Large ΔH Research Articles

Kinetic and thermodynamic analysis of ammonia electro-oxidation over alumina supported copper oxide (CuO/Al2O3) catalysts for direct ammonia fuel cells

In this work, ammonia electrooxidation (AEO) is studied to probe the electrochemical behavior of aqueous NH3 as direct fuel by implication of gamma alumina supported copper oxide catalysts (CuO/Al2O3). Precipitation and impregnation techniques are adapted to synthesize the substrate Al2O3, and loading of different ratios (1%, 2%, 3%, 4%) of active precursor i.e., CuO, respectively. Small average crystallite sizes (DAvg) in range of 1.46–6.76 nm and smaller particle sizes from micrographs proposed a superb activity of the catalysts. Modified GCE exhibited the excellent conductive properties towards standard redox probe K4[Fe(CN)6] and KCl, displaying a high active electrochemical surface area of up to 0.0021 cm2. Current profiles in response to increase in scan rates, concentrations of ammonia and temperature have been observed in 0.1 M KOH, thereby estimating the thermodynamic and kinetic constraints of the AEO process. Attributed to the electroactive properties towards AEO, CuO/Al2O3 is found to exhibit the desirable physiochemical properties owing to large oxidation current, large diffusion coefficient “D°” (3.6 × 10-9 cm2 s-1), large rate constant “ko” (1.2 × 10-5 cm s-1), large system entropy “ΔS” (-108 J K-1 mol-1), high change in enthalpy “ΔH” (72.3 J mol-1) and low activation energy “ΔG” (32.8 kJ mol-1). Resultingly, the oxidation of ammonia is found to be facile and robust by incorporation of CuO/Al2O3 catalysts owing to large ko, ΔH and ΔH. This study opened a gateway towards eco-benign and economical efficient energy generation and viable market entry of direct ammonia fuel cells.

Iron response elements (IREs)-mRNA of Alzheimer's amyloid precursor protein binding to iron regulatory protein (IRP1): a combined molecular docking and spectroscopic approach

The interaction between the stem-loop structure of the Alzheimer's amyloid precursor protein IRE mRNA and iron regulatory protein was examined by employing molecular docking and multi-spectroscopic techniques. A detailed molecular docking analysis of APP IRE mRNA∙IRP1 reveals that 11 residues are involved in hydrogen bonding as the main driving force for the interaction. Fluorescence binding results revealed a strong interaction between APP IRE mRNA and IRP1 with a binding affinity and an average binding sites of 31.3 × 106 M−1 and 1.0, respectively. Addition of Fe2+(anaerobic) showed a decreased (3.3-fold) binding affinity of APP mRNA∙IRP1. Further, thermodynamic parameters of APP mRNA∙IRP1 interactions were an enthalpy-driven and entropy-favored event, with a large negative ΔH (–25.7 ± 2.5 kJ/mol) and a positive ΔS (65.0 ± 3.7 J/mol·K). A negative ΔH value for the complex formation suggested the contribution of hydrogen bonds and van der Waals forces. The addition of iron increased the enthalpic contribution by 38% and decreased the entropic influence by 97%. Furthermore, the stopped-flow kinetics of APP IRE mRNA∙IRP1 also confirmed the complex formation, having the rate of association (kon) and the rate of dissociation (koff) as 341 μM−1 s−1, and 11 s−1, respectively. The addition of Fe2+ has decreased the rate of association (kon) by ~ three-fold, whereas the rate of dissociation (koff) has increased by ~ two-fold. The activation energy for APP mRNA∙IRP1 complex was 52.5 ± 2.1 kJ/mol. The addition of Fe2+ changed appreciably the activation energy for the binding of APP mRNA with IRP1. Moreover, circular dichroism spectroscopy has confirmed further the APP mRNA∙IRP1 complex formation and IRP1 secondary structure change with the addition of APP mRNA. In the interaction between APP mRNA and IRP1, iron promotes structural changes in the APP IRE mRNA∙IRP1 complexes by changing the number of hydrogen bonds and promoting a conformational change in the IRP1 structure when it is bound to the APP IRE mRNA. It further illustrates how IRE stem-loop structure influences selectively the thermodynamics and kinetics of these protein-RNA interactions.

Impact of regioregularity on the solubility parameters of poly(3‐hexylthiophene)

The solubilities of four P3HT samples of varying regioregularity (rr = 58%, 86%, 93%, and 96%) are measured in 18 organic solvents, and the solubility data are used to calculate the partial solubility parameters (SPs) (δD, δP, δH) of the samples using the functional solubility parameter (FSP) approach. The FSP results show that the δP and δH parameters increase with increasing regioregularity. The results reveal that the anomalously large δP and δH SPs regioregular P3HT previously reported in the literature are attributable to the microstructure (regioregularity) of the P3HT polymer, and that as that as the regioregularity of P3HT increases the polymer can engage in stronger associative interactions with the solvent. Complementary computational analyses of hexamer model systems using Gaussian16 and AIMll software, reveal variations in electronic characteristics and topological parameters consistent with the regioregular-dependent increases in δP and δH. This research may aid in modifying the fundamental and/or practical framework of SP theory to account for the effects of polymer microstructure and polymorphism.

Indoloquinoline Ligands Favor Intercalation at Quadruplex-Duplex Interfaces.

Quadruplex‐duplex (Q‐D) junctions are increasingly considered promising targets for medicinal and technological applications. Here, a Q‐D hybrid with a hairpin‐type snapback loop coaxially stacked onto the quadruplex 3’‐outer tetrad was designed and employed as a target structure for the indoloquinoline ligand SYUIQ‐5. NMR spectral analysis demonstrated high‐affinity binding of the ligand at the quadruplex‐duplex interface with association constants determined by isothermal titration calorimetry of about 107 M−1 and large exothermicities ΔH° of −14 kcal/mol in a 120 mM K+ buffer at 40 °C. Determination of the ligand‐bound hybrid structure revealed intercalation of SYUIQ‐5 between 3’‐outer tetrad and the neighboring CG base pair, maximizing π–π stacking as well as electrostatic interactions with guanine carbonyl groups in close vicinity to the positively charged protonated quinoline nitrogen of the tetracyclic indoloquinoline. Exhibiting considerable flexibility, the SYUIQ‐5 sidechain resides in the duplex minor groove. Based on comparative binding studies with the non‐substituted N5‐methylated indoloquinoline cryptolepine, the sidechain is suggested to confer additional affinity and to fix the alignment of the intercalated indoloquinoline aromatic core. However, selectivity for the Q‐D junction mostly relies on the geometry and charge distribution of the indoloquinoline ring system. The presented results are expected to provide valuable guidelines for the design of ligands specifically targeting Q‐D interfaces.

POD analysis of passage-layout effect on unsteady internal flow in a realistic blade serpentine coolant channel with low aspect ratios

Internal serpentine ribbed coolant channel with low aspect ratio (AR) has been widely employed to provide excellent thermal protection for advanced first-stage turbine blades. Various height differences (ΔHs) of adjacent passages along pressure side can be generated under the restriction of actual blade profile. The present work made an attempt to acquire detailed influences of ΔH on the time-averaged and transient secondary motions near sharp bends of low-AR coolant channels. An actual blade ribbed three-pass coolant channel with two ΔHs was chosen as experimental model. Planar time-resolved particle image velocimetry technique was applied to capture the transient flow fields in 21 typical cross sections. Time-averaged results were validated by standard PIV measurements and flow visualizations. Snapshot proper orthogonal decomposition was employed to extract the dominant flow structures and identify the underlying small-scale flow patterns in time-resolved velocity fields. The experimental results indicated the low-AR channel with real blade profile induces a new type of the secondary-vortex superposition downstream of bends, in comparison with the large-AR channels in open literature. The crucial reason is the ΔH along pressure side leads to an intensive impingement on the bend wall, which weakens the effects of bend, rib and curvature of suction side. Another new discovery is the effect of ΔH on unsteady internal flow. In the small ΔH case, the oscillatory frequency of the secondary vortices is nearly similar with the counter-rotating Dean-vortices; however, in the large ΔH case, the oscillatory frequency is much lower, and the rotating direction of vortices is same and unchanged in a period. Increasing inlet Reynolds number can significantly change the underlying small-scale flow patterns and improve the oscillatory frequency of large-scale structures, although the basic time-averaged flow patterns are nearly same. Effect of injection from auxiliary-hole (AH) on unsteady internal flow is obvious and the optimum mass flow ratio of AH-to-inlet is 5%.

Isothermal titration calorimetry as a useful tool to examine adsorption mechanisms of phosphate on gibbsite at various solution conditions

AbstractDespite extensive prior work with batch and spectroscopic methods, little information is available about adsorption thermodynamics of phosphate (P) on Al (oxyhydr)oxides. Here we directly quantified for the first time the adsorption heat of P on gibbsite at varying pH (3.0–9.0) and ionic strength (IS, 1–100 mM) with isothermal titration calorimetry (ITC). The obtained thermograms showed a strong effect of pH but a negligible influence of IS on the adsorption thermodynamics. Coupled with results from other methods including batch experiment, in‐situ attenuated total reflectance Fourier‐transform infrared spectroscopy, and nuclear magnetic resonance, the ITC data revealed contrasting mechanisms for P adsorption at pH 9.0 and 3.0. At pH 9.0, P adsorption was probably initiated by a reaction of P with Al(OH)4− (originated from the solubilization of gibbsite) with high equilibrium constant (k) and small exothermic enthalpy change (ΔH), followed by a ligand exchange between P and the > Al–O− (Al hydroxyl groups on gibbsite surface) with low k and relatively large exothermic ΔH. In contrast, P adsorption at pH 3.0 likely underwent a fast ligand exchange between P and the > Al‐OH2+ on gibbsite surface with high k and exothermic ΔH and then a slow surface precipitation of P with Al3+ due to gibbsite solubilization with low k and endothermic ΔH. This study provides complementary information to the prior knowledge on P adsorption on (hydr)oxides and demonstrates how to use ITC in combination with other methods for mechanistic examination of ions adsorption on the surfaces of soils and sediments.

Estimation of surface free energy and solubility parameters of Mg[sbnd]Al layered double hydroxides

The characteristic parameters, surface free energy (γ) and solubility parameters (δ), of Mg2AlNO3, Mg3AlNO3, Mg4AlNO3, Mg3AlCl, and Mg3AlCO3 layered double hydroxides (LDHs) were estimated using contact angle and liquid phase exfoliation (LPE) methods. The γ parameters include total (γst), dispersive (γsd), and polar (γsp) ones. The δ parameters include the Hildebrand parameter (δt,s) and the Hansen dispersive (δd,s), polar (δp,s), and hydrogen-bonding (δh,s) ones. Results show that the γst, γsd, and γsp of the five LDHs are ∼50.9–56.9, 35.2–39.4, and 15.6–18.8 mJ/m2, respectively, and the δt,s, δd,s, δp,s and δh,s of the LDHs except for the Mg3AlCO3 LDH are ∼31.4–32.9, 17.2, 9.5–10.0, and 24.5–26.1 MPa1/2, respectively. The compositions of LDHs have no obvious impact on all the characteristic parameters. In comparison with van de Waals attraction-bonded materials, the electrostatic attraction-bonded LDHs exhibit higher γst and δt,s values and, especially, much larger δh,s values. The characteristic parameter matching principle and the mixed-solvent strategy are suitable for the LPE of LDHs. Non-toxic glycol/PEG200 mixed liquids are found to be a good medium for the direct LPE of LDHs, producing monolayer nanosheets. To the best of our knowledge, this is the first report on the γ and δ parameters of LDHs. This work not only deepens the understanding of the feature of LDHs, but also provides effective liquid media for the direct LPE of LDHs.

Kinetico-Mechanistic Studies of Nucleoside and Nucleotide Substitution Reactions of Co(III) Complexes of Fully Alkylated Cyclen.

The solution chemistry of complex [Co{(Me)2(μ-ET)cyclen}(H2O)2](3+) containing a fully substituted tetraammine ligand designed for the avoidance of base-conjugated substitution mechanisms in the 6-8 pH range has been studied. The study should shed some light on the possible involvement of such Co(III) skeleton in inert interactions with biomolecules. The reactivity and speciation of the complex has been found similar to that of the parent cyclen derivative with the presence of mono- and bis-hydroxo-bridged species; at pH < 7.1, all reactivity has been found to be related to the aqua/hydroxo monomeric complexes. Under these pH conditions, the substitution reactions of the aqua/hydroxo ligands by chloride, inorganic phosphate, thymidine, cytidine 5'-monophosphate (5'-CMP), and thymidine-5'-monophosphate (5'-TMP) have been studied at varying conditions; ionic strength has been kept at 1.0 NaClO4 due to the high concentration of 2-(N-morpholino)ethanesulfonic acid (MES) or N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES) used to ensure buffering. Except for chloride, the process occurs neatly in a one or two step process, showing dissociatively activated substitution mechanisms, having in general large ΔH(⧧), positive ΔS(⧧), and values of ΔV(⧧) close to those corresponding to the liberation of an aqua ligand to the reaction medium. The actuation of noticeable encounter-complex formation equilibrium constants has been found to be the determinant for the reactions with nucleosides and nucleotides, a clear indication of the relevance of hydrogen-bonding interactions in the reactivity of these molecules, even in this highly ionic strength medium. For the substitution of the active aqua/hydroxo ligands with 5'-TMP, the first substitution reaction produces an Nthymine-bound 5'-TMP complex that evolves to a bis-5'-TMP with an Nthymine,Ophosphate-bonding structure. The formation of outer-sphere complexes between the dangling phosphate group of the Nthymine-bound 5'-TMP and the thymine moiety of another entering 5'-TMP has been found to be responsible for this fact, which leaves only the phosphate group for coordination available.

Identification of obligate C&lt;sub&gt;3&lt;/sub&gt; photosynthesis in Dendrobium

Dendrobium is one of the three largest genera in the Orchidaceae and is distributed throughout various habitats. We investigated photosynthesis in seven Dendrobium species and cultivars by comparing their leaf δ13C values, titratable acidity, and CO2 exchange in well-watered and drought-stressed conditions. In addition, the leaf thickness and mesophyll succulence index (Sm) were measured in well-watered conditions. Our results indicate that Dendrobium loddigesii is a typical obligate (or constitutive) CAM plant because the leaf δ13C values were -14.47 and -14.66‰ in both conditions, respectively. Others showed the leaf thickness of 0.31-0.89 mm and their δ13C values ranged from -25.68 to -28.37‰. These are not the CAM plants but they could not be classified as obligate C3 or C3/CAM intermediate plants. Dendrobium crepidatum and Dendrobium fimbriatum were further identified as the obligate C3 plants because the net CO2 uptake was positive during daytime and negative during nighttime in both conditions. In contrast, Dendrobium chrysotoxum, Dendrobium nobile, and D. nobile 'V1' and 'V4', showed no positive net CO2 uptake and low ΔH+ values during nighttime under well-watered conditions, indicating C3 photosynthesis. However, they showed the positive net CO2 uptake and large ΔH+ values during nighttime after drought-stress (21 or 28 days without H2O), indicating CAM photosynthesis. Therefore, these four species and cultivars were identified as C3/CAM intermediate (inducible or facultative) plants. In brief, obligate CAM, C3/CAM intermediate, and obligate C3 plant types all exist in the section of Dendrobium. To the best of our knowledge, this is the first report of the obligate C3 plants in Dendrobium, and these diverse photosynthetic pathways may explain their varied environmental adaptations.

Quantum Chemical Analysis about Deprotonation Reaction of Sulfonic Group of Modeled Hydrocarbon Membrane in Low Hydration Level

Hydrocarbon membranes (HC) are expected as a new polymer electrolyte membrane (PEM) of polymer electrolyte fuel cell (PEFC) by its high durability and low synthetic cost. There is one weak point in HC. That is low proton conductivity especially in low hydration level1 in comparison with perfluorosulfonic acid (PFSA) membrane. This difference between HC and PFSA is considered to be caused by high probability of deprotonation as well as membrane flexibility2. Thus, activation energy of deprotonation reaction (ΔEa) and energy difference before and after deprotonation (ΔH) are very important information to estimate the performance of membrane in the low hydration level. However, deprotonation mechanism of HC by water molecule(s) was not clarified yet. For PFSA, deprotonation process of sulfonic group by three water molecules has been already clarified by our study3. We showed characteristic reaction process in hydration level three. In this hydration level, sulfonic group of PFSA should be deprotonated smoothly by three water molecules via low ΔEa. ΔH in this reaction is enough low to prevent the reverse reaction. Thus, PFSA shows strong acidity. However, deprotonation mechanism and its properties about HC membrane are still unknown.Then, we attempted to clarify the deprotonation reaction about HC species by some water molecules in the same manner as PFSA. Benzenesulfonic acid (C6H5SO3H) was adopted as a model molecule of HC. A relative configuration between sulfonic group and water molecules in reactant, transition state and product for each hydration level was set in the same manner of the analysis of PFSA3. Each geometries, that is, reactant, transition state and product, were calculated by B3LYP/6-311+G(d,p) level of theory. The basis set super position error was corrected by counterpoise method. All of calculations were performed by Gaussian094, installed on super computer systems of Institute of Fluid Science, Tohoku University. All of geometries were confirmed as appropriate stable state or transition state by the normal vibration analysis.Energy changes along the reaction in hydration level from one to three were depicted in Fig. 1. All of energies were treated as relative energy; reactants are set as 0 kcal/mol.Fig. 1. Energy changes along the reaction in each hydration levels.The predicted ΔEas for hydration level one, two and three were 12.95, 10.02 and 4.16 kcal/mol, respectively. ΔEas were obviously decreased with increasing hydration level. ΔEas in these hydration levels were much higher than the heat energy around the 350K (≈ 0.7 kcal/mol), which was the operation temperature of PEFC. In hydration level three, predicted electronic energy of product was slightly larger than the reactant, which means that the product geometry which indicates SO3 - group, H3O+ and two water molecules is barely unstable by 0.55 kcal/mol. Considering the computational error of this study, however, it is hard to evaluate this slightly large ΔH Only we can conclude that deprotonated state in HC is not favorable qualitatively. In HC, thus, ΔEa of deprotonation reaction of sulfonic group and protonation reaction of SO3 - group are almost same. Then, those two reactions occur in the same probability. These results are different from PFSA’s results. ΔEas in PFSA species were 10.94, 5.24 and 0.70 kcal/mol in hydration level one, two and three, respectively. And ΔH in PFSA was - 3.72 kcal/mol in hydration level three. Compared with those results, both ΔEas and ΔH of HC are higher than those of PFSA. This difference is caused by the difference of its constituent. PFSA mainly consists of fluorocarbon which hasis strong electron withdrawing moiety. Due to the strong electronegativity electron withdrawing of fluorocarbon, both ΔEas and ΔH are very low. On the other hand, HC consists of phenyl group. Electron withdrawing ability of phenyl group is not so strong as fluorocarbon then both ΔEas and ΔHof HC are higher than those of PFSA.AcknowledgementThis work was financially supported by the New Energy and Industrial Technology Development Organization (NEDO). And authors also thank to the super computer systems of Institute of Fluid Science, Tohoku University.References (1) Rikukawa, M.; Sanui, K. Prog. Polym. Sci. 2000, 25, 1463. (2) Kopitzke, R. W.; Linkous, C. A.; Anderson, H. R.; Nelson, G. L. J. Electrochem. Soc. 2000, 147, 1677. (3) Sakai, H.; Tokumasu, T. The Journal of Physical Chemistry A 2014, 118, 275. (4) Frisch, M. J. et. al.; Revision C.01 ed.; Gaussian, Inc.: 2010.

Synthesis of Dabsyl-Appended Cyclophanes and Their Heterodimer Formation with Pyrene-Appended Cyclophanes

As a quencher-type host, dabsyl-appended cyclophanes bearing positively and negatively charged side chains (1a and 1b, respectively) were synthesized. Formation of cyclophane heterodimers of 1a with anionic fluorescent cyclophane bearing a pyrene moiety 2b was confirmed by fluorescence titration experiments. The 1:1 binding constant (K) of 1a toward 2b was calculated to be 1.6 × 10(5) M(-1). On the other hand, almost no complexation affinity of 1a toward cationic analogue of fluorescent cyclophane 2a was confirmed by the identical methods, indicating that electrostatic interactions became effective in the formation of cyclophane heterodimers. In addition, van't Hoff analysis applied to the temperature-dependent K values for the heterodimer formation gave negative enthalpy (ΔH) and entropy changes (ΔS). The large and negative ΔH values as well as small and also negative ΔS values showed that the complexation is an exothermic and enthalpy-controlled but not entropy-driven process. A similar trend of molecular recognition was also confirmed for formation of cyclophane heterodimers of 1b with 2a by the identical methods.

Thermodynamics of the binding of phenothiazines to human plasma, human serum albumin and alpha 1-acid glycoprotein: a calorimetric study.

A flow microcalorimetric study has been carried out to investigate the interactions between phenothiazine derivatives and human plasma, human serum albumin (HSA) and alpha 1-acid glycoprotein (AGP) at pH 7.4 and 37 degrees C. The direct analyses of enthalpic titration curves allowed the determination of the binding enthalpy change (delta H), the apparent binding constant (K), and the number of the binding sites (n), as well as the evaluation of the apparent free energy (delta G), and entropy (delta S) changes. The overall binding of phenothiazines was exothermic with negative delta H, which was compensated for by changes in delta S. The values of delta G were relatively insensitive to variation in the molecular details of the binding reaction. HSA possessed two classes of binding sites for phenothiazines. The first (n1 = 1), with high affinity (K1 = 10(5)-10(6) M-1) was characterized by small negative delta H and positive delta S values due to hydrophobic interaction. The second class of sites had a low affinity (K2 = 10(3)-10(4) M-1) and high capacity (n2 = 3-8) and contributed to the negative delta H and delta S values. The binding and thermodynamic parameters were influenced by the aliphatic side chain moieties on the phenothiazine nucleus. On the other hand, the drugs were bound to AGP at a single common binding site with a binding affinity of the order of 10(4)M-1, characterized by negative delta H and delta S values, which partially reflected the effect of a van der Waals' interaction.(ABSTRACT TRUNCATED AT 250 WORDS)

Variations of phase transition temperature and enthalpy in the systems Na2Mo1− x W x O4 and Na2Mo1− y S y O4

The samples in two material systems, Na2Mo1− x W x O4 and Na2Mo1− y S y O4, were prepared by using conventional solid reactions and characterized by X-ray diffractometer (XRD), energy dispersive X-ray spectroscopy (EDX), and difference scanning calorimetry (DSC). The XRD and EDX data indicated that all the samples in both systems were in the solid solution range. The DSC data indicated that in the system Na2Mo1− x W x O4, the solid–solid transition temperatures increased and in the system Na2Mo1− y S y O4, the solid–solid transition temperatures decreased. The total enthalpy (ΔH total) of the solid–solid transitions in the system Na2Mo1− x W x O4, decreased much less than that in the system Na2Mo1− y S y O4. This is probably because similar to Na2MoO4, the solid–solid transition of Na2WO4 has relatively large ΔH total, but Na2SO4 has much smaller solid–solid transition enthalpy. In order to modify the transition temperatures of Na2MoO4 and also to keep its relatively large ΔH total, it is necessary to choose a doping material with large ΔH total.

Thermodynamics of Membrane Partitioning and Self-association of Transmembrane Helices: Impact of Lipid Composition

Membrane partitioning and self-association of transmembrane helices (TMHs) have been extensively investigated using model TMHs to elucidate the driving forces of membrane protein folding. We designed the inert hydrophobic model TMH (AALALAA)3 to obtain thermodynamic parameters related to the steps. The formation of the antiparallel dimer was detected by fluorescence resonance energy transfer between fluorescent labeled peptides [1]. Stronger dimerization was observed in thicker membranes and at lower temperatures (ΔG = −9 – −26 kJ mol−1), driven by large negative ΔH values (−18 – −80 kJ mol−1). The enthalpy changes for helix-helix interaction can be well explained by electrostatic interaction between helix macrodipoles in different dielectric environments. The incorporation of cholesterol and PE also facilitated the dimerization by large negative ΔH values. The partitioning process was also investigated based on the transfer of the helix between vesicles [2]. Under hydrophobic mismatch conditions up to ∼7 Å, the helix partitioning became unfavorable up to +7 kJ mol−1, hampered by an increase in entropic (up to +20 kJ mol−1) and enthalpic (up to +66 kJ mol−1) terms in thinner and thicker membranes, respectively. The obtained thermodynamic parameters were reasonably explained assuming that hydrophobic mismatch induces aqueous exposure or membrane burial of the helix termini, respectively. I also present a design of a water-soluble TMH for the direct measurement of the partitioning process. Furthermore, I will introduce a novel method for quick fluorescent labeling of membrane proteins to detect TMH interactions in living cell membranes [3]. [1] Y. Yano and K. Matsuzaki, Biochemistry 45, 3370 (2006). [2] Y. Yano et al., Biochemistry 45, 3379 (2006). [3] Y. Yano et al., ACS Chem. Biol. 6, 341 (2008).

Energetics of (dis)assembly of the ternary SNARE complex

Energetics of (dis)assembly of the ternary SNARE complex

Translation Initiation Factors eIF4F and eIFiso4F from Wheat Germ Interact Differently with Potyvirus Genome‐linked Protein, VPg.

The binding between wheat germ eukaryotic translation initiation factors eIF4F or eIFiso4F and viral protein linked to the genome (VPg) of potyvirus were measured and compared. The fluorescence quenching data showed eIF4F (Kd = 219.41 nM) has about 2.6-fold less affinity than eIFiso4F (Kd = 84.34 nM) for VPg. However, eIF4F with tobacco etch virus (TEV) RNA (Kd = 79.0 nM) showed ~2.3-fold higher binding affinity as compared to eIFiso4F and TEV RNA (Kd = 178.0 nM). In order to determine if these reactions were competitive, addition of TEV RNA to the eIF4F-VPg and eIFiso4F-VPg complex was examined and found to increase the protein-protein affinity. The temperature dependent binding studies revealed that VPg binding to eIF4F and eIFiso4F is enthalpy-driven and entropy favorable with a large negative ΔH° (−27.83 and −31.03 kJmole−1), and positive ΔS° (34.12 and 32.32 Jmole−1K−1). A Lineweaver-Burk plot indicates noncompetitive or uncompetitive binding between VPg and TEV RNA for both eIF4F and eIFiso4F. Enhancement of eIF4F-VPg and eIFiso4F-VPg binding with the addition of TEV RNA suggests that translation initiation involving VPg occurs at internal ribosomal entry sites. Overall these results suggest the direct involvement of viral genome protein (VPg) in protein translation. Supported by Research Center in Minority Institution Award RR-03037 from the National Center for Research Resources of NIH and National Science Foundation Grant MCB 0413982.

Thermodynamics of CFTR Channel Gating: A Spreading Conformational Change Initiates an Irreversible Gating Cycle

CFTR is the only ABC (ATP-binding cassette) ATPase known to be an ion channel. Studies of CFTR channel function, feasible with single-molecule resolution, therefore provide a unique glimpse of ABC transporter mechanism. CFTR channel opening and closing (after regulatory-domain phosphorylation) follows an irreversible cycle, driven by ATP binding/hydrolysis at the nucleotide-binding domains (NBD1, NBD2). Recent work suggests that formation of an NBD1/NBD2 dimer drives channel opening, and disruption of the dimer after ATP hydrolysis drives closure, but how NBD events are translated into gate movements is unclear. To elucidate conformational properties of channels on their way to opening or closing, we performed non-equilibrium thermodynamic analysis. Human CFTR channel currents were recorded at temperatures from 15 to 35°C in inside-out patches excised from Xenopus oocytes. Activation enthalpies(ΔH ‡) were determined from Eyring plots. ΔH ‡ was 117 ± 6 and 69 ± 4 kJ/mol, respectively, for opening and closure of partially phosphorylated, and 96 ± 6 and 73 ± 5 kJ/mol for opening and closure of highly phosphorylated wild-type (WT) channels. ΔH ‡ for reversal of the channel opening step, estimated from closure of ATP hydrolysis–deficient NBD2 mutant K1250R and K1250A channels, and from unlocking of WT channels locked open with ATP+AMPPNP, was 43 ± 2, 39 ± 4, and 37 ± 6 kJ/mol, respectively. Calculated upper estimates of activation free energies yielded minimum estimates of activation entropies (ΔS ‡), allowing reconstruction of the thermodynamic profile of gating, which was qualitatively similar for partially and highly phosphorylated CFTR. ΔS ‡ appears large for opening but small for normal closure. The large ΔH ‡ and ΔS ‡ (TΔS ‡ ≥ 41 kJ/mol) for opening suggest that the transition state is a strained channel molecule in which the NBDs have already dimerized, while the pore is still closed. The small ΔS ‡ for normal closure is appropriate for cleavage of a single bond (ATP's beta-gamma phosphate bond), and suggests that this transition state does not require large-scale protein motion and hence precedes rehydration (disruption) of the dimer interface.

Quench rate and temperature effects on framework ordering in aluminosilicate melts

The effect of temperature on the structure of LiAlSiO4 and NaAlSiO4 liquids has been studied by 17O 3QMAS NMR spectroscopy using glass samples prepared with different fictive temperatures. The abundance of Al-O-Al and Si-O-Si bridging O linkages increases with increasing fictive temperature, indicating that the reaction 2 Al-O-Si ↔ Al-O-Al + Si-O-Si shifts to the right with increasing temperature in the liquid. The observed temperature dependence of Al-O-Al species abundance allows us to estimate the ΔH of this reaction to be 28.8 ± 11.3 kJ/mol for NaAlSiO4 glasses and 31.7 ± 13.3 kJ/mol for LiAlSiO4 glasses. Extrapolating our results to higher temperature, we estimate that 13.18% of the bridging O atoms occur as Al-O-Al in NaAlSiO4 and 15.25% in LiAlSiO4 at ~2000 K, as compared to 25% Al-O-Al predicted by a statistically random bridging O distribution in compositions with an Al/Si ratio of 1. Using the experimental data to estimate the contribution to configurational heat capacity from Al/Si disordering in NaAlSiO4 liquid, we find that redistribution of bridging O species with increasing temperature produces a negative dCP/dT above the glass transition and therefore cannot account for the observed positive dCP/dT. We discuss a generalized mechanism for structural rearrangement characterized by a small initial species concentration at Tg and a large ΔH, which produces a positive dCP/dT.

Overall Rate Analysis of the Gaseous Reduction of Stable Oxides Incorporating Chemical Kinetics, Mass Transfer, and Chemical Equilibrium

A unified mathematical relationship for the overall rate of gaseous reduction of a stable oxide that produces a volatile suboxide has been derived including the effects of a product gas in the bulk stream, chemical kinetics, mass transfer, and chemical equilibrium. The important effect of the small equilibrium constant is quantified. How the reaction conditions affect the overall rate at different asymptotic conditions is also elucidated. By using the obtained results, the conflicting previous claims on the rate-controlling mechanism for the hydrogen reduction of silica has been critically examined and reconciled. The small equilibrium constant of this reaction causes it to be rate controlled by mass transfer under typical conditions and its rate to be slow. How the presence of even a small amount of water vapor in the bulk gas greatly enhances the effects of the small equilibrium constant is elucidated from the mathematical derivation. In addition, the small equilibrium constant also causes the apparent activation energy of silica reduction by wet hydrogen to approach the ΔH° of reaction and that by dry hydrogen to approach ΔH°. Because of the large positive ΔH° value associated with reactions with a large positive ΔG° value, such a reaction has been mistaken to be rate controlled by chemical kinetics.

NMR study of the leaving ligand and solvent effects on the solvolysis of (μ-oxo)bis(μ-acetato)diruthenium(III) complexes

A mechanistic 1H NMR kinetic study on the solvolysis of complexes [Ru2(μ-O)(μ-O2CCH3)2(4-picoline)6](PF6)2 (1), [Ru2(μ-O)(μ-O2CCH3)2(pyridine)6](PF6)2 (2), and [Ru2(μ-O)(μ-O2CCH3)2(4-picoline)4(pyridine)2](PF6)2 (3) in DMSO-d6, CD3OD and CD3CN was carried out. All rate constants were calculated from the kinetic data, which fit in a series of reversible pseudo first order and second order reactions. The comparative kinetic study and the large ΔH‡ and positive ΔS‡ values for all solvolysis support the dissociative nature of the ligand exchange mechanism. The leaving ligand effect, determined by comparison of the forward reactions for complexes 1 and 3, showed that the rate constants for 4-picoline substitution are smaller than for pyridine. An apparent contradiction with the proposed mechanism for the reverse reaction in DMSO-d6 where the less basic ligand coordinates faster, was explained by a ligand–solvent interaction, which was demonstrated by our kinetics results, molecular modeling and through the determination of diffusion coefficients in DMSO-d6 and CD3OD using DOSY.