Deg-1 Mol-1 Research Articles

Effect of Co-substitution on the electrocatalytic properties of Ni1.5Fe1.5O4 for oxygen evolution in alkaline solutions

Some ternary ferrites having composition CoxNi1.5-xFe1.5O4 (0.0 ≤ x ≤ 1.25) have been synthesized by NH4OH co-precipitation method at 11.5 pH. Materials, so obtained, were tested for their electrocatalytic properties towards oxygen evolution reaction (OER) in the form film on Ni-support in alkaline solution. The study showed that the electrocatalytic properties the material increased with partial substitution of Co for Ni in the base oxide (Ni1.5Fe1.5O4). The value being highest with 0.5 mol Co-substitution. At E = 850 mV vs Hg/HgO in 1M KOH at 25 ºC, the electrode showed apparent current density 206.2 mA cm-2, which is about 3 times higher than the base oxide. The Tafel slope values were ranged between 46-82 mV decade-1. A pair of redox peak, an anodic (EPa = 522±28 mV) and corresponding cathodic (EPc= 356±9 mV), was observed in the cyclic voltammetry (CV) study of the material. The thermodynamic parameters namely, standard apparent electrochemical enthalpy of activation ( ), standard enthalpy of activation ( ) and standard entropy of activation ( ) for the oxygen evolution reaction (OER) have also been determined by recording anodic polarization curve in 1M KOH. The value of was observed to be almost similar with each oxide electrode. The values were highly negative and ranged between ~ -165 and ~ -207 J deg-1 mol-1. Phase and morphology of materials have been investigated by using physical techniques X-ray diffraction, infrared spectroscopy (IR) and scanning electron microscope (SEM), respectively.

Electrocatalytic Properties of Egg-white Sol-gel Derived MnxFe3-xO4 (0 ≤ x ≤ 1.5) for Alkaline Water Electrolysis

Some binary ferrites having molecular formula, MnxFe3-xO4 (0 ≤ x ≤ 1.5) have been synthesized by a simple and cost effective egg white sol-gel route. In each preparation, nitrates of constituent metal were taken as starting materials and egg white as gelling agent. The material, thus obtained was altered in the form of film electrode on the pre-treated Ni- support using an oxide-slurry painting tech-nique and investigated as electrocatalysts for the oxygen evolution reaction in KOH solutions. The study showed that Mn-substitution in the base oxide matrix influences the electrocatalytic activity of the material significantly and found to be greatest with 0.5 mol Mn-substitution. The reaction order has been found to be approximately unity with respect to OH¯ concentration. The Tafel slope values at low potential ranged between ~ 52 and ~ 65 mVdecade-1. The thermodynamic parameters namely, standard apparent electrochemical enthalpy of activa-tion (), standard enthalpy of activation () and standard entropy of activation () for the oxygen evolution reac-tion (OER) have also been determined. It has been observed that and values decreased with Mn-substitution in the Fe3O4 lattice. This decrease inandwas found to be greatest with 0.5 mol Mn. Thevalues were also found to be highly negative varying between ~ - 185 and – 211 J deg-1 mol-1.

The pressure-temperature phase diagram of hen lysozyme at low pH.

The equilibrium unfolding of hen lysozyme at pH 2 was studied as a function of pressure (0.1~700MPa) and temperature (−10°C~50°C) using Trp fluorescence as monitor supplemented by variable pressure 1H NMR spectroscopy (0.1~400MPa). The unfolding profiles monitored by the two methods allowed the two-state equilibrium analysis between the folded (N) and unfolded (U) conformers. The free energy differences ΔG (=GU–GN) were evaluated from changes in the wavelength of maximum fluorescence intensity (λmax) as a function of pressure and temperature. The dependence of ΔG on temperature exhibits concave curvatures against temperature, showing positive heat capacity changes (ΔCp=CpU–CpN= 1.8–1.9 kJ mol−1 deg−1) at all pressures studied (250~400 MPa), while the temperature TS for maximal ΔG increased from about 10°C at 250MPa to about 40°C at 550MPa. The dependence of ΔG on pressure gave negative volume changes (ΔV=VU–VN) upon unfolding at all temperatures studied (−86~−17 mlmol−1 for −10°C~50°C), which increase significantly with increasing temperature, giving a positive expansivity change (Δα~1.07mlmol−1 deg−1). A phase-diagram between N and U (for ΔG=0) is drawn of hen lysozyme at pH 2 on the pressure-temperature plane. Finally, a three-dimensional free energy landscape (ΔG) is presented on the p-T plane.

Optimization of Procedures for the Syntheses of Bisphosphinepalladium(0) Precursors for Suzuki−Miyaura and Similar Cross-Coupling Catalysis: Identification of 3:1 Coordination Compounds in Catalyst Mixtures Containing Pd(0), PCy3, and/or PMeBut2

Although Pd[PCy3]2, Pd[PMeBut2]2, Pd[PBut3]2, and a large number of similar bisphosphinepalladium(0) compounds are excellent cross-coupling catalysts, most are not readily synthesized and stored, and research involving their utilization generally relies on in situ synthetic processes that proceed at unknown rates and in unknown yields. This paper describes, in part, the development of an efficient synthetic route to the representative catalysts Pd[PCy3]2, Pd[PMeBut2]2, and Pd[PBut3]2 via reactions of Pd(η3-C3H5)(η5-C5H5) with 2 equiv each of PCy3, PMeBut2, or PBut3. The procedures, which may be generally useful, are optimized such that known concentrations of the bisphosphine catalysts may be generated quickly, reliably, and under mild conditions. That said, the chemistry of the 2:1 compounds can be surprisingly complex; although Pd[PBut3]2 shows no inclination to increase its coordination number, Pd[PCy3]2 and Pd[PMeBut2]2 react with added PCy3 or PMeBut2 to form 3:1 coordination compounds. Equilibrium constants for dissociation of the compounds PdL3 to PdL2 + free L (L = PCy3, PMeBut2) were measured over a range of temperatures, and from the resulting linear plots of ln KD vs 1/T, ΔH and ΔS values of 21 kJ mol-1 and 59 J deg-1 mol-1, respectively, were determined for the PCy3 system, and 23 kJ mol-1 and 86 J deg-1 mol-1, respectively, for PMeBut2. The enthalpy values provide a rough measure of the Pd−P bond dissociation energies, which are, as expected, considerably lower than both the single Pd(II)−phosphine and the single Pt(0)−phosphine bond energies that have been reported (both ∼55 kJ mol-1). The significant formation of the catalytically less active 3:1 compounds has serious implications for many of the published catalytic cross-coupling processes that involve catalyst formation via the slow reduction of palladium(II) precursors in the presence of excess phosphine; for many such systems, relatively little of the added palladium may actually be present as the catalytically active bisphosphinepalladium(0) compound.

Mechanistic Studies of Ubiquitin C-Terminal Hydrolase L1

Ubiquitin C-terminal hydrolases (UCHs) cleave Ub-X bonds (Ub is ubiquitin and X an alcohol, an amine, or a protein) through a thioester intermediate that is produced by nucleophilic attack of the Cys residue of a Cys-SH/His-Im catalytic diad. We are studying the mechanism of UCH-L1, a UCH that is implicated in Parkinson's disease, and now wish to report our initial findings. (i) Pre-steady-state kinetic studies for UCH-L1-catalyzed hydrolysis of Ub-AMC (AMC, 7-amido-4-methylcoumarin) indicate that k(cat) is rate-limited by acyl-enzyme formation. Thus, K(m) = K(s), the dissociation constant for the Michaelis complex, and k(cat) = k(2), the rate constant for acyl-enzyme formation. (ii) For K(assoc) (=K(s)(-)(1)), DeltaC(p) = -0.8 kcal mol(-)(1) deg(-)(1) and is consistent with coupling between substrate association and a conformational change of the enzyme. For k(2), DeltaS(++) = 0 and suggests that in the E-S, substrate and active site residues are precisely aligned for reaction. (iii) Solvent isotope effects are (D)K(assoc) = 0.5 and (D)k(2) = 0.9, suggesting that the substrate binds to a form of free enzyme in which the active site Cys exists as the thiol. In the resultant Michaelis complex, the diad has tautomerized to ion pair Cys-S(-)/His-ImH(+). Subsequent attack of thiolate produces the acyl-enzyme species. In contrast, isotope effects for association of UCH-L1 with transition-state analogue ubiquitin aldehyde suggest that an alternative mechanistic pathway can sometimes be available to UCH-L1 involving general base-catalyzed attack of Cys-SH by His-Im.

G148–GA3: A streptococcal virulence module with atypical thermodynamics of folding optimally binds human serum albumin at physiological temperatures

The third albumin binding domain of streptococcal protein G strain 148 (G148–GA3) belongs to a novel class of prokaryotic albumin binding modules that is thought to support virulence in several bacterial species. Here, we characterize G148–GA3 folding and albumin binding by using differential scanning calorimetry and isothermal titration calorimetry to obtain the most complete set of thermodynamic state functions for any member of this medically significant module. When buffered at pH 7.0 the 46-amino acid alpha-helical domain melts at 72 °C and exhibits marginal stability (15 kJ/mol) at 37 °C. G148–GA3 unfolding is characterized by small contributions to entropy from non-hydrophobic forces and a low Δ C p (1.1 kJ/(deg mol)). Isothermal titration calorimetry reveals that the domain has evolved to optimally bind human serum albumin near 37 °C with a binding constant of 1.4 × 10 7 M −1. Analysis of G148–GA3 thermodynamics suggests that the domain experiences atypically small per residue changes in structural dynamics and heat capacity while transiting between folded and unfolded states.

Improper hydrogen C-H...O bonds cause self-association of alpha, beta-enaminoketones containing fluorosubstituted alkyl groups.

Concentration- and temperature-dependent IR, NMR and dipole-moment studies on 4-N,N-dimethylamino-1,1,1-trifluoro-3-buten-2-one and two of its higher homologues showed that these compounds undergo reversible dimerization in nonpolar solvents. Antiparallel "closed" dimers are formed with a network of improper intermolecular C-H...O hydrogen bonds. Quantitative analysis of the 1H NMR data yielded delta H0 = -17.6 kJ mol-1 and delta S0 = -46.9 J deg-1 mol-1. The interactions observed are the strongest among those involving a C-H group reported so far. The complex described here is the first example of a cyclic complex stabilized by two improper C-H...O hydrogen bonds. The conclusions drawn from the solution and solid-state data were confirmed by ab initio calculations.

Peroxynitrite Reactions with Dimethylsulfide and Dimethylselenide: An Experimental Study

The kinetics of the peroxynitrite (ONOO- and HOONO, PN for both) reaction with (Me)2X (X = S and Se, dimethyl sulfide (DMS) and dimethylselenide (DMSe)) was studied. The reaction is first order with respect to both PN and (Me)2X concentrations. (Me)2XO and NO2- were formed with 100% yield based on PN in all cases except in the DMS reaction with HOONO, where the yield was ∼80%. DMSe is much more reactive than DMS toward both ONOO- and HOONO, while HOONO reacts much faster than ONOO- with both (Me)2X. The activation entropies (in cal deg-1 mol-1) and enthalpies (given in parentheses, in kcal/mol) for the reactions of DMS and DMSe with HOONO were determined to be −23 ± 3 (5.9 ± 0.5) and −20.5 ± 3 (4.6 ± 0.5), respectively. The corresponding values for the reaction with ONOO- were −29 ± 3 (9.7 ± 0.6) and −28 ± 3 (7.2 ± 0.5). A partial replacement of water with an organic cosolvent resulted in a decrease in the rate of reaction of ONOO- with both DMS and DMSe, while it had no effect on the rates of reaction wi...

A more unified picture for the thermodynamics of nucleic acid duplex melting: a characterization by calorimetric and volumetric techniques.

We use a combination of calorimetric and volumetric techniques to detect and to characterize the thermodynamic changes that accompany helix-to-coil transitions for five polymeric nucleic acid duplexes. Our calorimetric measurements reveal that melting of the duplexes is accompanied by positive changes in heat capacity (DeltaCP) of similar magnitude, with an average DeltaCP value of 64.6 +/- 21.4 cal deg-1 mol-1. When this heat capacity value is used to compare significantly different transition enthalpies (DeltaHo) at a common reference temperature, Tref, we find DeltaHTref for duplex melting to be far less dependent on duplex type, base composition, or base sequence than previously believed on the basis of the conventional assumption of a near-zero value for DeltaCP. Similarly, our densimetric and acoustic measurements reveal that, at a given temperature, all the AT- and AU-containing duplexes studied here melt with nearly the same volume and compressibility changes. In the aggregate, our results, in conjunction with literature data, suggest a more unified picture for the thermodynamics of nucleic acid duplex melting. Specifically, when compared at a common temperature, the apparent large differences present in the literature for the transition enthalpies of different duplexes become much more compressed, and the melting of all-AT- and all-AU-containing duplexes exhibits similar volume and compressibility changes despite differences in sequence and conformation. Thus, insofar as thermodynamic properties are concerned, when comparing duplexes, the temperature under consideration is as important as, if not more important than, the duplex type, the base composition, or the base sequence. This general behavior has significant implications for our basic understanding of the forces that stabilize nucleic acid duplexes. This behavior also is of practical significance in connection with the use of thermodynamic databases for designing probes and for assessing the affinity and specificity associated with hybridization-based protocols used in a wide range of sequencing, diagnostic, and therapeutic applications.

Association of terminal complement proteins in solution and modulation by suramin.

The association of terminal complement proteins was investigated by analytical ultracentrifugation and multi-angle laser light scattering. Native C8 and C9 formed a heterodimer in solution of physiological ionic strength with a free-energy change DeltaG degrees of -8.3 kcal/mol and a dissociation constant Kd of 0.6 microM (at 20 degrees C) that was ionic strength- and temperature-dependent. A van't Hoff plot of the change in Kd was linear between 10 and 37 degrees C and yielded values of DeltaH degrees = -12.9 kcal/mol and DeltaS degrees = -15.9 cal mol-1 deg-1, suggesting that electrostatic forces play a prominent role in the interaction of C8 with C9. Native C8 also formed a heterodimer with C5, and low concentrations of polyionic ligands such as protamine and suramin inhibited the interaction. Suramin induced high-affinity trimerization of C8 (Kd = 0.10 microM at 20 degrees C) and dimerization of C9 (Kd = 0.86 microM at 20 degrees C). Suramin-induced C8 oligomerization may be the primary reason for the drug's ability to prevent complement-mediated hemolysis. Analysis of sedimentation equilibria and also of the fluorescence enhancement of suramin when bound to protein provided evidence for two suramin-binding sites on each C9 and three on each C8 in the oligomers. Oligomerization could be reversed by high suramin concentrations, but 8-aminonaphthalene-1,3,6- trisulfonate (ANTS2- ), which mimics half a suramin molecule, could not compete with suramin binding and oligomerization suggesting that the drug also binds nonionically to the proteins.

Effect of Additives on Kinetic Thermal Stability of Polygalacturonase II from Aspergillus carbonarius: Mechanism of Stabilization by Sucrose

The thermal inactivation of polygalacturonase (PG II; Mw = 42 kDa) from Aspergillus carbonarius at optimum pH of the enzyme activity followed first-order kinetics both in the presence and in the absence of additives. The enzyme was stable below its optimum pH. Sodium chloride enhanced the thermal stability, the midpoint of thermal stability (Tm), shifted by 6 °C, and the half-life at Tm increased by 68-fold. Except for ethylene glycol, the other polyhydric alcohols enhanced thermal stability in a concentration-dependent manner; the stabilizing effect by polyhydric alcohols increased with increase in hydroxyl content of the polyol. The activation enthalpy increased in the presence of 1 M sucrose and 1 M NaCl from 77 to 116 kcal mol-1; activation entropy increased from 169 to 290 cal deg-1 mol-1. The net free energy change of stabilization in the presence of stabilizers was 3−4 kcal mol-1. The thermal inactivation of the enzyme involved conformational changes and unfolding of the enzyme molecule; the major secondary structural element, β structure, decreased and the tryptophan residues were exposed to solvent. Stabilization of enzyme by sucrose was by preventing unfolding of the enzyme. Keywords: Additives; polyhydric alcohols; polygalacturonase; pectinases; thermal stability; mechanism of stabilization

Why Does D2Bind Better Than H2? A Theoretical and Experimental Study of the Equilibrium Isotope Effect on H2Binding in a M(η2-H2) Complex. Normal Coordinate Analysis of W(CO)3(PCy3)2(η2-H2)

Vibrational data (IR, Raman and inelastic neutron scattering) and a supporting normal coordinate analysis for the complex trans-W(CO)3(PCy3)2(η2-H2) (1) and its HD and D2 isotopomers are reported. The vibrational data and force constants support the well-established η2-bonding mode for the H2 ligand and provide unambiguous assignments for all metal−hydrogen stretching and bending frequencies. The force constant for the HH stretch, 1.3 mdyn/Å, is less than one-fourth the value in free H2 and is similar to that for the WH stretch, indicating that weakening of the H−H bond and formation of W−H bonds are well along the reaction coordinate to oxidative addition. The equilibrium isotope effect (EIE) for the reversible binding of dihydrogen (H2) and dideuterium (D2) to 1 and 1-d2 has been calculated from measured vibrational frequencies for 1 and 1-d2. The calculated EIE is “inverse” (1-d2 binds D2 better than 1 binds H2), with KH/KD = 0.78 at 300 K. The EIE calculated from vibrational frequencies may be resolved into a large normal mass and moment of inertia factor (MMI = 5.77), an inverse vibrational excitation factor (EXC= 0.67), and an inverse zero-point energy factor (ZPE = 0.20), where EIE = MMI × EXC × ZPE. An analysis of the zero-point energy components of the EIE shows that the large decrease in the HH stretching frequency (force constant) predicts a large normal EIE but that zero-point energies from five new vibrational modes (which originate from translational and rotational degrees of freedom from hydrogen) offset the change in zero-point energy from the H2(D2) stretch. The calculated EIE is compared to experimental data obtained for the binding of H2 or D2 to Cr(CO)3(PCy3)2 over the temperature range 12−36 °C in THF solution. For the binding of H2 ΔH = −6.8 ± 0.5 kcal mol-1 and ΔS = −24.7 ± 2.0 cal mol-1 deg-1; for D2 ΔH = −8.6 ± 0.5 kcal/mol and ΔS = −30.0 ± 2.0 cal/(mol deg). The EIE at 22 °C has a value of KH/KD = 0.65 ± 0.15. Comparison of the equilibrium constants for displacement of N2 by H2 or D2 in the complex W(CO)3(PCy3)2(N2) in THF yielded a value of KH/KD = 0.70 ± 0.15 at 22 °C.

A Case of C−H Activation (Ortho Metalation) Which Is Reversible at 25 °C

The reaction of LiC2Ph with Ir(H)2ClL2 (L = PtBu2Ph) gives Ir(H)2(C2Ph)L2, which readily loses H2 to form the ortho-metalated species IrH(η2-C6H4PtBu2)(C2Ph)L. This molecule is unique in showing (sp2)C/H reductive elimination/oxidative addition, which is thermally reversible at 25 °C. Line shape analysis of the 31P{1H} NMR spectra yield ΔH⧧ = 12.3 (±0.4) kcal mol-1 and ΔS⧧ = −2.0 (±1.1) cal deg-1 mol-1 for this process. This implicates the 14-electron species Ir(C2Ph)L2 as highly reactive, yet more easily accessible, than IrClL2 (which ortho metallates essentially irreversibly). The strained four-membered ring in IrH(η2-C6H4PtBu2)(C2Ph)L reacts readily with PhC2H to give IrH(C2Ph)2L2, another unsaturated molecule essentially devoid of π-stabilization. The structure of this molecule, a square pyramid with apical hydride (proton NMR chemical shift −44 ppm), minimizes the Lewis acidity of such a species. Crystal data (−181 °C): a = 9.616(1) Å, b = 23.032(2) Å, c = 8.886(1) Å, α = 95.18(1)°, β = 99.03(1), γ = 98.16(1), with Z = 2 in space group P1̄.

Intramolecular Electron Transfer in Pentaammineruthenium(III)-Modified Cobaltocytochrome c

The iron in the heme group of horse-heart cytochrome c was replaced by cobalt according to established methods. The resulting cobalticytochrome c was subsequently modified at histidine-33 with a pentaammineruthenium group. Proof of correct derivatization was obtained by atomic absorption analysis of cobalt and ruthenium, differential pulse voltammetry, and enzymatic proteolysis analyzed by diode-array HPLC. Cobalt(II)-to-ruthenium(III) intramolecular electron transfer rates were measured as a function of temperature by electron pulse radiolysis. The azide radical (N3•) was used to oxidize the fully reduced form in order to generate the desired electron transfer precursor. The intramolecular electron transfer rate is 1.28 ± 0.04 s-1 at 25 °C (ΔH⧧ = 5.7 ± 0.2 kcal/mol, ΔS⧧ = −38.7 ± 0.5 cal/(deg mol)) for a driving force of 0.28 ± 0.02 eV. The results are compared with those for analogous pentaammineruthenium-modified, native iron, and zinc-substituted cytochromes c. The 0.4 eV increase in driving force for intramolecular electron transfer when iron is replaced by cobalt is largely compensated by an increase in reorganization energy.

Thermodynamic analysis of a designed three-stranded coiled coil.

The study and successful design of coiled-coil protein structural motifs have provided much insight into the rules governing protein folding and stability. In this work we use a thermodynamic approach to quantitate the rules that govern the specific oligomerization of coiled coils. We have designed a highly stable trimeric coiled coil by placing valine residues at each a position and leucine residues at each d position of the heptad repeating unit. The peptide forms a very stable trimer as determined by sedimentation equilibrium, and the concentration dependence of its circular dichroism spectrum follows a cooperative monomer/dimer/trimer equilibrium with the dimer state as a highly unstable intermediate. Its guanidinium chloride denaturation curve was collected at several peptide concentrations, and analysis of the data confirms the cooperativity of the trimerization process and provides a free energy of stabilization of - 18.4 kcal mol-1 for the trimer. The heat capacity, delta Cp, was measured by global analysis of thermal unfolding data collected at a number of guanidinium chloride concentrations. Guanidinium chloride induces cold denaturation in the thermal unfolding curves, providing a reasonably well-determined value for delta Cp of 750 cal deg-1 mol-1. This translates to a delta Cp of 8.6 cal deg-1 mol-1 per residue and corresponds well to that expected of a coiled coil with a well-defined tertiary structure.

Calorimetry of apolipoprotein-A1 binding to phosphatidylcholine-triolein-cholesterol emulsions

The thermotropic properties of triolein-rich, low-cholesterol dipalmitoyl phosphatidylcholine (DPPC) emulsion particles with well-defined chemical compositions (approximately 88% triolein, 1% cholesterol, 11% diacyl phosphatidylcholine) and particle size distributions (mean diameter, approximately 1000-1100 A) were studied in the absence and presence of apolipoprotein-A1 by a combination of differential scanning and titration calorimetry. The results are compared to egg yolk PC emulsions of similar composition and size. Isothermal titration calorimetry at 30 degrees C was used to saturate the emulsion surface with apo-A1 and rapidly quantitate the binding constants (affinity Ka = 11.1 +/- 3.5 x 10(6) M-1 and capacity N = 1.0 +/- 0.09 apo-A1 per 1000 DPPC) and heats of binding (enthalpy H = -940 +/- 35 kcal mol-1 apo-A1 or -0.92 +/- 0.12 kcal mol-1 DPPC). The entropy of association is -3070 cal deg-1 mol-1 protein or -3 cal deg-1 mol-1 DPPC. Without protein on the surface, the differential scanning calorimetry heating curve of the emulsion showed three endothermic transitions at 24.3 degrees C, 33.0 degrees C, and 40.0 degrees C with a combined enthalpy of 1.53 +/- 0.2 kcal mol-1 DPPC. With apo-A1 on the surface, the heating curve showed the three transitions more clearly, in particular, the second transition became more prominent by significant increases in both the calorimetric and Van't Hoff enthalpies. The combined enthalpy was 2.70 +/- 0.12 kcal mol-1 DPPC and remained constant upon repeated heating and cooling. Indicating that the newly formed DPPC emulsion-Apo-A1 complex is thermally reversible during calorimetry. Thus there is an increase in delta H of 1.17 kcal mol-1 DPPC after apo-A1 is bound, which is roughly balanced by the heat released during binding (-0.92 kcal) of apo-A1. The melting entropy increase, +3.8 cal deg-1 mol-1 DPPC of the three transitions after apo-A1 binds, also roughly balances the entropy (-3 cal deg-1 mol-1 DPPC) of association of apo-A1. These changes indicate that apo-A1 increases the amount of ordered gel-like phase on the surface of DPPC emulsions when added at 30 degrees C. From the stoichiometry of the emulsions we calculate that the mean area of DPPC at the triolein/DPPC interface is 54.5 A2 at 41 degrees C and 54.2 A2 at 30 degrees C. The binding of apo-A1 at 30 degrees C to the emulsion reduces the surface area per DPPC molecule from 54.2 A2 to 50.8 A2. At 30 degrees apo-A1 binds with high affinity and low capacity to the surface of DPPC emulsions and increases the packing density of the lipid domain to which it binds. Apo-A1 was also titrated onto DPPC emulsions at 45 degrees C. This temperature is above the gel liquid crystal transition. No heat was released or adsorbed. Furthermore, egg yolk phosphatidylcholine emulsions of nearly identical composition were also titrated at 30 degrees C with apo-A1 and were euthermic. Association constants were previously measured using a classical centrifugation assay and were used to calculate the entropy of apo-A1 binding (+28 cal deg-1 mol-1 apo-A1). This value indicates that apo-A1 binding to a fluid surface like egg yolk phosphatidylcholine or probably DPPC at 45 degrees C is hydrophobic and is consistent with hydrocarbon lipid or protein moities coming together and excluding water. Thus the binding of apo-A1 to partly crystalline surfaces is entropically negative and increases the order of the already partly ordered phases, whereas binding to liquid surfaces is mainly an entropically driven hydrophobic process.

Interaction of pyridine nucleotide substrates with Escherichia coli dihydrodipicolinate reductase: thermodynamic and structural analysis of binary complexes.

E. coli dihydrodipicolinate reductase exhibits unusual nucleotide specificity, with NADH being kinetically twice as effective as NADPH as a reductant as evidenced by their relative V/K values. To investigate the nature of the interactions which determine this specificity, we performed isothermal titration calorimetry to determine the thermodynamic parameters of binding and determined the three-dimensional structures of the corresponding enzyme-nucleotide complexes. The thermodynamic binding parameters for NADPH and NADH were determined to be Kd = 2.12 microM, delta G degree = -7.81 kcal mol-1, delta H degree = -10.98 kcal mol-1, and delta S degree = -10.5 cal mol-1 deg-1 and Kd = 0.46 microM, delta G degree = -8.74 kcal mol-1, delta H degree = -8.93 kcal mol-1, and delta S degree = 0.65 cal mol-1 deg-1, respectively. The structures of DHPR complexed with these nucleotides have been determined at 2.2 A resolution. The 2'-phosphate of NADPH interacts electrostatically with Arg39, while in the NADH complex this interaction is replaced by hydrogen bonds between the 2' and 3' adenosyl ribose hydroxyls and Glu38. Similar studies were also performed with other pyridine nucleotide substrate analogs to determine the contributions of individual groups on the nucleotide to the binding affinity and enthalpic and entropic components of the free energy of binding, delta G degree. Analogs lacking the 2'-phosphate containing homologs. For all analogs, the total binding free energy can be shown to include compensating enthalpic and entropic contributions to the association constants. The entropy contribution appears to play a more important role in the binding of the nonphosphorylated analogs than in the binding of the phosphorylated analogs.

Kinetic and Thermodynamic Analysis of RNA-Protein Interactions in the RNase P Holoenzyme from Escherichia coli

A gel retardation assay has been used to examine the kinetic and equilibrium properties of the interaction between C5 protein and M1 RNA in the formation of the ribonuclease P holoenzyme from Escherichia coli. The interaction is relatively insensitive to the identity of the monovalent anions present and to pH in the range 7.0-9.0, but it has a more critical requirement for specific monovalent and divalent cations: NH4+, K+, Mg2+, Ca2+, and Mn2+ all promote efficient formation of the complex. A positive delta S (+6.4 cal mol-1 deg-1) and a negative delta H (-11.3 kcal mol-1) combine to give a delta G equal to -13.3 kcal mol-1 at 37 degrees C in 0.42 M salt. The binding reaction is sensitive to the concentration of monovalent and divalent cations, with the affinity increasing with increasing ionic strength (delta log Ka/delta log [NH4+] = +2.7 +/- 0.1). The dependence of Kd on the ionic strength and the positive delta S suggests that hydrophobic and stacking interactions contribute significantly to the formation of the RNase P holoenzyme.

Energetics of thrombin-fibrinogen interaction.

The kinetic mechanism of thrombin-fibrinogen interaction has been elucidated by steady-state measurements of synthetic substrate hydrolysis by human alpha-thrombin in the presence of human fibrinogen used as a competitive inhibitor and sucrose used as a viscogenic agent. Sucrose greatly affects the FKm for thrombin-fibrinogen interaction, without altering the intrinsic properties of the system. Under conditions of pH 7.5 and 0.1 M NaCl, fibrinogen behaves like a sticky substrate for thrombin, with acylation being comparable to dissociation in the temperature range 20-37 degrees C. In the same temperature range, deacylation is much faster than acylation. The van't Hoff enthalpy of binding for thrombin-fibrinogen interaction is -24 +/- 3 kcal/mol and the entropy is -55 +/- 11 cal mol-1 deg-1. A chemical compensation effect is present in the binding of fibrinogen and synthetic amide substrates to thrombin, with the delta H and delta G values being linked through a linear relationship.

Interaction of thiourea with hydroxopentaaquorhodium(III) ion in ethanol-water

The rate of reaction between hydroxopentaaquorhodium-(III) and thiourea was studied spectrophotometrically in 30% v/v EtOH/H2O over the pH range 2.5–4.0 between 40° and 60°C. The rate was found to be independent of the incoming ligand concentration. The activation parameters (ΔH‡=113.8 kJ mol−1, ΔS‡=26.6 J deg mol−1) are consistent with a dissociative reaction involving a rate determining elimination of H2O from the hydroxopentaaquorhodium(III) cation followed by rapid thiourea ligation.