Binding group

If we consider our hypothetical receptor and its natural neurotransmitter, then we might reasonably predict which of a series of molecules would interact with the receptor and which would not. 

For example, consider the structures in Fig. 5.14. They all look different, but they all contain the necessary binding groups to interact with the receptor. Therefore, they may well be potential agonists or alternatives for the natural neurotransmitter. 

However, the structures in Fig. 5.15 lack one or more of the required binding groups and might therefore be expected to have poor activity. We would expect them to drift into the receptor site and then drift back out again, binding only weakly if at all. 

In fact, this seems unlikely when we consider that neurotransmitters appear to bind, pass on their message and then leave the binding site very quickly. In order to do that, there must be a fine balance in the binding forces between receptor and 

---

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). 

Compared with conventional type, the tentacle type exhibits a marked increase in selectivity. The results observed for preparative ion exchangers of tentacle type prompted the author to test this structural arrangement of the binding groups also for analytical materials based on porous silica. Figure 12 reveals that the tentacle-specific selectivity is fully preserved when the matrix is changed. 

One approach, pursued by two groups in particular, is to use model nucleobases that feature an additional metal-ion binding group. Gokel and co-workers have used this strategy with aza-crown derivatives (50,51). X-ray crystallography of 6is-(3-(l-thyminyl)propyl)-4,13-diaza-18-crown-6, 4, reveals the Na+ is coordinated by T-02 [Na-0 distances 2.488 and 2.468 Al (51). Aza-crown derivatives with multiple base substi-... 

Fig. 1. Common siderophore iron-binding groups catechol (Eq. (4)), hydroxamic acid (Eq. (5)), ot-hydroxycarboxylic acid (Eq. (6)), and hydroxypyridinone (Eq. (7)). Additional siderophore binding groups ...

Homopodal hexadentate siderophore mimics with various binding groups. 

These pFe values were calculated for [Fe3+]tot = 10 M, [L]t = 10 2M, and pH = 7.4. Tleteropodal hexadentate siderophore mimics with various binding groups. 

---