Preparation of density gradient solutions

The observed osmolality of OptiPrep depends on the mode of measurement (vapour pressure or freezing point); moreover the situation is complicated by the tendency of the iodixanol molecules to associate non-covalently in a concentrated aqueous solution. 

Measured values for its osmolality are thus lower than might be expected. Importantly however, when OptiPrep is diluted with a buffered isoosmotic solution, the iodixanol oligomers dissociate and all dilutions are isoosmotic.

 Under normal operating conditions therefore OptiPrep behaves as if it had an osmolality of approx 290 mOsm. 4. Preparation of density solutions Traditionally viruses have been purified in gradients containing high concentrations of sucrose, glycerol or CsCl. The particles have therefore been isolated in grossly hyperosmotic conditions. 

OptiPrep offers the opportunity to isolate them under isoosmotic conditions. In many instances the density of a virus in iodixanol will be considerably lower than that in CsCl and slightly lower than that in sucrose or glycerol. Commonly the solutions used to suspend viruses are phosphate-, Tris- or HEPES-buffered buffered saline (or 0.25 M sucrose). The solutions may contain low concentrations of additives such as EDTA (1 mM), KCl (2.5 mM) or MgCl2 (1 mM).