THE RATE OF WATER EQUILIBRATION IN VAPOR-DIFFUSION CRYSTALLIZATIONS -DEPENDENCE ON THE DISTANCE FROM THE DROPLET TO THE RESERVOIR

The rate of water equilibration in hanging-drop vapor-diffusion experi ments was studied as a function of the distance separating the hanging drop from the surface of the reservoir solution. Hanging drops of 1.0 0 M NaCl were allowed to partially equilibrate with reservoirs of 2.00 M NaCl at room temperature. Over the range of droplet-reservoir dista nces examined, 7.6-119.4 mm. the larger the distance that separated th e droplet and reservoir, the slower the droplet equilibrated with the reservoir. The variation of the rate of equilibration with droplet-res ervoir distance was non-linear: the rate was most sensitive to variati ons in the droplet-reservoir separation at short separations. A mathem atical model of the equilibration kinetics was developed that fits the experimental data. The model is based on the assumption that the rate -limiting step in vapor-diffusion equilibration is transit of water ac ross the vapor space. A simple device to vary the rate of water equili bration, and thereby optimize macromolecular crystal growth conditions , is described.