Aplicaciones boquimicas de la calorimetria

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ANNUAL REVIEWS
Ann.

Further

Rev. Phys. Chern. 1987.38: 463--88 Copyright © 1987 by Annual Reviews Inc. All rights reserved

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BIOCHEMICAL APPLICATIONS OF DIFFERENTIAL SCANNING
Annu. Rev. Phys. Chem. 1987.38:463-488. Downloaded from arjournals.annualreviews.org by Universidad Autonoma Metropolitana on 05/21/09. For personal use only.

CALORIMETRY JulianM. Sturtevant
Departments of Chemistry and of Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut 06511-8118
INTRODUCTION

Differential scanning calorimetry (DSC) and the closely related differential thermal analysis (DTA) have been widely employed during the last several decades in the thermodynamic study of processes that are initiated by either an increase or adecrease in temperature. This review focuses on biochemical applications of DSC. Macromolecular and polymolecular structures stabilized by the coop­ eration of numerous weak forces are important to most biochemical pro­ cesses. Since such highly cooperative structures undergo conformational or phase transitions upon being heated, significant information concerning these structures can be obtained byDSC. Small molecules cannot be studied by DSC unless they form aggregates showing intermolecular coop­ eration, as in crystals. This is illustrated in Table 1. Since the enthalpies of chemical processes rarely are as large as 20 cal g-l, it is evident that molecules having molecular weights, or molecular aggregates having aggregate weights, in the thousands of daltons are required to give tran­sitions sufficiently sharp for useful DSC observation. In a scanning calorimeter, one measures the specific heat of a system as a function of the temperature. For a solution, the apparent specific heat of the solute, e2, is given by the expression 1. where c is the specific heat of the solution,
c1

is that of the solvent, and

W2

463 0066-426X/87/1101-0463$02.00

464

STURTEVANTTable 1

Transition widths for a two-state

transition as observed by DSC for various
=

values of the transition enthalpy. Temperature

of half completion

50°C TemperaturetC
()(
=

Transition enthalpy
kcal mol I

0.1

()(

=

0.9

20 40

29 39

62 58

75

Annu. Rev. Phys. Chem. 1987.38:463-488. Downloaded from arjournals.annualreviews.org by Universidad AutonomaMetropolitana on 05/21/09. For personal use only.

60
IX =

43

extent of conversion.

is the weight fraction of the solute. Since the quantity c - C I is usually relatively small, for example, approximately -0.7% of Cl for a 1% aque­ ous solution of a protein, it is essential to employ a differential scheme of measurement in which c - C I is directly measured. This is accomplished in adifferential scanning calorimeter by using two closely matched cells filled with equal weights or, more usually, with equal volumes of solution and solvent. When one considers that in general a significant, or even major, fraction of the total change in apparent specific enthalpy is due to the simple heating or cooling of the solvent, it becomes evident that the highest possible sensitivity andaccuracy should be realized. The so-called excess apparent specific heat, Cex, is the amount by which the apparent specific heat during a transition involving the solute exceeds the baseline specific heat. A recurring problem in DSC, as in many other measurement techniques, is the determination of the appropriate baseline since, as is evident, no direct observation of it is possible during thetransition. Figure I shows a typical DSC curve observed for the reversible thermal denaturation of a globular protein. The apparent specific heat of the native form of the protein increases with increasing temperature while that of the denatured form is independent of temperature. The dashed curve, obtained by procedures outlined below, is the baseline specific heat, and Cex is as indicated. The...
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