Thermodynamic Parameters of Complexation

Figure 6.9 Effect of CITREM concentration on the molecular and thermodynamic parameters of complex protein-surfactant nanoparticles in aqueous medium (phosphate buffer, pH = 7.2, ionic strength = 0.05 M 20 °C) (a) extent of protein association, k = Mwcomplex/Mwprotem (b) structure-sensitive parameter, p (c) second virial coefficient, A2 (rnolal scale) (d) effective charge, ZE (net number n of moles of negative charges per mole of original sodium caseinate nanoparticles existing at pH = 7.2 (Mw = 4xl06 Da)). The indicated cmc value refers to the pure CITREM solution. Reproduced from Semenova et al. (2007) with permission.

Table 1 presents values of thermodynamic parameters of complexing reactions for a number of model systems obtained with the help of calorimetry and IR-spectro-scopy 14,34,35). 

Table 2. Thermodynamic parameters of complexing reactions for a series of epoxy amine systems5-16,17-29 30>...

Table 2. Thermodynamic parameters of complexation of calixpyrrole receptors with halides and dihydrogen phosphate anions (tetra-n-butylammonium as counterion) in acetonitrile, dichloromethane, A/,A/-dimethylformamide, dimethyl sulphoxide and propylene carbonate... 

Formation of an ionic tetracoordinate Si+ complex from an uncharged nucleophile and a functional silane is an exothermic process accompanied by a marked drop in entropy. Many qualitative observations indicated that these complexes are generated more readily at lower temperatures (78,242,252,256). Unfortunately, there are few data on the thermodynamic parameters of complex formation. From the temperature variation of the 29Si resonance position, Bassindale and Stout (252) determined the enthalpy and entropy of the formation of bis(iV,Af-trimethylsilyI)-imidazolium chloride (Table IV, entry 10). A similar procedure permitted Chaudhry and Rummer (242) to determine the enthalpy of formation of complexes of 2-trimethylsilyl-l,l,3,3,-tetramethylguanidine (Table IV, entries 6, 7). 

The thermodynamic parameters of complex-formation between i>ribose and Ca have been determined by Morel and coworkers and by Len-kinski and Reuben. The stability constant was found to be the result of a... 

The formation of complexes in solution was studied using mainly UV-VIS spectroscopy and conductometric, chromatographic, and calorimetric methods [14,28 and references therein]. These techniques allow determination of the thermodynamic parameters of complexation but do not give information on the geometry or the dynamic properties of the complex. 

Determination of the Stoichiometry and Stability of the Complexes 115 Tab. 5.2. Thermodynamic parameters of complexation of selected compounds by CyDs. 

In spite of very diverse successful practical applications, the mechanism of com-plexation and the relationship between structure and selectivity are still at best only partly solved and remain open for discussion. Thermodynamic studies could supply some valuable information facilitating an understanding of the physicochemical basis of the complexation processes. The GC modified with CyDs is one of a variety of experimental methods employed in the determination of thermodynamic quantities for the formation of CyD inclusion complexes (see Chapters 8-10). The thermodynamic parameters for separation of the enantiomers were determined for various derivatives of CyDs dissolved in various stationary phases [63-65] or as a Uquid derivatized form [66]. Interesting observations were made by Armstrong et al. [66]. The authors postulated two different retention mechanisms. The first involved classical formation of the inclusion complex with high thermodynamic values of AH, AAH, and AAS and a relatively low column capacity and the second loose, probably external, multiple association with the CyD characterized by lower AH, AAH, and AAS values. The thermodynamic parameters of complexation processes obtained from liquid and gas chromatography measurements are collected in Table 5.2. It is clear from those data that for all the compounds presented the complexation processes are enthalpy-driven since in all cases AH is more negative than TAS. 

Modern titration microcalorimetry is a powerful experimental method for the reliable determination of the thermodynamic parameters of complexation reactions in the fields of chemistry, biochemistry, pharmaceutics, and biology while differential... 

TABLE 14. Thermodynamic parameters of hydrogen-bonded complexes of p-FC6H4OH with sulphoxides and some other bases... 

The physical nature of the sulfate complexes formed by plutonium(III) and plutonium(IV) in 1 M acid 2 M ionic strength perchlorate media has been inferred from thermodynamic parameters for complexation reactions and acid dependence of stability constants. The stability constants of 1 1 and 1 2 complexes were determined by solvent extraction and ion-exchange techniques, and the thermodynamic parameters calculated from the temperature dependence of the stability constants. The data are consistent with the formation of complexes of the form PuSOi,(n-2)+ for the 1 1 complexes of both plutonium(III) and plutonium(IV). The second HSO4 ligand appears to be added without deprotonation in both systems to form complexes of the form PuSOifHSOit(n"3) +. ... 

The use of direct electrochemical methods (cyclic voltammetry Pig. 17) has enabled us to measure the thermodynamic parameters of isolated water-soluble fragments of the Rieske proteins of various bci complexes (Table XII)). (55, 92). The values determined for the standard reaction entropy, AS°, for both the mitochondrial and the bacterial Rieske fragments are similar to values obtained for water-soluble cytochromes they are more negative than values measured for other electron transfer proteins (93). Large negative values of AS° have been correlated with a less exposed metal site (93). However, this is opposite to what is observed in Rieske proteins, since the cluster appears to be less exposed in Rieske-type ferredoxins that show less negative values of AS° (see Section V,B). 

Thermodynamic Parameters of the Rieske Fragments from the be. Complexes of Bovine Heart (ISFb) (92) and Paracoccus dentrificans (ISFpd) (140) and of the Rieske-Type Ferredoxin FROM Benzene Dioxygenase (FdBED) (55)... 

No new absorption bands are observed in other cases, largely due to the fact that the strong absorptions of the aromatic donors obstruct the UV-spectral measurements. For the complex between CBr4 and TMPD, the quantitative analyses of the temperature and concentration-dependent absorptions of the new band at 380 nm afford the extinction coefficient of ct = 3.2 x 103 M 1 cm x, as well as the thermodynamic parameters for complex formation AH = - 4.5 kcalM x, AS = - 14 e.u., and Kda = 0.3 M x at 295 K. Such thermodynamic characteristics are similar to those of the dihalogen complexes of as well as those of other acceptors with aromatic donors. Similar results are also obtained for CBr4 associates with halide and thio-cyanide anions [5,53]. 

As we described in Chapter 3, the binding of reversible inhibitors to enzymes is an equilibrium process that can be defined in terms of the common thermodynamic parameters of dissociation constant and free energy of binding. As with any binding reaction, the dissociation constant can only be measured accurately after equilibrium has been established fully measurements made prior to the full establishment of equilibrium will not reflect the true affinity of the complex. In Appendix 1 we review the basic principles and equations of biochemical kinetics. For reversible binding equilibrium the amount of complex formed over time is given by the equation... 

Papisov et al. (1974) performed calorimetric and potentiometric experiments to determine the thermodynamic parameters of the complex formation of PMAA and PAA with PEG. They investigated how temperature and the nature of the solvent affected the complex stability. They found that in aqueous media the enthalpy and entropy associated with the formation of the PMAA/PEG complex are positive while in an aqueous mixture of methanol both of the thermodynamic quantities become negative. The exact values are shown in Table II. The viscosities of aqueous solutions containing complexes of PMAA and PEG increase with decreasing temperature as a result of a breakdown of the complexes. 

The most important property of cyclodextrins is in their ability to accommodate guest molecules within their cavity, which has a volume of 262 per molecule or 157 mL per mol of [3-CD (cavity diameter 6.0-6.5 A). In aqueous solution, this cavity is filled with molecules of water the displacement of which by a less polar guest leads to an overall decrease in free energy. Stability constants and thermodynamic parameters for complexation of a vast number of guest molecules can be found in ref. [3]. 

Table 3.5 Thermodynamic Parameters of Reaction of Cadmium(II)-Ammonia Complex with Ethylenediamine... 

Euture improvements of the methods presented here will include modifications that enable determination of the thermodynamic parameters of protein-ligand binding interactions. Eor example, ALIS-based Kd or off-rate measurements at varying temperatures could yield useful relationships between chemical structures and binding thermodynamics. Ready access to such information, especially for targets that otherwise require complex bioassays for their study, could posi-... 

Girard, M., Turgeon, S.L., Gauthier, S.F. (2003). Thermodynamic parameters of p-lacto-globulin-pectin complexes as assessed by isothermal titration calorimetiy. Journal of Agricultural andFood Chemistry, 51, 4450 1455. 

The mechanism of addition of amines to the epoxy ring considered above is part of the general problem of the mechanism of the reaction under study. The main feature of this mechanism is formation of different hetero- and autocomplexes of the starting substances and reaction products characterized by different reactivity5,13,14,33). These complexes may be formed both inter- and intramolecularly. The existence of these complexes has been proved and the thermodynamic parameters in complexing reactions have been determined 5.13,14,29,33-43)... 

The table demonstrates a good agreement between the thermodynamic parameters of the complexing reactions inside both the model and the real systems. Moreover, the absolute values of the thermodynamic parameters for the corresponding complex-... 

It is also common to measure by voltammetry the thermodynamic properties of purely chemical reactions that are in some way coupled to the electron transfer step. Examples include the determination of solubility products, acid dissociation constants, and metal-ligand complex formation constants for cases in which precipitation, proton transfer, and complexation reactions affect the measured formal potential. Also in these instances, studies at variable temperature will afford the thermodynamic parameters of these coupled chemical reactions. 

Titration calorimetry has been successfully employed in the determination of thermodynamic parameters for complexation (Siimer et al., 1987 Tong et al., 1991a). The technique has the advantage of employing direct calorimetric measurements and has been proposed as the most reliable method (Szejtli, 1982). It should be noted that the information derived from multistep series reactions is macroscopic in nature. In contrast to spectrophotometric methods that provide information concerning only the equilibrium constant(s), titration calorimetry also provides information about the reaction enthalpy that is important in explaining the mechanism involved in the inclusion process. 

Thermodynamic Parameters of the Complexation of Alkali Cations in Methanol at 25°C... 

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