Supramolecular complexes, thermodynamic

Self-organization systems under thermodynamic control (spontaneous processes with a negative free-energy change), such as supramolecular complexes, crystallization, surfactant aggregation, certain nano-structures, protein folding, protein assembly, DNA duplex. 

E. Leize, A. Jaffrezic, and A. Van Dorsselaer, Correlation between solvation energies and electrospray mass spectrometric response factors. Study by electrospray mass spectrometry of supramolecular complexes in thermodynamic equilibrium in solution, J. Mass Spectrom. 31 (1996), 537-544. 

An excellent, and far more thorough, treatment of binding constants can be found in Connors book of the same name [1] and an exhaustive treatment of supramo-lecular complexation thermodynamics has been undertaken by Schneider and Yatsimirsky [2], A highly relevant review comparing methods for determining supramolecular complex stabilities was also published in 1992 [3]. 

The rationale for the observed configuration (Scheme 3.29), is based on the X-ray structure of another a-carbamoyloxyorganolithium sparteine complex [185]. After deprotonation, the chelated supramolecular complex shown in the lower left is postulated. This structure contains an adamantane-like lithium-diamine chelate, and contains new stereocenters at the lithiated carbon and at lithium itself. Note that epimerization of the lithiated carbon would produce severe van der Waals repulsion between R and the lower piperidine ring, whereas epimerization at lithium produces a similarly unfavorable interaction between the same piperidine ring and the oxazolidine substituents. Thus, the carbamate is tailor-made for sparteine chelation of only one enantiomer of the a-carbamoyloxyorganolithium. These effects may provide thermodynamic stability to the illustrated isomer. To the extent these effects are felt in the transition state, they are also responsible for the stereoselectivity of the deprotonation. 

MicroCafs titration microcalorimeter is the most common instrument to have been used in many sdentific studies on the complexation thermodynamics of CyDs. Several important issues which should be taken into account on using a titration microcalorimeter will be discussed below for investigation of CyD complexation and supramolecular association in general. 

In addition to thermodynamic measurements. NMR spectroscopy provides an invaluable tool for kinetics and mechanistic measurements, giving access to a time range from the picosecond to years. The dynamic NMR studies of supramolecular complexes were recently reviewed. ... 

Though maximizing affinity is a profitable way to boost thermodynamic selectivity, there is an upper limit to it due to practical limitations. Taking the well-studied avidin-biotin system as a prototypical example of a high-affinity supramolecular complex (ATasj,"10 M ) and assuming the fastest possible association of host and guest (diffusion controlled rate sec ), one arrives at... 

While the conversion of a supramolecular complex back into its constituent components is part of the equilibration process and, as such, is contained in thermodynamic selectivity, any alternative transformation leading to a function is characterized by a rate and a corresponding monomolecular rate constant. Principally, it can refer to just one elementary reaction yielding the function, but more likely, function generation will be composed of several successive steps. In any case, a pathway is defined that features a certain activation barrier that, in turn, determines the total flux on this route. Typical kinetically controlled vectorial processes include the transport of compounds across membranes, the photosynthetic elec-... 

Another major difference between conventional solute transfer and supramolecular association is the crucial influence of the shape of the guest molecule in the latter case. This is clearly demonstrated upon complexation of P-CD with 2- and 3-phenylbutyric acids. The apparently trivial variation in the methyl position results in a profound difference in complexation thermodynamics. Despite virtually the same hydrophobicity (i.e., the distribution ratio between water and nonpolar organic solvent), the affinity of 3-phenylbutyric acid toward P-CD ( =402-430 M ) is more than four times larger than... 

Figure 8 Distribution of (A) thermodynamic effective molarity (EM) values for noncova-lent bond formation in supramolecular complexes (data for intramolecular H-bond formation in zinc porphyrin—pyridine complexes plotted separately as DMC) and (B) kinetic and (C) thermodynamic EM values for covalent bond formation.

This review describes thermodynamic EM for intramolecular nonco-valent interactions in the supramolecular complexes. Values of EM have been collected for approximately 500 different systems, and there are... 

APPENDIX COLLECTION OF THERMODYNAMIC EFFECTIVE MOLARITY VALUES FOR SUPRAMOLECULAR COMPLEXES... 

Perhaps the most apparent similarities with transition metal complexes are the main group metal ions and their supramolecular complexes. Here, the progression from the chelate to macrocyclic to cryptate effect can be readily recognized and the thermodynamics are, for the most-part, established. The term ionophore is often used for this class of hosts, originating from its use in biology for lipid-soluble molecules that serve as vehicles for transporting ions across membranes. 

Supramolecular complexes involve well-defined noncova-lent interactions between individual components generating emerging properties. Most supramolecular complexes are formed under thermodynamic control and are dynamic entities in solution. The power of nuclear magnetic resonance (NMR) spectroscopy for the study of supramolecular complexes arises from its unique capacity to probe the environment of individual atomic nuclei reporting on the structure and dynamics of the formed complexes. In particular, NMR spectra can provide information about the symmetry... 

Often, the course of a reaction is followed by NMR spectroscopy, and so kinetic data for the system is usually based on the monitoring of NMR integrals. However, the changes in the observed chemical shifts for the various species present in solution during the course of the reaction are not taken into account. By contrast, thermodynamic parameters for the supramolecular complexes involved are usually derived from independent NMR titration experiments, von... 

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