Density and electron density of aqueous cryoprotectant solutions at cryogenic temperatures for optimized cryoprotection and diffraction contrast

The glass-phase densities at T = 77 K of aqueous solutions of the common cryoprotective agents (CPAs) methanol, ethanol, 2-propanol, glycerol, 2-methyl-2,4-pentanediol (MPD), ethylene glycol, polyethylene glycol 200 and polypropylene glycol 425 were measured as a function of CPA concentration. Individual drops with volumes as small as ∼65 pl were rapidly cooled to achieve the glass phase, and their densities at T = 77 K were determined by cryoflotation. These densities were used to determine the glass-phase electron density of each solution and its volume thermal contraction between room temperature and 77 K. When combined with data for the critical cooling rates required to achieve the glass phase versus CPA concentration, these yield alternative measures of cryoprotectant effectiveness. These reference data will aid in minimizing sample stresses and mechanical damage in cryocrystallography, in cryogenic temperature X-ray imaging and in vitrification-based cryopreservation protocols, and in maximizing electron-density contrast between cryoprotectant solutions and biomolecules in cryogenic temperature small-angle X-ray scattering experiments and cryo-electron microscopy.