Host systems studies

Defect configurations in dilute alloys, studied up to now in the framework of multiple scattering theory, are such that a one-to-one correspondence exists between the atoms in the alloy and the reference system, the latter system regularly being the unperturbed host system. This one-to-one correspondence does not apply to the defect studied in substitutional electromigration, in which a host atom or an impurity can move to a neighbouring vacancy. 

As was the case for most other cytokines, medical appraisal/use of IL-2 was initially impractical due to the minute quantities in which it is normally produced. Some transformed cell lines, most notably the Jurkat leukaemia cell line, produces IL-2 in increased quantities, and much of the IL-2 used for initial characterization studies was obtained from this source. Large-scale IL-2 production was made possible by recombinant DNA technologies. Although the IL-2 gene/cDNA has now been expressed in a wide variety of host systems, it was initially expressed in E. coli, and most products being clinically evaluated are obtained from that source. As mentioned previously, the absence of glycosylation on the recombinant product does not alter its biological activity. 

We chose to show the application of techniques employed to study supramolecular dynamics to host systems that have defined binding sites and, therefore, form host-guest complexes with defined stoichiometries. CDs were chosen because they represent a host with only one binding site, and therefore can be viewed as a model system for hosts with the lowest degree of complexity. DNA was chosen because it provides multiple binding sites for small molecules, i.e. intercalative and groove binding, while the DNA is structurally fairly well defined. 

A recent study,209 in which previous results on the complexation of a series of non-centrosymmetrical guests with CDs were re-evaluated, suggested that the two observed relaxation processes could possibly be interpreted as a mechanism involving two parallel reactions inclusion of the guest through either the wide or narrow rim of the cyclodextrin. This mechanism was shown to lead to the same dependence of observed rate constants on concentration of cyclodextrin as the consecutive mechanism. This study showed that even for seemingly simple host systems the mechanistic details for complexation can be quite complex and still controversial. 

The dominant methodology is that based on covalent bonding of the chromophore to the polymer, as this shows much improved long term stability over simple guest-host systems. Guest-host polymers using NLO guests have been extensively studied in the area of photorefractive polymers (see section 5.6.3). 

A majority of the host systems mentioned so far have been shown to encapsulate halide guests as a consequence of the symmetrical nature of their binding pockets, which readily adapt to the spherical symmetry of the halide ion. A study of the bis-tren macrobicyclic ligand 19-6H, however, has revealed that, in addition to accommodating halide anions, it is also able to encapsulate azide, N, within its cylindrical cavity. Solution stability constant measurements indicate that the host... 

Supramolecular Chemistry tries to mimic these concepts, and numerous host molecules have been synthesized [2, 3]. For the understanding of host-guest systems, first the binding of a guest within a host was studied. Numerous... 

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