Binary and multi-component diffusion coefficients of protein solutions and role of mass transfer in the growth of protein crystals from solutions

Multi-component diffusion involving proteins is encountered frequently in biological processes. Examples include protein crystallization, dialysis associated with artificial kidneys, simultaneous diffusion of oxygen, sugars, and proteins in the blood, and the transfer of bile salts, fats, and amino acids in the small intestine. To analyze and interpret these phenomena, it is necessary to know the diffusion coefficient of the protein involved. The study of diffusion coefficients in super-saturated solutions is also fundamental in understanding the mechanism of crystal nucleation and crystal growth.;The purpose of this research was: (1) To measure the diffusion coefficient of lysozyme at pH 4.5 and 25°C in the binary system, lysozyme/water in the under-saturated and super-saturated solutions. (2) To measure the ternary diffusion coefficients of lysozyme at pH 4.5 and 25°C in the system lysozyme/NaCl/water over a wide range of concentration. (3) To use the binary and ternary diffusion data in a diffusive-convective transport model to examine the influence of convection on the growth rate of lysozyme and to establish the concentration profile of the protein and the salt around the growing crystal as a function of crystal size.;The results show that the diffusion coefficients coefficient of lysozyme in the binary system (salt-free buffer solution) decreased with increasing lysozyme concentration.;In the ternary system (the salt-buffer solution) the main term D 11 decreased slowly with increasing salt concentration and exceeded the binary diffusion coefficient D1 by 4 to 10%. The main term D22 was 1 to 16% less than binary diffusion coefficient, D 2. This behavior was attributed to the electro-neutrality constraint that must be maintained between the ions during the diffusion process.;The cross-term D21 increased sharply with increasing salt concentration. The cross-term D12 is very small and decreased slowly with increasing salt concentration. These results are consistent with the expected electrostatic interactions between lysozyme and salt in the diffusion process.;The crystal growth model, based on salt rejection at the protein crystal surface, showed that the growth rate and the interfacial concentration of lysozyme, with convection, remains nearly constant over a wide range of crystal sizes. In the absence of convection, both the growth rate and interfacial concentration decrease as the crystal grows larger.