Carrier stability constants

Cyclam is one of the most extensively investigated ligands in coordination chemistry [23]. Both cyclam and its 1,4,8,11-tetramethyl derivative in aqueous solution can be mono and diprotonated and can coordinate metal ions such as Co2+, Ni2+, Cu2+, Zn2+, Cd2+, and Hg2+ with very large stability constants [24]. Furthermore, cyclam and its derivatives have been studied in medical applications [25], as carrier of metal ions in antitumor [26], imaging contrast agents [27], and as antiHIV agents [28]. 

If the ligand (neutral carrier) is capable of completely enveloping the cations, the terms Ky and Eq of the membrane depend only on the ratio of the complex stability constants K JK of the ions with the ionophore. 

Table 2. Stability constants for some carrier antibiotic complexes (53, 56)... 

Figure 18 (a) Mechanism of ion pair transport mediated by a cation carrier (b) plot of initial transport rates (V) of cation picrates as a function of the logarithm of the stability constants... 

Pis, Pjs = stability constants of the carrier cation complexes in the outside solution (in water)... 

TABLE III. Stepwise Stability Constants for the Interaction of Some Carrier Molecules with Different Ions (Water or Ethanol3 in kg/mole)... 

These comparatively lipophilic ligands have been conceived as ion carriers for systems such as ion-selective electrodes, 29 therefore they do not have high stability constant values, but require fast complexation kinetics in order to achieve rapid equilibration. Nevertheless, it has been possible to recover crystalline species which often have present additional water molecules to help stabilize the crystal lattice. Coordinative participation of the carbonyl oxygen... 

The results (Table 10) show that the cryptands could act to produce carrier-mediated facilitated diffusion and there was no transport in the absence of the carrier. The rate of transport depended upon the cation and carrier, and the transport selectivity differed widely. The rates were not proportional to complex stability. There was an optimal stability of the cryptate complex for efficient transport, logKs 5, and this value is similar to that for valinomycin (4.9 in methanol). [3.2.2] and [3.3.3] showed the same complexation selectivity for Na+ and K+ but opposing transport selectivities. The structural modification from [2.2.2] to [2.2.C8] led to an enhanced carriage of both Na+ and K+ but K+ was selected over Na+. The modification changes an ion receptor into an ion carrier, and indicates that median range stability constants are required for transport. Similar, but less decisive, results have been found in experiments using open-chain ligands and crown ethers.498... 

Cations are known to be transported through membranes by synthetic macrocyclic polyethers as well as by antibiotics. When the rate-determining step is the ion extraction from the IN aqueous phase to the membrane phase, the transport rate increases with the increasing stability constant. On the other hand, when the rate-determining step is the ion-release from the membrane phase to the OUT aqueous phase, the carrier must reduce the stability constant in order to attain efficient decomplexation. Some polyether antibiotics feature... 

The interplay of complex stability and cation exchange kinetics is very important in the uses of supramolecular cation hosts. On the basis of their behaviour, we may distinguish between cation receptors (slow kinetics, large stability constants) and cation carriers (fast kinetics, lower stability). In the next section we will see how fast exchange kinetics make cation carriers highly useful in applications such as phase transfer catalysis. 

Transferrin binds to Al + and is the main protein carrier of Al + in the plasma. Displacement of the 10 times stronger binding Fe + is unnecessary because plasma transferrin is about 50 qM in unoccupied sites. On the basis of values of requisite stability constants, it was suggested that in blood plasma about 89% of Al + is bound to transferrin and 11% to citrate. This distribution is supported by direct measurements by a variety of methods. ... 

Li+ shows a considerably lower value. The rate and equilibrium constants of the complex formation of alkali ions with murexide in methanol are summarized in Fig. 11. These data clearly point out that murexide — both from the static and dynamic point of view — is an ideal candidate for the indication of alkali ions in methanol. The spectral shifts are easily detectable and characteristic. The stability constants are in a very convenient range (especially with regard to an investigation of the complex formation of the carriers). The rate of the complex formation is very high so that any change in the coupled reaction system can be followed almost instantaneously. 

Fig. 18. Rate constants, stability constants and heats of reaction for the complex formation of Na+ with carriers in methanol...

From this information on rate and stability constants of the various carrier complexes we may derive four rules for the design of an optimal carrier ... 

High rates are actually necessary for an efficient carrier action. A metal ion to be transported across a membrane must be accepted and delivered by the carrier as quickly as possible. The specificity is defined by the difference in stability constants of complex formation for different metal ions. The stability constant on the other hand determines the maximal values of rate for the delivery of the metal ion. If this rate of delivery would become slower than the actual transport rate all advantages gained by higher selectivity (high stability constant) would be lost (due to the slower rate of delivery). This condition requires the rate of complex formation to be almost diffusion controlled which CcUi only be facilitated via a conformation change of the carrier molecule. 

The distribution coefficient may be expressed as a function of the metal association (stabihty) constants in the LM solution, the association constants of metal ions with solvent environment in the feed and in the strip solutions and partition coefficients of the carrier and metal ion. In this case, the separation factor can be determined by stability constants of the metal complexes, formed with functional groups of carrier, if we assume that the metal ions are predominantly present (a) as free ions in the acid solution, so that complex concentrations can be disregarded and (b) as complexes in the LM solutions, so that free ion concentrations can be disregarded. 

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