Penetration of dyes into protein crystals

Experiments were carried out on 15 different protein crystals with the objective of estimating the rates of penetration of dye molecules into the crystals. The dyes were in the molecular-weight range 250–1000 Da and the protein crystals were of dimensions of 0.7 mm or greater. Experiments were also conducted on protein crystals grown between glass cover slips (separation 200 µm) that restricted the direction of diffusion. The rate of penetration of dyes into protein crystals depends very much on the degree of association between the dye and protein molecules. Dye penetration was not consistent with pure diffusion when the affinity of the protein for the dye was significant, and this was frequent. Penetration rates were less dependent on factors such as the molecular weight of the dye or the diffusion direction. For weakly interacting protein crystal/dye combinations, penetration was a fair measure of diffusivity and the observed rates were in the range 60–100 µm h⁻¹. For strongly interacting combinations, the rates of penetration were of the order of 15–30 µm h⁻¹.