Depletion Interactions and Protein Crystallization

In 1934 Desmond Bernal and Dorothy Crowfoot (later Hodgkin) discovered that crystals of pepsin, a digestive enzyme, give a well-resolved X-ray diffraction pattern [259]. It took 25 years before the first atomic stmctures of proteins using X-ray crystallography were determined. In 1958 Kendrew et al. published the 

Structure of the protein myoglobulin [260], which stores oxygen in muscle cells and in 1960 Perutz et al. [261] reported the structure of the protein hemoglobin, which transports oxygen in blood. 

The first requirement for protein structure determination with X-ray diffraction is to grow suitable crystals [262]. While great strides have been made in the 

It was shown that this remarkable phase behaviour could be understood on the basis of the sensitivity to the form of the pair potential of the phase diagram of small attractive colloidal particles [268-270]. Moreover, it was soon realized that successful protein crystallization depends on the location (protein concentration and temperature) in the phase diagram [271-275]. Control of protein crystal nucleation around the metastable liquid-liquid phase boundary appears key to the development of systematic crystallization strategies (for a concise review see [276]). This phase boundary can be manipulated by depletion interactions through the addition of non-adsorbing polymers such as polyethylene glycol. 

To illustrate the role of non-adsorbing polymer chains on the protein solution phase behaviour we discuss the results of adding the polymer PEO (also termed PEG) to a solution with the protein apoferritin by Tanaka and Ataka [277]. Apoferritin is an iron storage protein consisting of 24 subunits. The effective radius 

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A. Kulkami, C. Zukoski, Depletion interactions and protein crystallization, J. Ciyst. Growfli 232 (2001) 156-164. 

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