Lower Enthalpy Change Research Articles

Dicarboxylic acid salts as task-specific ionic liquids for reversible absorption of SO2 with a low enthalpy change

Six acid salt ionic liquids (ASILs), triethylbutylammonium dicarboxylates, have been synthesized to act as green materials for SO2 capture. The experimental results reveal that the ASILs can trap SO2 reversibly and chemically with a large capacity of 0.112 up to 0.232 (mass ratio) at 15.5 kPa and of 0.374 to 0.456 (mass ratio) at 100 kPa and 40 °C. Two of these ASILs are interestingly found to have a low viscosity that enables the fast mass transfer of SO2. A reaction mechanism is proposed to explain the chemical absorption based on FTIR spectra and structural calculations using the density functional theory. Thermodynamic analysis indicates the enthalpy of the reaction of SO2 with the ASILs is low (−29.9 and −42.2 kJ mol−1 for [N2224][dimaleate] and [N2224][dimalonate], respectively). Additionally, the ASILs have a high thermal stability, which favors their potential application in flue gas desulfurization.

Complex formation between primycin and ergosterol: entropy–driven initiation of modification of the fungal plasma membrane structure

The interaction of the antibiotic primycin with the main fungal sterol, ergosterol, was investigated in vitro in order to monitor the effect of primycin on the fungal plasma membrane at the molecular level. The thermodynamic parameters of complex formation were determined by measuring Rayleigh scattering as a signal sensitive to particle size. The Benesi-Hildebrand method validated the 1 : 1 stoichiometry of the primycin-ergosterol complexes. A very low enthalpy change (ΔH=-1.14 kJ mol(-1)) was measured during the complex formation, which itself cannot be responsible for the molecular association. However, the entropy production (ΔS=29.78 J mol K(-1)) observed during the complex formation can describe the molecular interaction. This effect is probably due to the partial destruction of the solvation shell of the interacting species before the interlinking of the molecules. The results highlight the importance of ergosterol as concerns the mode of effect of primycin in the treatment of fungal infections. As the entropy has a determinant role in the ergosterol-primycin interaction, this interaction exhibits a very high temperature dependence, with the important consequence that the effect exerted by primycin on the cell membranes increases with rising temperature, and the effect is therefore pronounced in fevered bodies.

Enthalpy/entropy driven activation of the first interquinone electron transfer in bacterial photosynthetic reaction centers embedded in vesicles of physiologically important phospholipids

The thermodynamics and kinetics of light-induced electron transfer in bacterial photosynthetic RCs are sensitive to physiologically important lipids (phosphatidylcholine, cardiolipin and phosphatidylglycerol) in the environment. The analysis of the temperature-dependence of the rate of the P +Q A −Q B→P +Q AQ B − interquinone electron transfer revealed high enthalpy change of activation in zwitterionic or neutral micelles and vesicles and low enthalpy change of activation in vesicles constituted of negatively charged phospholipids. The entropy change of activation was compensated by the changes of enthalpy, thus the free energy change of activation (≈ 500 meV) did not show large variation in vesicles of different lipids.

The conformational stability of the Streptomyces coelicolor histidine-phosphocarrier protein. Characterization of cold denaturation and urea-protein interactions.

Thermodynamic parameters describing the conformational stability of the histidine-containing phosphocarrier protein from Streptomyces coelicolor, scHPr, have been determined by steady-state fluorescence measurements of isothermal urea-denaturations, differential scanning calorimetry at different guanidinium hydrochloride concentrations and, independently, by far-UV circular dichroism measurements of isothermal urea-denaturations, and thermal denaturations at fixed urea concentrations. The equilibrium unfolding transitions are described adequately by the two-state model and they validate the linear free-energy extrapolation model, over the large temperature range explored, and the urea concentrations used. At moderate urea concentrations (from 2 to 3 m), scHPr undergoes both high- and low-temperature unfolding. The free-energy stability curves have been obtained for the whole temperature range and values of the thermodynamic parameters governing the heat- and cold-denaturation processes have been obtained. Cold-denaturation of the protein is the result of the combination of an unusually high heat capacity change (1.4 +/- 0.3 kcal.mol(-1).K(-1), at 0 m urea, being the average of the fluorescence, circular dichroism and differential scanning calorimetry measurements) and a fairly low enthalpy change upon unfolding at the midpoint temperature of heat-denaturation (59 +/- 4 kcal.mol(-1), the average of the fluorescence, circular dichroism and differential scanning calorimetry measurements). The changes in enthalpy (m(DeltaH(i) )), entropy (m(DeltaS(i) )) and heat capacity (m(DeltaC(pi) )), which occur upon preferential urea binding to the unfolded state vs. the folded state of the protein, have also been determined. The m(DeltaH(i) ) and the m(DeltaS(i) ) are negative at low temperatures, but as the temperature is increased, m(DeltaH(i) ) makes a less favourable contribution than m(DeltaS(i) ) to the change in free energy upon urea binding. The m(DeltaC(pi) ) is larger than those observed for other proteins; however, its contribution to the global heat capacity change upon unfolding is small.

Activity, Stability and Flexibility in Glycosidases Adapted to Extreme Thermal Environments

To elucidate the strategy of low temperature adaptation for a cold-adapted family 8 xylanase, the thermal and chemical stabilities, thermal inactivation, thermodependence of activity and conformational flexibility, as well as the thermodynamic basis of these processes, were compared with those of a thermophilic homolog. Differential scanning calorimetry, fluorescence monitoring of guanidine hydrochloride unfolding and fluorescence quenching were used, among other techniques, to show that the cold-adapted enzyme is characterized by a high activity at low temperatures, a poor stability and a high flexibility. In contrast, the thermophilic enzyme is shown to have a reduced low temperature activity, high stability and a reduced flexibility. These findings agree with the hypothesis that cold-adapted enzymes overcome the quandary imposed by low temperature environments via a global or local increase in the flexibility of their molecular edifice, with this in turn leading to a reduced stability. Analysis of the guanidine hydrochloride unfolding, as well as the thermodynamic parameters of irreversible thermal unfolding and thermal inactivation shows that the driving force for this denaturation and inactivation is a large entropy change while a low enthalpy change is implicated in the low temperature activity. A reduced number of salt-bridges are believed to be responsible for both these effects. Guanidine hydrochloride unfolding studies also indicate that both family 8 enzymes unfold via an intermediate prone to aggregation.

The molten phase conductance and the thermotropic phase transition in lead(II), zinc(II) and cadmium(II) 10-hydroxyoctadecanoates

The electrical conductances and heats of phase changes in lead(II), zinc(II) and cadmium(II) 10-hydroxyoctadecanoates are measured. The soaps are prepared by metathesis in alcohol solution from 10-hydroxyoctadecanoic acid which in turn is prepared from the oxidation of cis-9-octadecenoic (oleic) acid with concentrated sulphuric acid. The Arrhenius plot for lead(II) 10-hydroxyoctadecanoate shows a maximum, a behaviour that had earlier been interpreted to be due to complex formation. A similar plot for zinc(II) 10-hydroxyoctadecanoate is linear while that for cadmium(II) 10-hydroxyoctadecanoate presents a discontinuity which is interpreted, with the aid of the differential scanning calorimetric (DSC) spectra and optical examination, to be suggestive of the formation of an intermediate phase between the solid and the liquid phases. The low conductances of the soaps in comparison with the literature data on some metal caboxylates indicate the existence of extensive intermolecular hydrogen bonding in the molten phase. Both lead(II) and zinc(II) 10-hydroxyoctadecanoates melt directly from the solid to the liquid phase while their cadmium(II) counterpart passes through the gel-like phase before melting into the liquid. The mesophase of cadmium(II) 10-hydroxyoctadecanoate is transformed to the liquid phase with a relatively low enthalpy change and the behaviour is associated with the cylindrical micekkar structures of the liquid phase that have their formative stage from the gel-like mesophase.

Clustering reactions of HCNH +, HCNH +(N 2) and HCNH +(CH 4) with ethane: application to Titan atmosphere

Thermochemical data for several ion-molecule clustering reactions of HCNH +, HCNH +(N 2) and HCNH +(CH 4) with C 2H 6 were determined by high-pressure mass spectrometry in order to better understand the role of ethane in the chemistry of Titan’s atmosphere. Clustering equilibria were obtained in a gas mixture (pure N 2 with a small amount (10 −5 of the total pressure) of C 2H 6) irradiated by alpha particles in the temperature range 150–320 K. The enthalpy and entropy changes for the reactions HCNH +(C 2H 6) n +C 2H 6+N 2↔HCNH +(C 2H 6) n+1 +N 2 ( n=0, 1), HCNH +(C 2H 6)(N 2) n +N 2+N 2↔HCNH +(C 2H 6)(N 2) n+1 +N 2 ( n=0–2), HCNH +(CH 4)+C 2H 6+N 2↔HCNH +(CH 4)(C 2H 6)+N 2 and HCNH +(N 2)+C 2H 6+N 2↔HCNH +(N 2)(C 2H 6)+N 2 have been measured. For the HCNH +(C 2H 6) n series, the absolute enthalpy change for the trimer formation (4.5 kcal mol −1) is found to be much smaller than the relatively large enthalpy for the dimer formation (11.6 kcal mol −1). For the HCNH +(N 2) n (C 2H 6) series, the enthalpy change appears to decrease monotonically from n=1 to 3, showing no discontinuity in the attachment energy of the ligand molecule. The measured low enthalpy changes suggest that the nature of the bonding remains mainly electrostatic for these cluster’s formations as it is for the formation of HCNH +(CH 4)(C 2H 6). The free energy changes Δ G° for the substitution of N 2 or CH 4 by C 2H 6 in HCNH +(N 2) n ( n=1, 2), HCNH +(CH 4) and HCNH +(N 2)(C 2H 6) are negative for T=90 K and T=165 K. The ligand-exchange reaction leading to HCNH +(C 2H 6) and HCNH +(C 2H 6) 2 is possible in Titan’s lower atmosphere (100–150 km). These processes could lead to the formation of heavier cyanides and eventually large aerosols.

Ion-molecule clustering reactions of HCNH + ion with N 2 and/or CH 4 in N 2CH 4 mixtures

Thermochemical data for several ion-molecule clustering reactions of HCNH + ion with N 2 and (or) CH 4 have been studied by high-pressure mass spectrometry in order to better understand the chemistry of the atmosphere of Titan. Clustering equilibria were obtained in gas mixtures irradiated by α-particles in the temperature range 145–300 K. The enthalpy and entropy changes for the reactions HCNH +(N 2) n + CH 4+N 2↔HCNH +(N 2) n (CH 4)+N 2 ( n = 0–2), HCNH +(N 2) n−1 (CH 4) + N 2 + N 2↔HCNH +(N 2) n + (CH 4)+N 2 ( n = 1, 2), HCNH +(CH 4)+CH 4+N 2 ↔HCNH +(CH 4)+N 2 have been measured. The low enthalpy changes suggest that the nature of the bonding remains mainly electrostatic for these reactions. Enthalpy changes as a function of n for X +. (N 2) n and X +(CH 4) n ions (X + = CH 5 +, C 2H 5 + and C 3H 7 +) were compared with those for HCNH +(N 2) n and HCNH +(CH 4) n ions. The free energy changes for the step-wise replacement of N 2 by CH 4 in HCNH +(N 2) n (CH 4) n clusters ( n and m = 0, 1) have been determined. The ligand-exchange reaction HCNH +(N 2)+CH 4↔HCNH +(CH 4)+N 2, the first step of a possible path for the formation of organic-nitrogen compounds, could exist for T ∼ 160 K in Titan's atmosphere (∼ 150 km). Switching reactions from HCNH +(N 2)(CH 4) involving N 2, CH 4 and C 2H 2 and leading to HCNH +(C 2H 2) 2 could also be a path yielding heavier organic-nitrogen compounds in the lower Titan's atmosphere.

Thermodynamics of maltose binding protein unfolding.

The maltose binding protein (MBP or MalE) of Escherichia coli is the periplasmic component of the transport system for malto-oligosaccharides. It is used widely as a carrier protein for the production of recombinant fusion proteins. The melting of recombinant MBP was studied by differential scanning and titration calorimetry and fluorescence spectroscopy under different solvent conditions. MBP exhibits a single peak of heat absorption with a delta(Hcal)/delta(HvH) ratio in the range of 1.3-1.5, suggesting that the protein comprises two strongly interacting thermodynamic domains. Binding of maltose resulted in elevation of the Tm by 8-15 degrees C, depending of pH. The presence of ligand at neutral pH, in addition to shifting the melting process to higher temperature, caused it to become more cooperative. The delta(Hcal)/delta(HvH) ratio decreased to unity, indicating that the two domains melt together in a single two-state transition. This ligand-induced merging of the two domains appears to occur only at neutral pH, because at low pH maltose simply stabilized MBP and did not cause a decrease of the delta(Hcal)/delta(HvH) ratio. Binding of maltose to MBP is characterized by very low enthalpy changes, approximately -1 kcal/mol. The melting of MBP is accompanied by an exceptionally large change in heat capacity. 0.16 cal/K-g, which is consistent with the high amount of nonpolar surface--0.72 A2/g--that becomes accessible to solvent in the unfolded state. The high value of delta Cp determines a very steep delta G versus T profile for this protein and predicts that cold denaturation should occur above freezing temperatures. Evidence for this was provided by changes in fluorescence intensity upon cooling the protein. A sigmoidal cooperative transition with a midpoint near 5 degrees C was observed when MBP was cooled at low pH. Analysis of the melting of several fusion proteins containing MBP illustrated the feasibility of assessing the folding integrity of recombinant products prior to separating them from the MBP carrier protein.

Thermodynamic effects of mutations on the denaturation of T4 lysozyme

We investigated the folding of substantially destabilized mutant forms of T4 lysozyme using differential scanning calorimetry and circular dichroism measurements. Three mutations in an alpha-helix in the protein's N-terminal region, the alanine insertion mutations S44[A] and K48[A], and the substitution A42K had previously been observed to result in unexpectedly low apparent enthalpy changes of melting, compared to a pseudo-wild-type reference protein. The pseudo-wild-type reference protein thermally unfolds in an essentially two-state manner. However, we found that the unfolding of the three mutant proteins has reduced cooperativity, which partially explains their lower apparent enthalpy changes. A three-state unfolding model including a discrete intermediate is necessary to describe the melting of the mutant proteins. The reduction in cooperativity must be considered for accurate calculation of the energy changes of folding. Unfolding in two stages reflects the underlying two-subdomain structure of the lysozyme protein family.

Arctic life adaptation—II. the function of musk ox ( Ovibos muschatos) hemoglobin

1. 1. The hemoglobin system from musk ox ( Ovibus muschatos) has been characterized from the functional point of view with special regard to the effect of organic phosphates and temperature. 2. 2. The results are similar to those previously obtained in the case of reindeer and confirm that hemoglobins from arctic animals may display very low enthalpy change for the reaction with oxygen. 3. 3. This finding is considered an example of molecular adaptation of respiratory pigments to extreme environmental conditions.

Surface reaction of triethanolamine on cement clinkers

Studying the adsorption of triethanolamine from solutions, the chemisorption of the adsorbate on the surface of the cement clinker can be detected by flow microcalorimetric measurements. The results supported by thermal desorption measurements definitely proved that the triethanolamine molecules practically form a bimolecular adsorption layer on the surface of cement clinker in the saturation range, and the first adsorption layer is bound irreversibly to the clinker species. The difference between the displacement enthalpies on the original clinker samples and on those pretreated with triethanolamine was found to be almost one order of magnitude indicating that the sorption of triethanolamine on the surface of the original clinker sample produces a rather high change of enthalpy which is principally caused by chemisorption. Significantly lower enthalpy changes were measured if triethanolamine had been adsorbed previously on the surface of the clinker, and in this case heat effects corresponding to physical adsorption can only be detected.

Structure and thermotropic behavior of phosphatidylserine bilayer membranes

The structure and thermotropic properties of a homologous series of diacylphosphatidylserines (PS) in the anhydrous and hydrated states have been examined with low-angle X-ray diffraction and differential scanning calorimetry. In the anhydrous state at low temperatures both acidic PS and its NH4+ salts exhibit lamellar bilayer crystal forms that transform to liquid-crystalline hexagonal (type II) structures at higher temperatures. The crystal leads to liquid-crystal transition temperature increases with increasing chain length, the transition temperature of an NH4+-PS being higher than that of its corresponding acidic form. In contrast, the transition enthalpies of the acidic PS are higher than those of the NH4+ salt forms. Hydrated cyclic PS and NH4+-PS exhibit reversible lamellar gel leads to liquid-crystal transitions. In this case the acidic form undergoes this chain length dependent transition at a higher temperature, but with a lower enthalpy change, than the NH4+-PS. Both below and above the hydrocarbon chain melting transition, hydrated lamellar bilayer structures are present. The temperature-composition phase diagram of the NH4+-dimyristoyl-PS/H2O system has been studied in detail. The chain melting transition decreases with increasing hydration, reaching a limiting value of 39 degrees C. X-ray diffraction shows that both the bilayer gel structure and the bilayer liquid-crystal form take up water continuously (i.e., no hydration limit), a characteristic of lipid bilayers with a net charge. Electron-density profiles a NH4+-dimyristoyl-PS at different hydration levels permit detailed analysis of the structural parameters of the PS bilayer.

THE HIGH SPIN SPECIES IN SOLUTION OF DEUTEROHEMIN WITH TWO IMIDAZOLES COVALENTLY LINKED TO PORPHYRIN RING AND ITS EQUILIBRIUM WITH THE LOW SPIN SPECIES

Abstract MCD and NMR studies revealed that deuterohemin is, in part, in a five-coordinated high spin form in solution where two imidazoles bound covalently. For the chemical equilibrium between the high spin and the main low spin six-coordinated species, standard enthalpy and entropy changes were −11×103Jmol−1 and −52 K−1mol−1, respectively.

Addition of pentenes to slowly reacting mixtures of hydrogen and oxygen at 480°C

A detailed analysis has been made of the products formed when 1% of pent-1-ene and of cis-pent-2-ene are added to slowly reacting mixtures of H2+ O2 in aged boric-acid-coated vessels at 480°C. The primary products are formed through four different types of attack on the pentenes of almost equal importance, radical addition of H, OH and HO2, and H atom abstraction from the pentenes by all three radicals. Penta-1,3-diene is the only C5 diene observed, in marked contrast to an earlier study of pentene oxidation. From measurements of the initial relative yields of penta-1,3-diene and buta-1,3-diene, a value of k2= 2.1 × 106 dm3 mol–1 s–1 at 480°C is obtained. [graphic omitted]CH2CH3+ O2→ CH2CH—CHCHCH3+ HO2. This is the first value determined for a reaction of this type and is at least ten times lower than rate constants for the formation of an alkene from an alkyl radical. This decrease is consistent with the considerably lower enthalpy change in reaction (2).