Microcalorimetry

Microcalorimetry is an extremely sensitive technique that determines the heat emitted or adsorbed by a sample in a variety of processes. Microcalorimetry can be used to characterize pharmaceutical solids to obtain heats of solution, heats of crystallization, heats of reaction, heats of dilution, and heats of adsorption. Isothermal microcalorimetry has been used to investigate drug-excipient compatibility [82]. Pikal and co-workers have used isothermal microcalorimetry to investigate the enthalpy of relaxation in amorphous material [83]. Isothermal microcalorimetry is useful in determining even small amounts of amorphous content in a sample [84]. Solution calorimetry has also been used to quantitate the crystallinity of a sample [85]. Other aspects of isothermal microcalorimetry may be obtained from a review by Buckton [86]. 

Modern microcalorimetry is a powerful and indispensable experimental technique that allows us to simultaneously determine the enthalpy change and equilibrium constant from a single experimental run [1]. Usually, in supramolecular chemistry, biochemistry, biotechnology, pharmacology, and medicinal chemistry, reactants are available in limited quantities and are very expensive, hence it is reasonable to increase the sensitivity of the instrument (and consequently reduce the amount of reactant required for a single calorimetric run), rather than employ less sensitive ones. Indeed, the sensitivity of calorimeters has been dramatically improved, particularly in recent years, to the level of a few microcalories or even less. 

It should be noted that calorimetry is one of the oldest physicochemical experimental methods with a history of more than a century of scientific appUcation for which numerous experimental devices and techniques have been designed, tested, and applied. Since the first extensive review of the use of microcalorimetry in the fields of biochemistry, biotechnology, and biology by Calvet and Prat in 1956 [2], a wide variety of different microcalorimeters have been developed and employed in various branches of the life sciences. 

Despite the great diversity in the design of microcalorimeters and the experimental procedures described in the literature [1-10], only two microcalorimetric methods have found widespread application in cydodextrin (CyD) studies and drug-design research. These two methods are differential scanning calorimetry (DSC) and isothermal titration microcalorimetry (ITC). DSC and ITC can be con- 

Cyclodextrins and Their Complexes. Edited by Helena Dodziuk Copyright 2006 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-31280-3 

Application of Differential Scanning Calorimetry in Cydodextrin and Drug Studies 

During the measurement of any sample using microcalorimetry, the heat flows associated with all the reactions that are occurring simultaneously within the sample are recorded. Although this allows many complex reactions which are outside the scope of other analytical tools, to be studied, it also means that poor sample preparation can lead to erroneous heat flow signals and it may be that the heat flow signal is influenced greatly by an effect other than that which is the intended subject of the study. Consequently, the use of microcalorimetry 

In general, calorimetry is highly suited to the study of pharmaceutical systems because the technique is very sensitive to changes induced by formulation or processing. This section deals with some potential applications of microcalorimetry to the characterisation of pharmaceutical systems that are undergoing physical changes. 

Calorimetry is the science of measuring heat changes from chemical reactions and physical events. DSC is described in Section 16.3 and has been a staple method of analysis for materials scientists. Classical DSC instruments and classical calorimetric titrimetry instruments (Sections 16.5 and 16.6) often lack the sensitivity required for the study of biological samples, where processes like the folding or unfolding of a protein may exchange only microjoules of heat. A new class of ultrasensitive microcalorimetry instrumentation has been developed primarily for studies in the life sciences, where sample amounts may be extranely limited. 

Modern calorimetry has transformed and advanced to such a degree that today its acute sensitivity and flexibility has made it an indispensable tool throughout the life sciences. Ultrasensitive calorimetry now plays an integral role in biotechnology, providing researchers with essential information on the thermodynamics, structure, stability, and functionality of proteins, nucleic acids, lipids, and other biomolecules. Ultrasensitive calorimetry is a vital tool for R D in pharmaceuticals, genetics, energy, and materials—in almost any area where the measurement and controlled manipulation of substances and interactions are required at the molecular level. 

Ultrasensitive calorimeters enable researchers and product developers to gain thermodynamic information that was previously difficult or even impossible to gain with earlier models or by other techniques. The capabilities of advanced, ultrasensitive calorimeters are enormously helpful because of the amount and accuracy of the data they extract, as well as their abilities to obtain data using very small samples. 

DSC measures heat as a function of changing temperature. It is typically used to discern a wide range of thermal transitions in biological systems and the thermodynamic parameters associated with these changes. ITC is typically used for monitoring a chemical reaction initiated by the addition of a binding component and has become the method of choice for characterizing biomolecular interactions. 

Instruments are available from several manufactmers, including TA Instruments and GE Healthcare Life Sciences. Some companies use micro in their instrument names, some use nano, and some use the Greek letter p in their names. For the sake of simplicity, the term micro will be used unless a specific instrument is discussed by name. 

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Numerous attempts to determine the equilibrium constants using titration microcalorimetry failed, due to solubility problems encountered at the higher concentrations of catalyst and dienophile that are required for this technique. 

Usually the acid-base properties of poly electrolyte are studied by potentiometric titrations. However it is well known, that understanding of polyelectrolyte properties in solution is based on the knowledge of the thermodynamic properties. Up to now, there is only a small number of microcalorimetry titrations of polyelectrolyte solutions published. Therefore we carried out potentiometric and microcalorimetric titrations of hydrochloric form of the linear and branched polyamines at 25°C and 65°C, to study the influence of the stmcture on the acid-base properties. 

In addition we studied the complexation of ClC+ by the polyamines using microcalorimetry and potentiometry. The enthalpy changes measured are presented as function of the degree of protonation and the amount of CiT bound. 

Methyl iso-butyl ketone, 279-281 Microcalorimetry, 237 Molar conductivity, 155, 160 Molar volume, 156, 160... 

The change in the degree of filling of carboxylic CP with protein is reflected in the character of contributions of AH and T AS. The method of differential microcalorimetry was used for the investigation of the thermal effects at varying a in the Hb-CP (Table 10). 

R. L. Moss and L. Whalley Heat-Flow Microcalorimetry and Its Application to Heterogeneous Catalysis P. C. Gravelle... 

This method depends on the fact that bacteria like all living organisms produce heat when they metabolize. Because of the small amount of heat produced, especially sensitive calorimetric devices are required hence the name microcalorimetry. The specimen to be evaluated is diluted with a nutrient medium and, if microorganisms are present and can metabolize, heat is produced and can be measured. An interesting offshoot of this technique is the fact that differing organisms produce different heat outputs and this may provide a means of identification. Microcalorimetry may enable organisms to be detected and possibly identified in 3 hours. 

BeezerA.E. 19 0) Biological Microcalorimetry. London Academic Press. 

Figure 7.5. Sticking coefficients along with differential heats of adsorption as measured by microcalorimetry for ethylene and acetylene on Rh(lOO). [Adapted from R. Kose, W.A. Brown and D.A. King, Chem. Rhys. Lett. 311 (1999) 109.]...

HO - Si =)4]. The defective MFI matrix has also been observed for TS-1. It has been demonstrated by IR and microcalorimetry that the insertion of Ti heteroatoms in the MFI lattice has a mineralizing effect causing the progressive reduction of the internal defects [24,83,84]. 

Merritt WR, Champion PJ, Hawkings RC (1957) The half-life of °Pb. Can J Phys 35 16 Pickett DA, Mnrrell MT, Williams R.W (1994) Determination of femtogram qnantities of protactinium in geological samples by thermal ionization mass spectrometry. Anal Chem 66 1044-1049 Robert J, Miranda CF, Mnxart R (1969) Mesure de la periode dn protactininm-231 par microcalorimetrie. Radiochim Acta 11 104-108... 

Robert J, Miranda CF, Mrrxart R (1969) Mesure de la periode du protactinirrm-231 par microcalorimetrie. Radiochim Acta 11 104-108... 

Moisture isotherms were, up until recently, not considered part of preformulation. However, with the advent of microcalorimetry, moisture isotherms and surface areas may be determined with mg quantities of drug substance [29,30]. 

High-performance liquid chromatography (HPLC) is the usual chemical method employed, although older methods, such as thin layer chromatography (TLC) can be of use. Microcalorimetry can also be of use [52]. 

Bauerle and Seelig [395] studied the structural aspects of amlodipine (weak base, primary amine pKa 9.26 [162]) and nimodipine (nonionizable) binding to phospholipid bilayers, using NMR, microcalorimetry, and zeta-potential measurements. They were able to see evidence of interactions of amlodipine with the cis double bond in the acyl chains. They saw no clear evidence for (=P—O- 1 H N—) electrostatic interactions. 

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