Small Enthalpy Research Articles

High pressure NMR reveals active-site hinge motion of folate-bound Escherichia coli dihydrofolate reductase.

A high-pressure (15)N/(1)H two-dimensional NMR study has been carried out on folate-bound dihydrofolate reductase (DHFR) from Escherichia coli in the pressure range between 30 and 2000 bar. Several cross-peaks in the (15)N/(1)H HSQC spectrum are split into two with increasing pressure, showing the presence of a second conformer in equilibrium with the first. Thermodynamic analysis of the pressure and temperature dependencies indicates that the second conformer is characterized by a smaller partial molar volume (DeltaV = -25 mL/mol at 15 degrees C) and smaller enthalpy and entropy values, suggesting that the second conformer is more open and hydrated than the first. The splittings of the cross-peaks (by approximately 1 ppm on (15)N axis at 2000 bar) arise from the hinges of the M20 loop, the C-helix, and the F-helix, all of which constitute the major binding site for the cofactor NADPH, suggesting that major differences in conformation occur in the orientations of the NADPH binding units. The Gibbs free energy of the second, open conformer is 5.2 kJ/mol above that of the first at 1 bar, giving an equilibrium population of about 10%. The second, open conformer is considered to be crucial for NADPH binding, and the NMR line width indicates that the upper limit for the rate of opening is 20 s(-)(1) at 2000 bar. These experiments show that high pressure NMR is a generally useful tool for detecting and analyzing "open" structures of a protein that may be directly involved in function.

Langmuir-Blodgett films of cellulose ethers containing carbazole

Spread monolayers of cellulose derivatives containing side chain carbazole chromophores are investigated. Structural parameters, such as side chain length and the degree of carbazole substitution, are systematically varied. At lower carbazole content, surface pressure-area isotherms exhibit a liquid analogous phase at molecular areas similar to those observed for cellulose alkyl ethers. A characteristic constant pressure plateau is also evident. As the carbazole content is increased, isotherms shift to lower molecular areas, implying the close parallel packing of chromophore units in the monolayers. An increase in carbazole content also provokes an increase in surface pressure, as well as changes in isotherm shape. For monolayers that show an isotherm plateau, the equilibrium transition surface pressure is found to decrease with increasing temperature. Relatively small positive enthalpy and entropy changes are calculated for the transition. These values are consistent with the identification of the phase transition as bilayer formation. Monolayer transfer to solid substrates was attempted for all samples. Transfer ratios close to unity can be achieved by the optimisation of transfer conditions. Because of the differences in isotherm shape, derivatives with higher carbazole content are transferred at low surface pressure (5 mN·m–1), while (dodecyl)3.0 cellulose and samples of lower carbazole content are transferred at 12 mN·m–1.

A study of complexes Mg(NH 3) n+· and Ag(NH 3) n+, where n = 1–8: competition between direct coordination and solvation through hydrogen bonding

Density functional calculations at B3LYP/6-31+G(d) and B3LYP/DZVP are reported for Mg(NH 3) n +·, where n = 1–6 and for some solvated ions Mg(NH 3) n +· … NH 3 ( n = 1–3, 6). After correction for basis set superposition errors, the enthalpies for sequential addition of NH 3 to Mg +· resulting from direct coordination to the metal are 38.1, 26.6, 21.2, 13.7, 12.1, and 11.3 kcal mol −1. The free energies for these same addition reactions are all negative, although for complexes with n ≥ 4 the values are very small. Attempts at optimising structures with higher coordination numbers all resulted in the formation of solvated octahedral complexes. Enthalpies for solvation through hydrogen bonding to one of the ligated NH 3 molecules are all less than 16 kcal mol −1 and decrease rapidly as the number of ligated NH 3 molecules increases. Molecular orbital calculations at B3LYP/DZVP have been used to optimise structures for ions Ag(NH 3) n +, where n = 1–6. The five-coordinate and six-coordinate structures have very small binding enthalpies (4.3 and 2.6 kcal mol −1) and the free energies for formation of these ions are positive. The binding energies for the addition of the first and second NH 3 molecules added to Ag + are 40.1 and 36.1 kcal mol −1, while those for the third and fourth additions are much smaller (15.1 and 11.0 kcal mol −1). Adducts up to n = 3 have been detected in electrospray experiments. The first three adducts of Ag + with NH 3 have been formed in the selected ion flow tube apparatus and multicollision induced dissociation experiments show Ag(NH 3) 3 + to have a lower binding enthalpy than both Ag(NH 3) 2 + and Ag(NH 3) +.

Structural changes in poly(ethylene terephthalate) induced by mechanical milling

Poly(ethylene terephthalate) (PET) has been subjected to high-energy ball milling (mechanical milling, MM) at three different temperatures. The resulting milled powder is characterized by molecular weight measurements, differential scanning calorimetry and wide-angle X-ray scattering. Regardless of the initial degree of crystallinity or milling temperature employed, MM apparently yields an “oriented amorphous” PET morphology in which the PET chains are locally aligned but rotationally disordered. This conclusion is based on the persistence of the (100) peak in otherwise amorphous x-ray patterns from milled PET. Thermograms of milled PET exhibit a small, broad crystallization exotherm and a large melting endotherm. The unusually small crystallization enthalpy is attributed to the local orientation of PET molecules present in the oriented amorphous morphology. Only minor rotations and translations of oriented PET molecules are needed to put the chains into register and, hence, the crystalline state. Evidence is also presented to suggest that extended-chain crystals of PET are produced upon crystallization of mechanically milled PET.

Partially unfolded equilibrium state of hen lysozyme studied by circular dichroism spectroscopy.

Equilibrium unfolding of hen egg white lysozyme as a function of GdnCl concentration at pH 0.9 was studied over a temperature range 268.2-303.2 K by means of CD spectroscopy. As monitored by far- and near-UV CD at 222 and 289 nm, the lack of coincidence between two unfolding transition curves was observed, which suggests the existence of a third conformational species in addition to native and unfolded states. The three-state model, in which a stable intermediate is populated, was employed to estimate the thermodynamic parameters for the GdnCl-induced unfolding. It was found that the transition from the native to intermediate states proceeds with significant changes in enthalpy and entropy due to an extremely cooperative process, while the transition from the intermediate to unfolded states shows a low cooperativity with small enthalpy and entropy changes. These results indicate that the highest energy barrier for the GdnCl-induced unfolding of hen lysozyme is located in the process from the native state to the intermediate state, and this process is largely responsible for the cooperativity of protein unfolding.

Caffeine and nicotinamide enhances the aqueous solubility of the antimalarial agent halofantrine

The aqueous solubility of the antimalarial agent halofantrine in phosphate buffers pH 5.9 and 7.0 (ionic strength 0.08) is increased by the addition of caffeine and nicotinamide. Solubility is increased to a greater extent in the presence of caffeine (12.5–125 mM) than nicotinamide (125 mM –2.0 M). The greatest increase in solubility was observed at pH 5.9 where the basal solubility of halofantrine rose from 0.91 to 435 μM when 125 mM caffeine was added. Phase solubility studies support the formation of a 1:1 complex between caffeine and halofantrine which is characterised by a K 1:1 constant of 2.75×10 3 M −1 (pH 5.9). A less stable 1:1 complex is formed at pH 7.0 ( K 1:1=6.37×10 3 M −1). Differential scanning calorimetry of solid mixtures of caffeine and halofantrine showed the absence of the endotherms of the two drugs and the appearance of a distinct endotherm (with a smaller enthalpy) characteristic of the complex. An analysis of the 1H-NMR spectra of mixtures of caffeine and halofantrine revealed perturbations in the chemical shifts of the methyl group and proton at positions 4 and 8 of caffeine, and a change in splitting pattern of the H 9 proton of the phenanthrene ring in halofantrine.

Donor−Acceptor Complexes of Alkylcarbazole and Dicarbazolylalkane Donors with the Acceptors Tetracyanoethylene and Tetranitromethane

Electron donor−acceptor complexes of conformationally flexible 1,n-dicarbazolylalkanes (C12H8N−(CH2)n−NC12H8), where n = 1−5, were examined. Carbazole, methylcarbazole, ethylcarbazole, and cyanoethylcarbazole also were studied as monochromophoric analogues for comparison. In dichloromethane solution, the dicarbazolylalkanes form 1:1 complexes with the terminal carbazolyl chromophores acting as independent donors when n ≥ 2. With the acceptor tetranitromethane (TNM), the carbazoles form contact complexes displaying small positive enthalpies of formation. In contrast, stable complexes form with the acceptor tetracyanoethylene (TCNE). Crystalline TCNE complexes were isolated for the bichromophoric donors with n = 2−4. The solid complexes and their uncomplexed donor components were analyzed by single-crystal X-ray diffraction. The solid-state stoichiometries of (carbazolyl donor):(TCNE acceptor) were found to depend on the donor conformation. Dicarbazolylalkane donors separated by two or four methylene units ...

Synthesis of laterally substituted bistolane liquid crystals

Methods for synthesizing bistolane liquid crystal materials with lateral methyl and ethyl substituents are presented. Some of the bistolanes are nematic at room temperature. These highly conjugated mesogens exhibit wide nematic ranges, small enthalpies of fusion, high birefringence and modest viscosity. Their potential applications for flat panel displays employing light scattering or Bragg reflection and for infrared optically phased arrays are foreseeable.

Temperature Dependence of Chloride Complexation for the Trivalent f-Elements

Abstract The influence of elevated temperatures on the formation of 1:1 chloro complexes for Eu3+ and Am3+ are reported. Using a solvent extraction technique, stability constants for the equilibrium M(aq) 3++Cl(aq) – MCl(aq) 2+ have been measured in the temperature range of 25–75 °C. Modest increases in 1 are observed, and small positive enthalpies for these reactions are reported. These data are discussed in the context of previous reports for the trivalent lanthanide and actinide chloro systems.

Thermochemistry of stuffed quartz-derivative phases along the join LiAlSiO4-SiO2

Enthalpies of drop-solution (∆Hdrop-soln) of a suite of stuffed quartz-derivative phases with the composition Li1-xAl1-xSi1+xO4 (0 ≤ x ≤ 1) have been measured in molten 2PbO·B2O3 at 974 K. Substitution of Si4+ for Li++Al3+ results in more exothermic enthalpies of drop-solution, which is consistent with behavior seen in other crystalline and glassy aluminosilicates. Al/Si ordering serves to stabilize these phases, and long-range ordering for compositions with x approximately <0.3 can be discerned in both calorimetric data and in structural data obtained by electron and synchrotron X-ray diffraction (XRD). In contrast, a structural but not an energetic discontinuity is apparent at x ≅ 0.65, which corresponds to a compositionally induced α-β quartz transition with a small enthalpy of transformation.

Phase transition properties of polyethylene glycol-cellulose blends and their miscibility in mixed solvents

The phase transition properties of blends of polyethylene glycol (PEG) with cellulose (CELL) prepared from solution in N,N-dimethylacetamide/lithium chloride (DMAC/LiCI) and those from solution in dimethylsulfoxide/paraform- aldehyde (DMSO/PF) were found to be completely different. The differences of the phase transition properties were probably related to the different miscibilities of these two polymers in the two solvent systems. In DMAC/LiCl, the miscibility of CELL and PEG was limited; the composite obtained exhibited a solid- liquid phase transition and had a small phase transition enthalpy. However, in DMSO/PF, these two polymers had a high level of miscibility; the composite obtained exhibited a solid-solid phase transition and had a large phase enthalpy. It is suggested that the differences of miscibility and the phase transition properties were caused by the different dissolving mechanisms of CELL and the different interactions in these two solutions.

Facile formation of azines from reactions of singlet methylene and dimethylcarbene with precursor diazirines: theoretical explorations

The reactions of the singlet methylene (1a) and dimethylcarbene (1b), with their diazirine precursors, diazirine (2a), and dimethyldiazirine (2b), have been studied theoretically using ab initio and density functional theory. The reaction has no activation barriers for the parent system (1a + 2a) and proceeds via a reactive complex and a transition state with a small negative enthalpy of activation Δ Hnot =298 = -1.1 kcal mol-1, ΔSnot =298 = -34.4 cal mol-1 K-1, ΔG°298 = 9.2 kcal mol-1) for the dimethyl derivatives (1b + 2b). The formation of N-methylene diazirinium ylides (3a,b) is exothermic by 64-80 kcal mol-1. The isomer, 1,3-diazabicyclo[1.1.0]butane (4a), is more stable (5-12 kcal mol-1) than isomer 3a, but can neither be formed by direct thermal reaction of 1a with 2a nor undergo the direct rearrangement into formaldazine (5a). The rearrangement of ylides 3a,b into azines 5a,b proceeds by conrotatory C3-N1 ring opening. The predicted activation barrier of ca. 15 kcal mol-1 for the ring opening in ylide 3b is in excellent agreement with experimental data. The formation of pyridinium ylides from carbenes and pyridine is also studied.Key words: diazirinium ylide, ab initio MO (molecular orbital) theory, density functional theory, pyridinium ylide, CIS (singles configuration interaction) transition energies.

High birefringence and wide nematic range bis-tolane liquid crystals

Several dialkyl and fluoro bis-tolane liquid crystals were synthesized and their physical properties evaluated. These highly conjugated liquid crystals exhibit a wide nematic range, small heat fusion enthalpy, high birefringence, and relatively low viscosity. Their excellent compatibility with commercial cyano mixtures makes these liquid crystals attractive for many electro-optic applications.

Facile formation of azines from reactions of singlet methylene and dimethylcarbene with precursor diazirines: theoretical explorations

The reactions of the singlet methylene (1a) and dimethylcarbene (1b), with their diazirine precursors, diazirine (2a), and dimethyldiazirine (2b), have been studied theoretically using ab initio and density functional theory. The reaction has no activation barriers for the parent system (1a + 2a) and proceeds via a reactive complex and a transition state with a small negative enthalpy of activation Δ Hnot =298 = -1.1 kcal mol-1, ΔSnot =298 = -34.4 cal mol-1 K-1, ΔG°298 = 9.2 kcal mol-1) for the dimethyl derivatives (1b + 2b). The formation of N-methylene diazirinium ylides (3a,b) is exothermic by 64-80 kcal mol-1. The isomer, 1,3-diazabicyclo[1.1.0]butane (4a), is more stable (5-12 kcal mol-1) than isomer 3a, but can neither be formed by direct thermal reaction of 1a with 2a nor undergo the direct rearrangement into formaldazine (5a). The rearrangement of ylides 3a,b into azines 5a,b proceeds by conrotatory C3-N1 ring opening. The predicted activation barrier of ca. 15 kcal mol-1 for the ring opening in ylide 3b is in excellent agreement with experimental data. The formation of pyridinium ylides from carbenes and pyridine is also studied.Key words: diazirinium ylide, ab initio MO (molecular orbital) theory, density functional theory, pyridinium ylide, CIS (singles configuration interaction) transition energies.

Formation and isomerization of cis, cis-Ir(H) 2(--Ph)(CO)(PPh 3) 2 and cis, cis-Ir(H)(--Ph) 2(CO)(PPh 3) 2 in oxidative addition of hydrogen and phenylacetylene to trans-Ir(CO)(--Ph)(PPh 3) 2

Oxidative addition of H–R (H--Ph and H 2) to trans-Ir(--Ph)(CO)(PPh 3) 2 ( 2) gives the initial products, cis, cis-Ir(H)(--Ph) 2(CO)(PPh 3) 2 ( 3a) and cis, cis-Ir(H) 2(--Ph)(CO)(PPh 3) 2 ( 3b), respectively. Both cis-bis(PPh 3) complexes, 3a and 3b undergo isomerization to give the trans-bis(PPh 3) complexes, trans, trans-Ir(H)(--Ph) 2(CO)(PPh 3) 2 ( 4a) and cis, trans-Ir(H) 2(--Ph)(CO)(PPh 3) 2 ( 4b). The isomerization, 3b→ 4b is first order with respect to 3b with k 1=6.37×10 −4 s −1 at 25°C under N 2 in CDCl 3. The reaction rate ( k 1) seems independent of the concentration of H 2. A large negative entropy of activation (Δ S ≠=−24.9±5.7 cal deg −1 mol −1) and a relatively small enthalpy of activation (Δ H ≠=14.5±3.3 kcal mol −1) were obtained in the temperature range 15∼35°C for the isomerization, 3b→ 4b under 1 atm of H 2.

Energetics of solvent and ligand-induced conformational changes in alpha-lactalbumin.

The energetics of structural changes in the holo and apo forms of a-lactalbumin and the transition between their native and denatured states induced by binding Ca2+ and Na+ have been studied by differential scanning and isothermal titration microcalorimetry and circular dichroism spectroscopy under various solvent conditions. Removal of Ca2+ from the protein enhances its sensitivity to pH and ionic conditions due to noncompensated negative charge-charge interactions at the cation binding site, which significantly reduces its overall stability. At neutral pH and low ionic strength, the native structure of apo-alpha-lactalbumin is stable below 14 C and undergoes a conformational change to a native-like molten globule intermediate at temperatures above 25 degrees C. The denaturation of either holo- or apo-alpha-lactalbumin is a highly cooperative process that is characterized by an enthalpy of similar magnitude when calculated at the same temperature. Measured by direct calorimetric titration, the enthalpy of Ca2+-binding to apo-LA at pH 7.5 is -7.1 kJ mol(-1) at 5.0 degrees C. which is essentially invariant to protonation effects. This small enthalpy effect infers that stabilization of alpha-lactalbumin by Ca2+ is primarily an entropy driven process, presumably arising from electrostatic interactions and the hydration effect. In contrast to the binding of calcium, the interaction of sodium with apo-LA does not produce a noticeable heat effect and is characterized by its ionic nature rather than specific binding to the metal-binding site. Characterization of the conformational stability and ligand binding energetics of alpha-lactalbumin as a function of solvent conditions furnishes significant insight regarding the molecular flexibility and regulatory mechanism mediated by this protein.

Induced Acceleration of Phosphine Exchange in Metal Carbonyls by Pendant Groups of Coordinated Polyphosphines. Two Dangling Phosphine Arms Are Much Better Than One1

The kinetics and thermodynamics of isomerization of the linkage isomers (OC)5W[η1-PPh2CH2CH(PPh2)2] (1) and (OC)5W[η1-PPh2CH(PPh2)CH2PPh2] (2) in chloroform have been studied by 31P{1H} NMR over the temperature range of 10−55 °C. Conversion of 1 to 2 is exothermic (ΔH = −12.25 ± 0.1 kJ mol-1) and is accompanied by a large decrease in entropy (ΔS = −28.2 ± 0.3 J mol-1 K-1). The reaction proceeds much faster than expected [k = (1.18 ± 0.01) × 10-5 s-1] at 25 °C, and its small activation enthalpy (ΔH⧧ = 92.6 ± 1.9 kJ mol-1) and large negative activation entropy (ΔS⧧ = −28.2 ± 6.2 J mol-1 K-1) suggest an associative mechanism. The reaction is 4 orders of magnitude faster than the isomerization of (OC)5W[η1-PPh2CH2CH2P(p-tol)2] (3) to (OC)5W[η1-P(p-tol)2CH2CH2PPh2] (4). Exchange of coordinated and dangling phosphines is faster than chelation for each of the four complexes, but chelation of 1 or 2 is much faster than chelation of 3 or 4. It appears that the second dangling phosphine arm present in 1 and 2 accel...

A fast quantitative modelling of ns laser ablation based on non-stationary averaging technique

A semi-analytical approach to a quantitative analysis of thermal ns laser ablation is presented. The nonlinear heat equation is reduced to three ordinary differential equations for the surface temperature, spatial width of the enthalpy distribution, and the ablated depth. Due to its speed and flexibility, the method provides a convenient tool for the fast analysis of experimental data. The influence of different factors on ablation curves (ablated depth h vs. fluence φ) is studied. Analytical formulas for threshold fluence φ th and h( φ) dependences are discussed. The ablation curves reveal three regions of fluence: Arrhenius, linear, and the screening region. Small vaporization enthalpy results in a sub-linear but faster than logarithmic h( φ) dependence. Weakly-absorbing materials may exhibit two different ablation regimes – without or with ablation of the heated subsurface layer. The method is applied to the analysis of the single pulse ablation of polyimide. The single thermal evaporation energy ≈1.51 eV describes satisfactory the ablation at 248, 308 and 351 nm wavelengths.

Swelling of sodium chloride filled polybutadiene networks in water, water/ acetone and water/THF mixtures

The swelling of polybutadiene (PB) networks of different cross-link densities filled with finely divided sodium chloride (NaCI) particles are studied in water, water/acetone and water/THF mixtures. Swelling over periods of 2 months are recorded. The degree of swelling was observed to increase continuously in water during this period, whereas in water/acetone mixtures it reached a peak followed by deswelling and it converged to a maximal value in water/THF mixtures. The highly cross-linked samples exhibited unusually high degrees of swelling, and calculations indicated full stretching of the network chains. The enthalpic contribution to the chemical potential of the networks in equilibrium with solvent was determined using the phantom network model of rubber elasticity theory. The swollen network-salt system was approximated by the lattice model of polymer solutions. The positive contributions of water and acetone to the chemical activity of the swollen network were offset by the negative contribution of dissociation of salt in water, resulting in a small negative enthalpy of mixing. The highest rate of diffusion of solvent was observed for the PB/water/THF ternary system. Diffusion rates of pure water and water/acetone mixture were much smaller and of comparable magnitude. Mechanical measurements of stress and strain at break, and dynamic mechanical measurements of storage and loss moduli of filled and unfilled samples showed that filling with NaCl did not change the mechanical properties of the networks significantly, in spite of the dramatic change in the swelling behaviour.

Elastic, thermoelastic and piezoelastic properties of crystals of the KSCN family (KN3, KHF2, KSCN, NH4HF2, NH4SCN)

Abstract Large single crystals of KN3, KHF2, NH4HF2, KSCN and NH4SCN, which are structurally related to those of the CsCl type, were grown from aqueous solutions by controlled evaporation. Elastic, thermoelastic and piezoelastic constants were determined from ultrasonic resonance frequencies of thick plane-parallel plates. In addition, the tensors of thermal expansion were measured. Elastic properties and thermal expansion reflect the close relationship to the CsCl-type structure. The anisotropy is mainly caused by the preferential orientation of the long axes of the anionic dumb-bells within one distinct plane. KSCN exhibits weak elastic anomalies approaching the phase transition at 413 K. These effects, which are much smaller than those found in the high-temperature phases of the alkali cyanides, indicate, together with the almost missing hysteresis and the small transition enthalpy, a transition of mainly second order. The variation of the elastic behaviour of KHF2 and NH4SCN passing through the transition at 471 K and 367 K, respectively, express first-order processes.