Ligand layer

Information may be stored in the architecture of the receptor, in its binding sites, and in the ligand layer surrounding the bound substrate such as specified in Table 1. It is read out at the rate of formation and dissociation of the receptor—substrate complex (14). The success of this approach to molecular recognition ties in estabUshing a precise complementarity between the associating partners, ie, optimal information content of a receptor with respect to a given substrate. 

Estimation of ligand thickness. The simplest method consists in measuring the thickness s of the ligand layer on space-filling molecular models. When X-ray structures are available, the thickness may be obtained from crystal packing data. However, ligands often have regions of different thicknesses and contain crevasses in which solvent molecules may or may not penetrate. 

Fig. 15. Calculated effect of thickness s of ligand layer on the ion selectivity of electrically neutral model ligands...

Wilson and Johnston [100] have studied another common case of passivated clusters, namely gold clusters ( =38,44,55) protected by an outer layer of thiol ligands. Much larger clusters of this type can be produced routinely in solution, with various types of ligands [101-104]. Wilson and Johnston treated the ligand layer only implicitly, but they could show that for the case of Au55 the bare cluster preference of an icosahedral over a cuboctahedral shape is reversed in the presence of a ligand layer. Experimental inference [102] may point in the same direction. 

Clearly, much work remains to be done in this subarea. Evolutionary operators need to be refined in order to deal efficiently with clusters and their ligand layers separately, as well as simultaneously. Suitable levels of theory for the interparticle forces have to be found and tested. And, of course, the size gap between application calculations and experiments needs to be closed. 

The introduced groups can be further modified and activated, e.g. with carbodiimide or divinylsulfone chemistry, which enables the surface to bind to e.g. amino groups of proteins. However, also the activation of the protein (or any other amino or thiol-group carrier) using the same chemistry is a means to couple covalently to the surface. On the other hand for many applications simple non-covalent absorption to a plasma-etched polymer is sufficient to bind the ligand-layer to the polymer surface adapting know-how from the ELISA-technology. 

Typically, the water-soluble nanoparticles prepared via wet-chemical approaches are surface-capped with a ligand layer, which is formed either in situ or through postmodification of the as-prepared nanoparticles. According to the properties of the capping layer, the methods for conjugation of the biomolecules are varied. 

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