not form an α-helix is that the Hse13-COO^? … His12⁺ salt-bridge competes successfully with a helix stabilizing salt-bridge (Glu9^? … His12⁺).S-peptide (residues 1 to 20 of ribonuclease A) does form an α-helix with properties similar to those of the C-peptide (lactone) helix, which shows that the lactone ring of C-peptide lactone is not needed for helix formation.These results support the hypothesis that a Glu9^? … His12⁺ salt-bridge stabilizes the C-peptide (lactone) helix, and they show that specific interactions between side-chains can be important in preventing as well as in promoting α-helix formation.     

Combination therapy with bone marrow stromal cells and FK506 enhanced amelioration of ischemic brain damage in rats

AimsWe previously reported that cysteinyl leukotriene receptor 2 (CysLT₂) mediates ischemic astrocyte injury, and leukotriene D₄-activated CysLT₂ receptor up-regulates the water channel aquaporin 4 (AQP4). Here we investigated the mechanism underlying CysLT₂ receptor-mediated ischemic astrocyte injury induced by 4-h oxygen-glucose deprivation and 24-h recovery (OGD/R).Main methodsPrimary cultures of rat astrocytes were treated by OGD/R to construct the cell injury model. AQP4 expression was inhibited by small interfering RNA (siRNA). The expressions of AQP4 and CysLTs receptors, and the MAPK signaling pathway were determined.Key findingsOGD/R induced astrocyte injury, and increased expression of the CysLT₂ (but not CysLT₁) receptor and AQP4. OGD/R-induced cell injury and AQP4 up-regulation were inhibited by a CysLT₂ receptor antagonist (Bay cysLT2) and a non-selective CysLT receptor antagonist (Bay u9773), but not by a CysLT₁ receptor antagonist (montelukast). Knockdown of AQP4 by siRNA attenuated OGD/R injury. Furthermore, OGD/R increased phosphorylation of ERK1/2 and p38, whose inhibitors relieved the cell injury and AQP4 up-regulation.SignificanceThe CysLT₂ receptor mediates AQP4 up-regulation in astrocytes, and up-regulated AQP4 leads to OGD/R-induced injury, which results from activation of the ERK1/2 and p38 MAPK pathways.     

Dynamics and energetics of water permeation through the aquaporin channel

Structural properties of water inside bovine aquaporin-1 are investigated by molecular simulation. The calculations, which are based on the recently determined X-ray structure at 2.2 A resolution (Sui et al., Nature 2001;414:872-878), are carried out on one monomeric subunit immersed in a water-n-octane-water bilayer. Molecular dynamics (MD) simulations suggest that His182, a fully conserved residue in the channel pore, is protonated in the delta position. Furthermore, they reveal a highly ordered water structure in the channel, induced by the electrostatic properties of the protein. Multiple-steering MD simulations are used to calculate the free-energy of water diffusion. To the best of our knowledge, this represents the first free-energy calculation based on the new, high-resolution structure of the pore. The calculated barrier is 2.5 kcal/mol, and it is associated to water permeation through the Asn-Pro-Ala (NPA) region of the pore, where water molecules are only hydrogen-bonded with themselves. These findings are fully consistent with those based on the previous MD studies on the human protein (de Groot and Grubmüller, Science 2001;294:2353-2357).     

Origins of proton transport behavior from selectivity domain mutations of the aquaporin-1 channel

The permeation free-energy profile and maximum ion conductance of proton transport along the channel of three aquaporin-1 (AQP1) mutants (H180A/R195V, H180A, and R195V) are calculated via molecular dynamics simulations and Poisson-Nernst-Planck theory. The proton dynamics was described by the multistate empirical valence bond (MS-EVB) model. The results reveal three major contributions to the overall free-energy barrier for proton transport in AQP1: 1), the bipolar field, 2), the electrostatic repulsion due to the Arg-195 residue, and 3), the dehydration penalty due to the narrow channel pore. The double mutation (H180A/R195V) drastically drops the overall free-energy barrier by roughly 20 kcal/mol via simultaneously relaxing the direct electrostatic interaction (by R195V) and dehydration effect (by H180A).     

Amide 1H n.m.r. study of the folding of ribonuclease C-peptide

The folding of ribonuclease A 1–13 (C-peptide) in H₂O near 0°C has been monitored by means of the amide and side chain NH proton resonances. The C-peptide carboxylate at low temperature forms, in a significant amount, a folded structure similar to the one that the 1–19 S-peptide adopts in the same conditions (3–13 α-helix). A quantitative comparison between helix stabilities of the lactone and carboxylate forms of C-peptide and S-peptide is reported. It is concluded that the proposed His 12⁺ … Hse 13 (COO^? salt bridge, which competes with the one-turn stabilizing salt bridge His 12⁺ … Glu 9^? in the C-peptide carboxylate, does not suppress helix formation as previously suggested but it merely reduces its stability. The behaviour of the N⁵-H resonance of the Arg 10⁺ side chain provides evidence for its implication in a further stabilizing interaction, most probably with Glu 2^?.     

Some insights into the binding mechanism of the GABAA receptor: a combined docking and MM-GBSA study

Gamma-aminobutyric type A receptor (GABA_AR) is a member of the Cys-loop family of pentameric ligand gated ion channels (pLGICs). It has been identified as a key target for many clinical drugs. In the present study, we construct the structure of human 2α₁2β₂γ₂ GABA_AR using a homology modeling method. The structures of ten benzodiazepine type drugs and two non-benzodiazepine type drugs were then docked into the potential benzodiazepine binding site on the GABA_AR. By analyzing the docking results, the critical residues His102 (α₁), Phe77 (γ₂) and Phe100 (α₁) were identified in the binding site. To gain insight into the binding affinity, molecular dynamics (MD) simulations were performed for all the receptor–ligand complexes. We also examined single mutant GABA_AR (His102A) in complexes with the three drugs (flurazepam, eszopiclone and zolpidem) to elucidate receptor–ligand interactions. For each receptor–ligand complex (with flurazepam, eszopiclone and zolpidem), we calculated the average distance between the C_α of the mutant residue His102A (α₁) to the center of mass of the ligands. The results reveal that the distance between the C_α of the mutant residue His102A (α₁) to the center of flurazepam is larger than that between His102 (α₁) to flurazepam in the WT type complex. Molecular mechanic-generalized Born surface area (MM-GBSA)-based binding free energy calculations were performed. The binding free energy was decomposed into ligand-residue pairs to create a ligand-residue interaction spectrum. The predicted binding free energies correlated well (R ²?=?0.87) with the experimental binding free energies. Overall, the major interaction comes from a few groups around His102 (α₁), Phe77 (γ₂) and Phe100 (α₁). These groups of interaction consist of at least of 12 residues in total with a binding energy of more than 1 kcal mol^?1. The simulation study disclosed herein provides a meaningful insight into GABA_AR–ligand interactions and helps to arrive at a binding mode hypothesis with implications for drug design.     

Studies on the Changes of Meat Fats by Various Processings

Data are presented to show the gas chromatographic identification of a total of 18 saturated aliphatic γ- and δ-lactones obtained from melted beef depot fat, namely, δ-C₆, γ-C₇, γ-C₈, γ-C₉, and a homologous series of γ- and δ-lactones of the even-carbon numbers C₁₀ to C₁₆ and of smaller amount of the odd-carbon numbers C₁₁ to C₁₅. These lactones were isolated by steam distillation and silicic acid adsorption chromatography, and identified through gas chromatography and infrared spectroscopy.

Lactones obtained had a peach-like flavor, and it was suggested that lactones were important in heated beef fat as the flavor compounds.     

Computational modelling and molecular dynamics simulations of a cyclic peptide mimotope of the CD52 antigen complexed with CAMPATH-1H antibody

A peptide mimotope of the CD52 antigen with the sequence T₁SSPSAD₇ has been co-crystallised with the CAMPATH-1H antibody. Molecular dynamics (MD) simulations in explicit water of the T₁SSPSAD₇ peptide in both antibody free and bound states showed that the peptide's β-turn remained stable in the bound state but it was eliminated in the free state. Based on the observation that Thr₁ and Ala₆ residues made close contacts through their side chain, a new peptide mimotope is proposed: (S,S)-C₁SSPSCD₇. Thr₁ and Ala₆ residues have been mutated in Cys residues and a disulphide bond has been imposed. The new analogue has been simulated in both antibody bound and free states with MD in explicit water. It was found that the peptide remained in the stable β-turn conformation, both in complexed and free states. The difference in configurational entropy was estimated to be 0.15 kJ/K/mol. However, despite the structural similarity, the cyclic analogue lost more than 25% of its buried surface area contact with the antibody and a couple of critical hydrogen bond interactions were broken. It is concluded that design of cyclic analogues that mimic the bound conformation of peptides should be carefully performed and conformational ‘freezing’ does not necessarily guarantee better binding.     

Roles of the Hydrophobic Gate and Exit Channel in Vigna radiata Pyrophosphatase Ion Translocation

Membrane-embedded pyrophosphatase (M-PPase) hydrolyzes pyrophosphate to drive ion (H⁺ and/or Na⁺) translocation. We determined crystal structures and functions of Vigna radiata M-PPase (VrH⁺-PPase), the VrH⁺-PPase–2P_i complex and mutants at hydrophobic gate (residue L555) and exit channel (residues T228 and E225). Ion pore diameters along the translocation pathway of three VrH⁺-PPases complexes (P_i-, 2P_i- and imidodiphosphate-bound states) present a unique wave-like profile, with different pore diameters at the hydrophobic gate and exit channel, indicating that the ligands induced pore size alterations. The 2P_i-bound state with the largest pore diameter might mimic the hydrophobic gate open. In mutant structures, ordered waters detected at the hydrophobic gate among VrH⁺-PPase imply the possibility of solvation, and numerous waters at the exit channel might signify an open channel. A salt-bridge, E225–R562 is at the way out of the exit channel of VrH⁺-PPase; E225A mutant makes the interaction eliminated and reveals a decreased pumping ability. E225–R562 might act as a latch to regulate proton release. A water wire from the ion gate (R-D-K-E) through the hydrophobic gate and into the exit channel may reflect the path of proton transfer.     

Arabidopsis thaliana NIP7;1: an anther-specific boric acid transporter of the aquaporin superfamily regulated by an unusual tyrosine in helix 2 of the transport pore

Plant nodulin-26 intrinsic proteins (NIPs) are members of the aquaporin superfamily that serve as multifunctional transporters of uncharged metabolites. In Arabidopsis thaliana, a specific NIP pore subclass, known as the NIP II proteins, is represented by AtNIP5;1 and AtNIP6;1, which encode channel proteins expressed in roots and leaf nodes, respectively, that participate in the transport of the critical cell wall nutrient boric acid. Modeling of the protein encoded by the AtNIP7;1 gene shows that it is a third member of the NIP II pore subclass in Arabidopsis. However, unlike AtNIP5;1 and AtNIP6;1 proteins, which form constitutive boric acid channels, AtNIP7;1 forms a channel with an extremely low intrinsic boric acid transport activity. Molecular modeling and molecular dynamics simulations of AtNIP7;1 suggest that a conserved tyrosine residue (Tyr81) located in transmembrane helix 2 adjacent to the aromatic arginine (ar/R) pore selectivity region stabilizes a closed pore conformation through interaction with the canonical Arg220 in ar/R region. Substitution of Tyr81 with a Cys residue, characteristic of established NIP boric acid channels, results in opening of the AtNIP7;1 pore that acquires a robust, transport activity for boric acid as well as other NIP II test solutes (glycerol and urea). Substitution of a Phe for Tyr81 also opens the channel, supporting the prediction from MD simulations that hydrogen bond interaction between the Tyr81 phenol group and the ar/R Arg may contribute to the stabilization of a closed pore state. Expression analyses show that AtNIP7;1 is selectively expressed in developing anther tissues of young floral buds of A. thaliana, principally in developing pollen grains of stage 9-11 anthers. Because boric acid is both an essential nutrient as well as a toxic compound at high concentrations, it is proposed that Tyr81 modulates transport and may provide an additional level of regulation for this transporter in male gametophyte development.     

Nitric oxide conduction by the brain aquaporin AQP4

Involvement of aquaporins in gas conduction across the membrane and the physiological significance of this process have attracted marked attention from both experimental and theoretical studies. Previous work demonstrated that AQP1 is permeable to both CO₂ and O₂. Here we employ various simulation techniques to examine the permeability of the brain aquaporin AQP4 to NO and O₂ and to describe energetics and pathways associated with these phenomena. The energy barrier to NO and O₂ permeation through AQP4 central pore is found to be only ～3 kcal mol^?1. The results suggest that the central pore of AQP4, similar to that of AQP1, can indeed conduct gas molecules. Interestingly, despite a longer and narrower central pore, AQP4 appears to provide an energetically more favorable permeation pathway for gas molecules than AQP1, mainly due to the different orientation of its charged residues near the pore entrance. Although the low barrier against gas permeation through AQP4 indicates that it can participate in gas conduction across the cellular membrane, physiological relevance of the phenomenon remains to be established experimentally, particularly since pure lipid bilayers appear to present a more favorable pathway for gas conduction across the membrane. With an energy well of ?1.8 kcal mol^?1, the central pore of AQP4 may also act as a reservoir for NO molecules to accumulate in the membrane. Proteins 2010. © 2009 Wiley‐Liss, Inc.     

Enhancement of proton conductance by mutations of the selectivity filter of aquaporin-1

Prevention of cation permeation in wild-type aquaporin-1 (AQP1) is believed to be associated with the Asn-Pro-Ala (NPA) region and the aromatic/arginine selectivity filter (SF) domain. Previous work has suggested that the NPA region helps to impede proton permeation due to the protein backbone collective macrodipoles that create an environment favoring a directionally discontinuous channel hydrogen-bonded water chain and a large electrostatic barrier. The SF domain contributes to the proton permeation barrier by a spatial restriction mechanism and direct electrostatic interactions. To further explore these various effects, the free-energy barriers and the maximum cation conductance for the permeation of various cations through the AQP1-R195V and AQP1-R195S mutants are predicted computationally. The cations studied included the hydrated excess proton that utilizes the Grotthuss shuttling mechanism, a model “classical” charge localized hydronium cation that exhibits no Grotthuss shuttling, and a sodium cation. The hydrated excess proton was simulated using a specialized multi-state molecular dynamics method including a proper physical treatment of the proton shuttling and charge defect delocalization. Both AQP1 mutants exhibit a surprising cooperative effect leading to a reduction in the free-energy barrier for proton permeation around the NPA region due to altered water configurations in the SF region, with AQP1-R195S having a higher conductance than AQP1-R195V. The theoretical predictions are experimentally confirmed in wild-type AQP1 and the mutants expressed in Xenopus oocytes. The combined results suggest that the SF domain is a specialized structure that has evolved to impede proton permeation in aquaporins.     

Molecular simulation of shale gas adsorption and diffusion in inorganic nanopores

We studied the structural and dynamical properties of methane and ethane in montmorillonite (MMT) slit pore of sizes 10, 20 and 30 Å using grand canonical Monte Carlo and classical molecular dynamics (MD) simulations. The isotherm, at 298.15 K, is generated for pressures up to 60 bar. The molecules preferentially adsorb at the surface as indicated by the density profile. In case of methane, we observe only a single layer, at the pore wall, whose density increases with increasing pressure. However, ethane also displays a second layer, though of low density in case of pore widths 20 and 30 Å. In-plane self-diffusion coefficient, D_‖, of methane and ethane is of the order of 10^? 6 m²/s. At low pressure, D_‖ increases significantly with the pore size. However, D_‖ decreases rapidly with increasing pressure. Furthermore, the effect of pore size on D_‖ diminishes at high pressure. Ideal adsorbed solution theory is used to understand the adsorption behaviour of the binary mixture of methane (80%) and ethane (20%) at 298.15 K. Furthermore, we calculate the selectivity of the gases at various pressures of the mixture, and found high selectivity for ethane in MMT pores. However, selectivity of ethane decreases with increase in pressure or pore size.     

Folded Structures in Protonated Reduced Dipeptides

Reduced dipeptides with the general formula RCO-Xaa- rXbb-N⁺HR′R′′ (rXbb, reduced analogue of residue Xbb: NH-C^α HR¹ -C_r H₂) are shown to adopt a folded conformation in solution and in the solid state. The protonated reduced amide bond is an active proton donor capable of interacting with a peptide carbonyl to give a strong hydrogen bond topologically equivalent to the i+2 or i+3? i interaction. The resulting conformation is similar to the γ- or β-turn structure found in peptides and proteins.