Consequences of complex formation

Another consequence of complex formation is the possibility of having alternate reaction mechanisms. For example, Cu+ is normally unstable in aqueous solutions. (In fact, it does not even... 

The flavonols and their glycosides contribute to specific taste characteristics such as bitterness and astringency in berry fruits and their products (Shahidi and Naczk, 1995). The molecular structure of flavonols lacks the conjugated double bonds of the anthocyanins, and they are thereby colorless. They may, however, contribute to discoloration of berry fruits, as they are readily oxidized by O-phenoloxidase in the presence of catechin and chlorogenic acid. Discoloration may also occur as a consequence of complex formation with metallic ions. On the other hand, the flavonol glycoside rutin is known to form complexes with anthocyanins, thus stabilizing the color of these compounds. 

The changes in the interaction of the solvent molecules that occur as a consequence of complex formation are well reflected by the solvation entropies. From the solvation entropies determined by Parker and Watts [Pa 74] in various solutions, it emerges that in aprotic solvents the solvation entropy generally has a larger negative value, indicative of a higher degree of order, than in protic solvents. 

The incompatibility of PEGs with medicaments is often discussed in terms of their ability to form complex compounds. The possibility of forming addition compounds is due to the ether oxygen in PEGs. Since these complex compounds often depend on concentration, are reversible, and in addition, are frequently affected by the presence of water, it is no easy matter to establish the consequences of complex formation. It is necessary to carry out tests in individual cases to establish whether release of these active substances has been inhibited. A delayed... 

Let us now examine the consequences of the formation of a donor-acceptor bond in a little more detail. If the donor - acceptor bond is completely covalent, then we record net transfer of one unit of charge from the donor to the acceptor as a direct consequence of the equal sharing of the electron pair between the two centres. This result leaves a positive charge on the donor atom and a negative charge on the acceptor atom. The limiting ionic and covalent descriptions of a complex cation such as [Fe(H20)6] are shown in Fig. 1-1. 

As mentioned above, in contrast to classic antioxidant vitamins E and C, flavonoids are able to inhibit free radical formation as free radical scavengers and the chelators of transition metals. As far as chelators are concerned their inhibitory activity is a consequence of the formation of transition metal complexes incapable of catalyzing the formation of hydroxyl radicals by the Fenton reaction. In addition, as shown below, some of these complexes, for example, iron- and copper-rutin complexes, may acquire additional antioxidant activity. 

Kinetics and mechanisms of complex formation have been reviewed, with particular attention to the inherent Fe +aq + L vs. FeOH +aq + HL proton ambiguity. Table 11 contains a selection of rate constants and activation volumes for complex formation reactions from Fe " "aq and from FeOH +aq, illustrating the mechanistic difference between 4 for the former and 4 for the latter. Further kinetic details and discussion may be obtained from earlier publications and from those on reaction with azide, with cysteine, " with octane-and nonane-2,4-diones, with 2-acetylcyclopentanone, with fulvic acid, and with acethydroxamate and with desferrioxamine. For the last two systems the various component forward and reverse reactions were studied, with values given for k and K A/7 and A5, A/7° and A5 ° AF and AF°. Activation volumes are reported and consequences of the proton ambiguity discussed in relation to the reaction with azide. For the reactions of FeOH " aq with the salicylate and oxalate complexes d5-[Co(en)2(NH3)(sal)] ", [Co(tetraen)(sal)] " (tetraen = tetraethylenepentamine), and [Co(NH3)5(C204H)] both formation and dissociation are retarded in anionic micelles. 

It seems that there is probably greater availability of positively charged residues on the adsorbed protein for electrostatic interaction with sulfate groups of the anionic polysaccharide. This could lead to a greater extent of neutralization of dextran sulfate as a result of complex formation, and consequently to a lower thermodynamic affinity of the complexes for the aqueous medium and a lower value of the ( -potential for emulsion droplets in bilayer emulsions. 

The carbon dioxide molecule exhibits several functionalities through which it may interact with transition metal complexes and/or substrates. The dominant characteristic of C02 is the Lewis acidity of the central carbon atom, and the principle mode of reaction of C02 in its main group chemistry is as an electrophile at the carbon center. Consequently, metal complex formation may be anticipated with basic, electron-rich, low-valent metal centers. An analogous interaction is found in the reaction of the Lewis acid BF3 with the low-valent metal complex IrCl(CO)(PPh3)2 (114). These species form a 1 1 adduct in solution evidence for an Ir-BF3 donor-acceptor bond includes a change in the carbonyl stretching frequency from 1968 to 2067 cm-1. 

The chirality of metal helicates can be demonstrated experimentally by X-ray crystal structure determinations and in solution by NMR spectroscopy. Addition of chiral shift reagents such as [Eu(tfc)3] (tfc = 3-(trifluoromethylhydroxymethylene)-(+)-camphorato) to selected helicates results in the splitting of some of the ligand signals as a consequence of the formation of diastereomeric complexes with the shift reagent. Such splitting is not observed for the free ligands, which are achiral. 

Thermodynamics of complex formation, discussed in detail by Schmidtchen in the present volume, hold many surprises.[32] In comparing enthalpic with entropic parameters one should not forget that AG and AS depend on the chosen units for relative concentration, which can be either mol/1, or can be given in dimensionless mole fractions in consequence the usual partition of absolute numbers for AH and AS becomes to some degree arbitrary (see e.g. ref. 2d, p. 24, p. 106). In addition, complexation is usually characterized by sizeable changes of heat capacity, making the thermodynamic partitions temperature-dependent. 

The formation of decanal - starch - complexes with time at different temperatures is presented in Figure 1. The reaction is completed in a matter of minutes, and a stable equilibrium is obtained. Isotherms of complex formation are shown in Figure 2. The complex formation starts at very low concentrations and comprises appreciable amounts of ligand. We are actually studying these reactions in some detail and are interested in the consequences of these interactions for taste and odor perception. As reported elsewhere, the complexed ligands, if present in dry state, have a remarkably increased chemical stability (25). 

A further consequence of the ease of complex formation may be a more rapid copolymerization, although the complex may form an adduct or await the input of sufficient energy to open. 

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