Applications of differential scanning calorimetry to enzyme kinetics

Differential scanning calorimetry has been investigated for its application to the experimental study of the temperature dependence of the kinetics of enzyme—substrate reactions. A reaction cell was developed which allows mixing of enzyme and substrate solutions directly in the calorimeter. The device was used to study the zero-order reaction of acetylcholinesterase with acetylcholine and the first-order reaction of α-chymotrypsin with N-acetyl-l-alanine methyl ester. The reaction cell is found to be satisfactory in the isothermal mode for both first- and zero-order reactions and in the scanning mode for the zero-order reactions but not for the first-order reaction. Limitations of the design are described for general enzyme kinetic studies.     

Determination of kinetic parameters for the reaction between phenol and formaldehyde by differential scanning calorimetry

The kinetic parameters of the complex reaction between phenol and formaldehyde in the presence of sodium hydroxide (NaOH) have been obtained by differential scanning calorimetry (DSC). The two dominant reactions appear to be addition of formaldehyde to phenol with formation of o-hydroxymethyl-phenol and subsequent condensation of the latter. For both reactions, the activation energy (E_a), reaction order and rate constants at different temperatures have been determined. E_a for addition changes from 23·7 to 19·3 kcal mole^?1 and for condensation from 22·9 to 19·1 kcal mole^?1 when the amount of NaOH is increased from 0·25 to 1·00 per cent. The reaction order for addition is 2 and for condensation 1. Thus DSC appears useful for studying the kinetics of more complex polymerization reactions.     

Thermal behaviour of milk powders studied by differential thermal analysis and heat flow calorimetry

The technique of heat flow calorimetry was used to study the thermal behaviour of different milk powders above 20°C. Exothermic reactions of different types occurred when measurements were made with the samples in sealed cells. According to our interpretation of the calorimetric curves, these reactions correspond to: crystallization of amorphous lactose, Maillard reaction between milk proteins and lactose, oxidation of milk fat and decomposition of lactose. Their relative importance is discussed. In addition, the technique of high-pressure differential thermal analysis was used for studying spontaneous ignition and combustion of milk powders, under a large oxygen excess. The role of fat, carbohydrate and protein decomposition is underlined. Some self-ignition temperature values are given.     

Isothermal differential scanning calorimetry on an exothermic phenomenon during thermal decomposition of hydromagnesite 4 MgCO3 · Mg(OH)2 · 4 H2O

An exothermic phenomenon and a simultaneous rapid evolution of a small amount of carbon dioxide at ?500°C during thermal decomposition of hydromagnesite 4 MgCO₃ · Mg(OH)₂ · 4 H₂O was studied by isothermal DSCTG in a carbon dioxide atmosphere. It was quantitatively confirmed that the exothermic phenomenon was due to crystallization of MgCO₃ from the amorphous phase and that the evolution of carbon dioxide was due to decomposition of the MgCO₃ by the heat of crystallization (?3.4 kcal mole^?1.     

The influence of polymer morphology on polymer film electrochemistry

The electrochemical behavior of functionalized polystyrene-coated electrodes shows a marked dependence on the nature of the electrolyte ions. Scanning electron microscope and surface profile measurements are presented which show that changes in polymer film volume and morphology accompany electrochemical oxidation. Changing polymer morphology by doping the films with soluble monomers during preparation is shown to produce large changes in electrochemical response. Diffusion coefficients were determined for a neutral organic dye dopant in each of the polymer films investigated, and these correlate very well with the oxidation overpotentials observed electrochemically. The nature of polymer film/solvent interactions and the mechanism by which counter ions penetrate the polymer phase is discussed and is related to other physical properties of amorphous polymers in terms of free volume concepts.     

Spin-rotation relaxation times for methyl compounds with hindrance barriers at 65°C

The ¹³C spin-rotation relaxation times of methyl groups with a range of potential barriers to hindered rotation were measured for five compounds at 65°C. The data follow a pattern similar to those obtained earlier at 10°C and 38°C, and agree well with the theoretical results of Mendonca et al.     

Polymer-polymer interaction parameters in solvent/polymer/polymer ternary systems

Empirical equations defining the relationships between b₂₃ and χ₂₃ interaction parameters for the ternary systems toluene (1)/poly(dimethylsiloxane), PDMS, (2)/polystyrene, PS, (3) and benzene (1)/PDMS(2)/PS(3) have been determined. The dependences of both interaction parameters on the mean molecular weights of the macromolecular components have also been studied. A single dependence of b₂₃ (or χ₂₃) upon the mean molecular weight is found when both polymeric components have intermediate to high molecular weights. However, that single dependence is not found when the molecular size of polymer (2) is low.     

Vapor pressures by differential thermal analysis: application to some nickel chelates

A simple and rapid DTA procedure is described for determining the vapor pressures of metal-organic compounds. We have applied the method to six volatile nickel β-ketoamine chelates which have proved useful, in previous work, for the chemical vapor deposition of nickel metal. The volatilities of these compounds are strongly influenced by the configuration (cis or trans planar) about the bonded metal atom and by the presence or absence of fluorine as part of the organic portion of the molecules. Some of the chelates have vapor pressures of several hundred torr at temperatures where thermal decomposition is not significant.     

Mechanism and kinetics of the reaction of F with trivalent phosphorus iodide using electron spin resonance detection

The reaction of iodobistrifluoromethylphosphine, P(CF₃)₂I, with atomic fluorine was studied by fast-flow ESR methods. The initial stage of the reaction is the abstraction of I by F, to form IF with a bimolecular rate constant of (1.0 ± 0.3) × 10¹⁴ cm³ mol^?1 s^?1 at 297 K. In the presence of excess F, stepwise addition to the phosphorous occurs. In the presence of excess P(CF₃)₂I, the reaction P(CF₃)₂ + IF → P (CF₃)₂F + I appears to take place. This reaction rate is slow relative to P(CF₃)₂ + F → P(CF₃)₂F.     

Transport and kinetics at microheterogeneous electrodes: Part III. Application to the photoelectrochemical decomposition of water

The transport and kinetics in a system involving two sets of microheterogeneous electrodes, which are designed to use solar radiation to split water into H₂ and O₂, are described. A reaction scheme involving fourteen different processes is considered. The conditions for an efficient system are derived. It is shown that the main problems are electrode selectivity and the loss of H₂ and O₂ by back reactions.     

A high pressure differential thermal analysis apparatus

A high pressure differential thermal analysis apparatus is described which is capable of operation in the pressure range from 1-600 atm of nitrogen gas and at temperatures from 25 to 500°C. Use of the apparatus is illustrated by the deaquation reactions of CuCl₂·2H₂0 and CoSO₄·7H₂O at pressures from 1 to 69 atm.     

Reaction kinetics in thermal analysis. Part 1. The sensitivity of kinetic equations to experimental errors. A mathematical analysis

The frequent publication of contradicting or meaningless kinetic parameters and the resulting criticism of the “ill-conditioned nature” of non-isothermal reaction kinetics led the authors to an examination of the sensitivity of kinetic parameters to experimental errors. Using simple mathematical deductions, conditions were given at which about 10% precision of the kinetic parameters can easily be achieved. To obtain a graphic picture about the information content of a thermoanalytical curve and the effect of the systematic measurement errors, mathematical relationships were deduced to show the dependence of the kinetic parameters on the formal (geometric) characteristics of the thermoanalytical curves.     

Determination of silver in dore metal by weight titration with equivalence-point detection by differential electrolytic potentiometry

Use of weight titrimetry, with differential electrolytic potentiometry for detection of the equivalence point, instead of gravimetry, has been critically tested for the determination of silver in doré metal. The electrode response in the presence of possible interferences (Sb, As, Se, Cu, Ni, Fe, Pb and Co) was studied by voltammetry. Titration curves for six different samples were obtained and their differences from those for pure silver were interpreted. Positive errors were found to occur in the presence of 0.1% Fe(III) and Se(IV) but not in the presence of 1% Cu(II). The results obtained by gravimetry and weight titrimetry were compared. The imprecision of the proposed method was between 0.03 and 0.05% (relative standard deviation); that of gravimetry was between 0.05 and 0.08%. The absolute difference from the gravimetric results was between + 0.05 and + 0.08%. The time for analysis of one sample was between 60 and 80 min.     

A pyrolysis/gas chromatography—mass spectrometry and thermogravimetry/differential thermal analysis study of bis(diethyldithiocarbamato)diphenyl tin (IV)

The P/GC—MS and TG/DTA analysis, in an inert atmosphere, of bis(diethyldithiocarbamato)diphenyl tin (IV) indicates that the thermal decomposition proceeds in two consecutive stages. Loss of the dithiocarbamate ligands and the phenyl groups occurs within the temperature range of 210–380°C leaving tin (II) sulphide as residue. The stability of the phenyl radical as a reactive intermediate, together with the unidentate bonding of the dithiocarbamate ligand, dramatically influence the major mode of thermal decomposition.     

A simultaneous electrical conductivity—differential thermal analysis apparatus

A highly sensitive simultaneous electrical conductivity—differential thermal analysis apparatus is described. The apparatus can simultaneously record both the electrical conductivity and differential thermal analysis curves of a sample under the same conditions of furnace heating rate and atmosphere. Application of the apparatus is illustrated by curves of CuSO₄·5H₂O, [Co(NH₃)₅H₂0]Br₃ and Cu₂HgI₄.