Cryptates decomplexation

I would like to extend Prof. Simon s characterizations of these beautiful new molecules to include a description of the effects on lipid bilayers of his Na+ selective compound number 11, which my post-doctoral student, Kun-Hung Kuo, and I have found to induce an Na+ selective permeation across lipid bilayer membranes [K.-H. Kuo and G. Eisenman, Naf Selective Permeation of Lipid Bilayers, mediated by a Neutral Ionophore, Abstracts 21st Nat. Biophysical Society meeting (Biophys. J., 17, 212a (1977))]. This is the first example, to my knowledge, of the successful reconstitution of an Na+ selective permeation in an artificial bilayer system. (Presumably the previous failure of such well known lipophilic, Na+ complexing molecules as antamanide, perhydroan-tamanide, or Lehn s cryptates to render bilayers selectively permeable to Na+ is due to kinetic limitations on their rate of complexation and decomplexation). 

The kinetics and dynamics of crvptate formation (75-80) have been studied by various relaxation techniques (70-75) (for example, using temperature-jump and ultrasonic methods) and stopped-flow spectrophotometry (82), as well as by variable-temperature multinuclear NMR methods (59, 61, 62). The dynamics of cryptate formation are best interpreted in terms of a simple complexation-decomplexation exchange mechanism, and some representative data have been listed in Table III (16). The high stability of cryptate complexes (see Section III,D) may be directly related to their slow rates of decomplexation. Indeed the stability sequence of cryptates follows the trend in rates of decomplexation, and the enhanced stability of the dipositive cryptates may be related to their slowness of decomplexation when compared to the alkali metal complexes (80). The rate of decomplexation of Li" from [2.2.1] in pyridine was found to be 104 times faster than from [2.1.1], because of the looser fit of Li in [2.2.1] and the greater flexibility of this cryptand (81). At low pH, cation dissociation apparently... 

From the temperature dependence of the rate of decomplexation /c i = 1/t the activation parameters and AS can finally be determined. Figure 5 displays some temperature-dependent Na spectra for complexation between Na and cryptand C222. (64) Free enthalpies and entropies of activation for complexation between Li" and cryptands C211 and C221 as well as between Na" and cryptands C222 are in Table IV. While for the lithium cryptates the enthalpies of... 

Kinetic parameters for lithium and sodium cryptate [I ] decomplexation (61,64)... 

The first A V determinations for a cryptate have been reported in a conduc-timetric SF study of the decomplexation of [Na(C221)] under acidic conditions where one or both of the reactions paths (16) and (18) operate depending on the... 

Table 9,2, Decomplexation (k s ) and Complexation (k dm mol s ) Rate Constants for Thallium(I) Cryptates in a Range of Solvents at 298,2...

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