Edwin A. Lewis

Isothermal titration calorimetry: experimental design, data analysis, and probing macromolecule/ligand binding and kinetic interactions.

Isothermal titration calorimetry (ITC) is now routinely used to directly characterize the thermodynamics of biopolymer binding interactions and the kinetics of enzyme-catalyzed reactions. This is the result of improvements in ITC instrumentation and data analysis software. Modern ITC instruments make it possible to measure heat effects as small as 0.1 microcal (0.4 microJ), allowing the determination of binding constants, Ks, as large as 10(8) - 10(9)M(-1). Modern ITC instruments make it possible to measure heat rates as small as 0.1 microcal/sec, allowing for the precise determination of reaction rates in the range of 10(-12) mol/sec. Values for K(m) and k(cat), in the ranges of 10(-2) - 10(3) microM and 0.05 - 500 sec(-1), respectively, can be determined by ITC. This chapter reviews the planning of an optimal ITC experiment for either a binding or kinetic study, guides the reader through simulated sample experiments, and reviews analysis of the data and the interpretation of the results.

A multiple-system, multi-channel diffusion denuder sampler for the determination of fine-particulate organic material in the atmosphere

Abstract Measurement of the loss of semi-volatile organic compounds from particles collected with a filter is carried out by comparing the amounts collected by comparable filter pack and diffusion denuder samplers. The sorbents used to collect organic compounds in the denuder and sorbent filters must have the same efficiency for collection of all gas-phase organic compounds present. Interpretation of the data requires that the efficiency of collection of gas-phase compounds by the denuder be known. In theory this can be accomplished by determination of the deposition pattern of all organic compounds collected in the denuder, but in practice this is very difficult if the organic material consists of a wide variety of compounds. An alternative approach is to determine the breakthrough of organic compounds in a sampler with a particle-collection filter preceding the denuder and sorbent filter. In such a sampler only gas-phase organic material enters the denuder. We have developed both a multi-channel parallel plate diffusion denuder sampler and a comparable sampler in which the denuder is preceded by a filter. Samples can be collected with the multisystem sampler at a flow rate of 35 sLpm. The denuder surfaces and the sorbent filters are made from sheets of an activated charcoal-impregnated filter paper. Collection of semi-volatile compounds by the samplers has been characterized and the systems have been field tested. The samplers are now being used for the routine collection and determination of semi-volatile organic compounds in particles at Canyonlands National Park in southeastern Utah. Available data from this field program show significant losses of particulate organic compounds on a quartz filter due to volatilization during sampling.

Thermodynamics of binding of guest molecules to α- and β-cyclodextrins

Values of ΔG, ΔH, and ΔS are reported for the interaction of eighteen molecules and ions with α-cyclodextrin and of four molecules with β-cyclodextrin. The conclusions of this study are (1) the cyclodextrin–ClO4– complex has been shown to bind a cation, (2)α- and β-cyclodextrin have equilibrium constants for binding the same guest that are similar but the enthalpy and entropy changes are quite different in some cases, and (3) changes in ΔH are largely compensated for by changes in ΔS and it is suggested that this effect is due principally to the nature of the solvent, i.e. water.

Determination of the particle size distribution and chemical composition of semi-volatile organic compounds in atmospheric fine particles with a diffusion denuder sampling system

Abstract Correct assessment of the factors associated with visibility impairment is dependent on accurate determination of the chemical composition as a function of size of particles present in the atmosphere. Collection of particles on a filter results in underestimation of particulate organic compounds due to losses from the semi-volatile organic fraction during sample collection, i.e. a “negative sampling artifact”. These semi-volatile organic compounds lost from particles collected on a filter can be correctly measured using a diffusion denuder sampling system. This paper describes a multi-system, multi-channel, high-volume diffusion denuder sampler for the accurate determination of the particle size distribution and chemical composition of semi-volatile organic compounds in atmospheric fine particles. Data obtained in Provo, UT and Los Angeles, CA on the chemical composition and particle size distribution of semi-volatile organic compounds in atmospheric fine particles indicate that the majority of the semi-volatile organic compounds lost from particles during sampling were present in particles in the 0.4–0.8 μm size range in the atmospheres sampled. Organic compounds lost from the particles during sampling include paraffinic compounds, aromatic compounds and organic acids and esters.

n-hexane as a model for compressed simple liquids

Isobaric thermal expansivities, αp, ofn-hexane have been measured by pressure-controlled scanning calorimetry from just above the saturation vapor pressure to 40 MPa at temperatures from 303 to 453 K and to 300 MPa at 503 K. These new data are combined with literature data to obtain a correlation equation for αp valid from 240 to 503 K at pressures up to 700 MPa. Correlation equations are developed for the saturated vapor pressure, specific volume, and isobaric heat capacity of liquid n-hexane from 240 to 503 K. Calculated volumes, isobaric and isochoric specific heat capacities. isothermal compressibilities, and thermal coefficients of pressure are presented for the entire range of pressure and temperature. The pressure-temperature behavior of these quantities is discussed as a model behavior for simple liquids without strong intermolecular interactions.

Kinetics of Drug Decomposition by Heat Conduction Calorimetry

The application of heat conduction calorimetry to the determination of decomposition mechanisms and rates for drugs is shown to be a rapid and generally useful method. The application of the method to determine the nature of the decomposition reaction, sources of systematic errors in the method, the equations relating the calorimetric signal to the kinetics of the reaction, and some examples of results are presented and discussed.

Modeling complex equilibria in isothermal titration calorimetry experiments: thermodynamic parameters estimation for a three-binding-site model.

Isothermal titration calorimetry (ITC) is a powerful technique that can be used to estimate a complete set of thermodynamic parameters (e.g., K(eq) (or ΔG), ΔH, ΔS, and n) for a ligand-binding interaction described by a thermodynamic model. Thermodynamic models are constructed by combining equilibrium constant, mass balance, and charge balance equations for the system under study. Commercial ITC instruments are supplied with software that includes a number of simple interaction models, for example, one binding site, two binding sites, sequential sites, and n-independent binding sites. More complex models, for example, three or more binding sites, one site with multiple binding mechanisms, linked equilibria, or equilibria involving macromolecular conformational selection through ligand binding, need to be developed on a case-by-case basis by the ITC user. In this paper we provide an algorithm (and a link to our MATLAB program) for the nonlinear regression analysis of a multiple-binding-site model with up to four overlapping binding equilibria. Error analysis demonstrates that fitting ITC data for multiple parameters (e.g., up to nine parameters in the three-binding-site model) yields thermodynamic parameters with acceptable accuracy.

The chemical composition of environmental tobacco smoke III. Identification of conservative tracers of environmental tobacco smoke

Several components of environmental tobacco smoke (ETS) have been determined in experiments conducted in a 30 m3 Teflon chamber. The effect of residence time in the chamber on the particle size distribution, mass of particles, gas-particle equilibria, and chemical composition of both the gas and particle phases of environmental tobacco smoke has been studied. Many organic compounds present in tobacco smoke condensate are not found in the particle phase in environmental tobacco smoke but are present in the gas phase. Concentrations of several nitrogen containing organic compounds, identified in the chamber experiments, have been determined in several indoor environments where environmental tobacco smoke was present. These compounds were not found in indoor environments where environmental tobacco smoke was not present. The studies in the indoor environments indicate that gas phase nicotine and myosmine are removed from indoor environments at a much faster rate than gas phase 3-ethenylpyridine or 2-ethenylpyridine or particle-phase nicotine or cotinine. Gas-phase 3-ethenylpyridine and particle-phase nicotine are proposed as tracers of environmental tobacco smoke.

Fine particulate chemical composition and light extinction at Canyonlands National Park using organic particulate material concentrations obtained with a multisystem, multichannel diffusion denuder sampler

The concentration of fine particulate carbonaceous material has been measured over a 1-year period at the Interagency Monitoring of Protected Visual Environments (IMPROVE) Canyonlands National Park, Utah sampling site using a Brigham Young University organic sampling system (BOSS) multisystem, multichannel diffusion denuder sampler. Samples were collected on the IMPROVE schedule of a 24-hour sample every Wednesday and Saturday. The concentrations of particulate C, determined using only a quartz filter pack sampling system, were low by an average of 39%, as a result of the loss of semi-volatile organic compounds from the particles collected on quartz filters during sampling. The loss was higher during the summer than during the winter sampling periods. The BOSS and IMPROVE quartz filter carbon measurements were in agreement except for a few samples collected during the summer. The fine particulate carbonaceous material concentrations determined using the BOSS have been combined with concentrations of particulate elemental C (soot), sulfate, nitrate, crustal material, and fine and coarse particulate mass from the IMPROVE sampling system, as well as relative humidity, light absorption, and transmissometer measurements of light extinction from IMPROVE. Extinction budgets have been calculated using multilinear regression analyses of the data set. Literature data were used to estimate the change in the mass extinction coefficients for the measured species as a function of relative humidity. The results show carbonaceous material to be the principal contributor to light extinction due to particles during the study period, with the major contributor to light extinction being light-absorbing carbonaceous material. However, the periods of maximum light extinction are associated with high humidity and the associated increased scattering of light due to particulate sulfate during the winter. The effect of particulate organic compounds on light extinction is greatest in the summer and smallest in the winter.

An automated calorimeter for the measurement of isobaric expansivities and of isothermal compressibilities of liquids by scanning pressure from 0.1 to 400 MPa at temperatures between 303 and 503 K

Abstract A pressure-scanning calorimeter operating in the pressure range from 0.1 to 400 MPa at temperatures from 303 to 503 K is described. Information collection from, and control of, the calorimeter is done with a computer and a stepping motor. The calorimeter was tested by determining the isobaric expansivity of n -hexane over nearly the entire range of accessible pressures and temperatures. Expansivities for a mixture of n -hexane and hexane isomers are also presented. The sample cell and pressurizing equipment associated with the calorimeter can also be used to determine the compressibilities of liquids near room temperature. A method for calibration of the system for this purpose is demonstrated by use of n -hexane as a calibrating fluid.