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Using dilatometry, we examined the crystallization kinetics of the jack bean storage protein, canavalin. We found that the kinetic rate law is first order in the supersaturation. At pH values above the pI = 5.2, where the protein is negatively charged, the apparent rate constant, k, increases as the pH approaches the pI, decreases with increasing temperature, and increases with increasing salt concentration. The rate constant is essentially independent of the initial protein concentration. Precisely the same patterns of behavior were observed by Kim et al. [Mol. Phys. 2003, 101, 2677−2686] in dilatometer experiments involving the crystallization of lysozyme even though lysozyme molecules are positively charged under crystal growth conditions. By combining dimensional analysis with a simple theory of protein crystal nucleation and growth, we show that k ≈ ω/c s, where ω is the rate coefficient that controls the rate of advance of the crystal facets, and c s is the solubility of the protein in the growth solution. We determined the dependence of ω upon temperature and salt concentration by analyzing the observations of Forsythe and Pusey [J. Cryst. Growth 1994, 139, 89−94] of the rate of advance of the facets of growing lysozyme crystals. To the extent that our data overlap that of Forsythe and Pusey, we are able to show for both canavalin and lysozyme that the dependence of k, ω, and c s upon these variables is consistent with k ≈ ω/c s.

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