Receptor cavity

Complementarity. To a first approximation, complementarity should take two forms (Fig. 1). Firstiy, the shape and size of the receptor cavity must complement the form of the substrate. Secondly, there must be a chemical complementarity between the binding groups lining the interior of the cavity and the external chemical features of the substrate (15). 

The optimum B4 value for N4-substituents, 4.5 A coincides very well not only with that for N substituents of adenylate cytokinins, 5.2 A, shown in the previous section, but also with the value of 4.7 A observed for another class of compounds, N4-substituted 4-amino-2-methylpyrrolo[2,3-d]pyrimidines (24)47) which show either cytokinin or anticytokinin activity depending upon the structure of N4-substituents. These results in combination may help to think of the size of structure which should sterically fit well the cytokinin receptor cavity. 

The variation in activity is governed mainly by the steric effect of substituents within the compounds examined in both series. However, the mode of interactions seems to be different, as revealed by the different steric parameters incorporated. The significance of the Arw in Eq. 52 may be that the meso-substituents interact in such a manner that they come into contact with the receptor surface of walls, whereas that of the AVW in Eq. 53 may indicate the importance or fitting into a cavity or pocket on the receptor surface in the region where the (//-substituents direct. The AVW value for C1(0.0) is nearest to the optimum steric condition (AVW 2 —0.24) calculated from Eq. 53. Thus, (//-Cl substituents in lindane are thought to be those which best fit the receptor cavity among the (//-substituents. 

Many systems are described in which changing the cavity shape by allosteric effects allows the cation binding ability and the selectivity of the receptor to be modified and controlled. For instance a photo-responsive cis/trans isomerizable azobenzene unit has been introduced in macrocyclic structures in order to change the receptors cavity shape, leading to a photo-control of ion extraction. 

Recently the Nolte group has constructed a series of non-classical amphiphiles consisting of ruthenium-bipyridine centers functionalized with a receptor cavity viz., water-soluble metallo-hosts (Figure 7.22).15... 

Other closely related microheterogeneous environments such as micelles [70] or tailored electron relays capable of micellization upon reduction [71], operate by related hydrophilic-hydrophobic interactions in controlling photosensitized ET processes. Similarly, separation of photoproducts at the molecular level, by means of hydrophobic interactions, has been accomplished by utilizing cyclodextrin receptors [66, 72]. This host component selectively associates one of the photoproducts into the hydrophobic receptor cavity and consequently back ET is retarded. 

The highest stability of the sensor-analyte complex is achieved when the substrate fits perfectly in the hole within the receptor. The analyte does not have to fit the receptor cavity perfectly efficient binding can be achieved by careful design of the receptor. In the case of metal ion sensors the receptor must contain a proper type and number of donor atoms angular orientation and directionality of lone electron pairs are also of crucial importance [5]. 

There exist variations on aforementioned procedures. Equations with parameters raised to a power, such as Xf, may be considered as linear in a variable Zj (= X, ). For example, a squared parameter might be used in the case of an extremum occurring in a plot of a property versus some parameter such as the molecular volume. Such a relation might be expected if there exists maximum molecular size for fitting into a receptor cavity. Other mathematical functions of the primitive descriptors can likewise be used to generate terms. Linear regression models are preferred because the terms in Eq. [1] have relatively simple physical interpretations. 

A subset of points, uniformly distributed on the molecular surface, is selected from this surface. The default value is usually set to 100, which is sufficient to describe drug-like molecules and receptor cavities. However, it is possible to select a lower number of surface nodes if thought appropriate for the situation. 

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