Re: [ccp4bb]: osmolality values of cryoprotectants

[Edward Berry <eaberry@lbl.gov>]

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> Tommy Wang wrote:
> 
> Does anyone know where I may find osmolality values for the commonly used crystallization agents and cryoprotectants? I
> checked the CRC handbook of chemistry and physics and also searched the web to no avail. If such a table is unavailable, does
> anyone know how to derive the value from freezing point depression?
> 
> Thanks in advance
> Tommy
> 

The molality is equal to the freezing point depression in degrees C divided by 1.86,
if I'm not mistaken.

The depressed freezing point is taken as the point where the solvent *begins* 
to crystallize: as freezing progresses the molality gets higher and higher and the 
temperature gets lower and lower. Be careful to avoid supercooling. If you
cool slowly while recording temperature with a strip chart recorder, 
it should go a little below the freezing point, jump up to the freezing point,
then slowly go down as the osmolarity increases. Extrapolate back to the 
intersection with the initial cooling curve to get the initial freezing point.

A slower but perhaps more accurate way would be to perform a vapor diffusion
experiment, allowing a droplet containing a known amount of the solute
to equilibrate against a much larger reservoir of known osmolarity, and
measuring the volume or weight of the droplet at equilibrium. Then assume
the osmolarity of droplet and reservoir are the same, calculate concentration
from the final volume to get the osmotic coefficient. The kinetics of the 
equilibrium would be of interest to crystallographers, as well as the 
final point.

If you get data for various PEGS by either method I would be interested in
the results. Ideally we could calculate the concentration of a cheap solute
to use in the reservoir rather than our precious screening solutions.
And if we know that a droplet of our geometry takes 4 weeks to equilibrate 
but we get crystals overnight, we can reallize we are essentially 
crystallizing by the "batch" method not by vapor diffusion.

Ed