Decomplexation rates

In the context of Scheme 11-1 we are also interested to know whether the variation of K observed with 18-, 21-, and 24-membered crown ethers is due to changes in the complexation rate (k ), the decomplexation rate (k- ), or both. Krane and Skjetne (1980) carried out dynamic 13C NMR studies of complexes of the 4-toluenediazo-nium ion with 18-crown-6, 21-crown-7, and 24-crown-8 in dichlorofluoromethane. They determined the decomplexation rate (k- ) and the free energy of activation for decomplexation (AG i). From the values of k i obtained by Krane and Skjetne and the equilibrium constants K of Nakazumi et al. (1983), k can be calculated. The results show that the complexation rate (kx) does not change much with the size of the macrocycle, that it is most likely diffusion-controlled, and that the large equilibrium constant K of 21-crown-7 is due to the decomplexation rate constant k i being lower than those for the 18- and 24-membered crown ethers. Izatt et al. (1991) published a comprehensive review of K, k, and k data for crown ethers and related hosts with metal cations, ammonium ions, diazonium ions, and related guest compounds. 

These results are at variance with Lauger s interpretation of the saturation current-voltage characteristics for valinomycin-K+ in phos-phatidylserine bilayer membranes, which he purports to be due to limiting decomplexation rates. 

Table 3.9 Complexation and decomplexation rate constants of picrate salts for representative compounds under standard conditions.

Table 3.9 shows that all of the complexation rates are very fast on the human time scale and cannot even be followed by H NMR spectroscopy (effectively, reactions occur immediately upon mixing). Decomplexation rates for more flexible hosts are also fast on the human time scale, but may be accessible by H NMR, while for spheraplexes the reverse reaction is very slow even on the human time scale at 25 °C. 

Table 14.1 Maltodextrin complexes with the transport protein of maltose (from Quiocho 1989) (reproduced with kind permission from the International Union of Pure and Applied Chemistry). K

Until recently, the role of slow rates of cation release in LM transport was unclear. From U NMR studies, it is known that the complexes can be kinetically stable [65, 66] and, as a consequence, decomplexation rates can be very slow. Influence of slow rates of alkah metal cations release was raised in transport through a BLM [58,64,67-69]. Recently, at cation transport experiments with different calix crown ether derivatives [42,70], was proven that the rate of decomplexation can be rate controlling in the transport through SLMs. 

The overall mass-transfer rate can be limited by any of the diffusion resistances in the three liquid phases (diffusion-limited transport) and/or chemical reaction (complexation/decomplexation) rate resistances on the membrane-solution interfaces (reaction rate-limited transport). 

J. Yoon, D. J. Cram, Decomplexation rate comparisons of hemicarceplexes whose single unique host bridge is changed in length and blocking power, Chem. Commun., 1997, 1505-1506. 

A critical point in NMR determinations of thermodynamic data is to have a proton whose chemical environment changes sufficiently upon binding to lead to a change in chemical shift, that is, 5//free 7 SHbound- As has been discussed above, binding determinations by NMR can be divided into two cases those that are slow on the NMR timeframe and those that are fast. The observed com-plexation and decomplexation rates (25) are those of uni-molecular processes (units s ), and it is the slower of these two processes that we must compare with the rate of NMR data acquisition. Our previous discussion emphasized how the timeframe of the NMR experiment was dependent on the external field strength of the instrument, but it is also important to note that whether a complexation is observed to be fast or slow on the NMR timescale depends on the chemical shift difference between the free and the bound state. The key equation is (37), which defines the boundary between fast and slow timescales, that is, the observed rate constant for coalescence ( coai) of the signals for the proton in question in the free and the bound state ... 

An interesting way to analyze contribntions that make up constrictive binding is to measure the steric kinetic isotope effects (SKIEs) by comparing decomplexation rates of hemicarceplexes with isotopically labeled guests. The physical origin of a steric isotope effect is the smaller zero-point energy (ZPE) of the C-D bond as compared to that... 

Reaction-Rate Limited Transport of Monovalent Ions. From H NMR studies it is known that decomplexation rates can be very slow and, as a consequence, complexes can be kinetically stable (35,36). Until recently the role of slow rates of cation release in SLM transport was unclear. Lehn et al (24) and Fyles (37) theoretically raised the question of the influence of slow rates of alkali metal cation release on transport through a BLM. Experimentally, this phenomenon has only been observed by Yoshida et al (38). They showed that cation transport through a BLM mediated by polynactin was limited by the rate of cation release from the membrane. In 1994, Echegoyen stated that in SLM transport the rate of cation release from the membrane could never... 

The results shown in Table 3 and 4 both indicate that the transport of KCIO4 by carrier 7 is mainly limited by a slow decomplexation rate (o>l), while carrier 6 is mainly diffusion limited (a < 1). 

Parameters that Affect Kinetically Limited Transport. Both the diffusional process and cation release are influenced by the membrane solvent, support and anion in kinetically limited transport 40, 44). Using the membrane solvents listed in Table 6, the diffusion coefficient D and the decomplexation rate constant k were determined for the transport of KCIO4 through NPOE/Accurel by 1,3-dimethoxycalix[4]crown-5 (carrier 9). The value for a is 1.9 and the activation energy Eg is 61 kJ moV The transport is therefore considered as a kinetically limited process... 

Figure 15. Relationship Between the Decomplexation Rate Constant and the Solvent Polarity for the Transport of KCIO4 by Carrier 9 (Adapted from ref. 43).

The effect of the support on the transport kinetics has been examined by normalizing the decomplexation rate constant for the porosity of the membrane k = k 0 (40, 44), Consequently, a can be calculated from the rate of decomplexation Iq, and the diffusion coefficient and compared with other supports. From the... 

The complexation and corresponding decomplexation rates are the same at equilibrium, d[C]... 

In addition to the structural requirement, there are dynamic requirements that must be considered. The binding of the cation on one side (source side) of the membrane should be rapid and the stability of die complex should be great. Inside the membrane, only the strength of the complex is important because decomplexation is unfavorable in the lipid environment and therefore not a signifrcant factor. On the other (receiving) side of the membrane, complex stability should be weak and the decomplexation rate should be rapid so that cation release is facile. The kinetic and thermodynamic requirements for cation carriers on opposite sides of the membrane are thus incongruous. 

If we consider that the stability constant for the complex is K< =k,. .pi.,/kA., pi. the requirement for a carrier is that the same molecule have a large and small decomplexation rate at the same time. Since this is impossible, compounds are chosen that achieve transport by a compromise of the complexation and decomplexation rate constants. One means of circumventing this problem is to use a ligand (binder) that can be made to form a more or less stable complex by a rapid chemical modification. This is usually referred to as "switching." Mechanisms to switch a ligand s binding ability based on ionization, thermal, photochemical, and redox properties have been described in the literature. 

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