The chemistry of irreversible capture.

The specific recognition and binding of biological molecules by antibodies is fundamentally important. Natural antibodies are multivalent, having at least two identical ligand-binding sites; this permits them to bind tightly at cell surfaces, which present multiple copies of their target ligands. Antibodies that bind to soluble monovalent ligands, such as most small molecules, do not share this multivalent advantage. Nor do engineered fragments of antibodies, such as single-chain Fv proteins or Fab fragments, which generally possess only a single ligand-binding site. Engineered monovalent antibody/ligand pairs that retain the binding specificity of the antibody, but do not dissociate, are promising components of new delivery systems. These are based on a combination of genetic manipulation of the protein and chemical synthesis of appropriate ligands, examples of which are reviewed here.