CHAPERONE SALTS, POLYETHYLENE-GLYCOL AND RATES OF EQUILIBRATION IN VAPOR-DIFFUSION CRYSTALLIZATION

The kinetics of water-vapor equilibration in macromolecular crystalliz ation were investigated for sitting droplets of aqueous polyethylene g lycol (PEG) 8000 as a function of concentration. Equilibrations, set u p with initial concentrations of PEG in the droplet at half those in t he reservoir, were very slow for concentrations of relevance to the ma cromolecular crystal growth problem. At 301 K, 24 mu l droplets at ini tial concentrations of 2.5, 5.0 and 7.5%(w/v) PEG require 12, 5, and 3 weeks to reach equilibrium, respectively. On the other hand, the addi tion of modest quantities of sodium chloride to both droplet and reser voir increases the rate of equilibration for aqueous PEG sitting dropl ets significantly. At 293 K, droplets with initial volumes of 24 mu l and PEG concentrations of 5%(w/v) require 12 weeks to reach equilibriu m, while droplets of the same volume and initial concentrations of 5%( w/v) PEG and 200 mM NaCl require less than two weeks to reach equilibr ium. The slow vapor-diffusion equilibrations of pure PEG solutions, an d the subsequent increase in these rates with colligative agents such as salt, are a consequence of the non-ideality of aqueous PEG solution s. These results are of interest both from a practical and a theoretic al viewpoint. They underscore the importance of kinetic factors in mac romolecular crystal growth, help to explain apparent inconsistencies o f outcome in PEG-mediated crystallizations, and yield another methodol ogy for the optimization of crystal growth conditions, namely the cont rol of the kinetics of equilibration using colligative agents.