Covalent bonds receptor-ligand

Fig. 9 Utility of de-PEGylation technology in liposomes, (a) PEG derivative possessing a lipid moiety. The covalent bond between PEG and the lipid moiety can be cleaved by stimuli such as those within the acid environment of cancer and inflammation, (b) After binding the target cell via specific recognition of the receptor by the ligand, PEG molecules on the surface of the liposome are cleaved. The release of PEG facilitates membrane fusion of the liposome and liposome decomposition, resulting in efficient drug delivery...

That being the case, it is appealing to refer to anion receptors as ligands. However, traditionally the commonly understood definition of ligand has been a Lewis base capable of forming a coordinate-covalent bond with a metal ion. Since for anions, the ligand actually behaves as a Lewis acid, the definition needs to be expanded to include both Lewis acid and Lewis base behavior. 

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