Abstract
The ability to conjugate proteins to colloidal gold sols provides a wide variety of probes for electron microscopy. Antibodies, protein A, protein G, lectins, enzymes, toxins, and other proteins have all been conjugated to colloidal gold. The nature of the interaction between the colloidal gold and the protein is poorly understood. Proteins are conjugated to gold sols by adjusting the pH of the gold sol to approximately 0.5 pH unit higher than the pI of the protein being conjugated. This chapter gives a general method for conjugating proteins to colloidal gold as well as more specific methods for conjugating antibodies and protein A to colloidal gold.