A water mop

There are nuances in terminology, culture, and default assumptions among different branches of science. What appears natural in one area often goes against the grain and intuition in another, particularly when it is difficult to put forth well justified arguments at a molecular level.

In the past, I commented on the peculiar properties of PEG (polyethylene glycol). In my discussions with colleagues in protein crystallography, I have always found it odd to hear references to PEG as the “dehydrating agent”. It is especially uncomfortable when PEG is administered in an aqueous buffer to a vessel containing protein crystals. But our biological colleagues have no trouble with this concept at all. Below is a reference to the propensity of PEG to exert “extreme dehydration” on protein crystals. I used this paper in my talk (IRTG conference here in Toronto) earlier this week. While no one has issues with heterogeneous molecular sieves as water mops, many a chemist cringe at the thought of dehydration in water, given the homogeneous nature of one of the parties to the interaction. While this sounds like an oxymoron, dehydration of this sort is something protein crystallographers are rather comfortable with. In the paper below, PEG-driven dehydration is called upon to drastically alter the quality of protein crystals.

mop.jpg

http://scripts.iucr.org/cgi-bin/paper?S1744309111048706

 

2 thoughts on “A water mop

  1. PEGs are not a dehydratation agent, but you can make highly concentrated solutions of them (high osmolarity) without perturbing much the structure of liquid water – if you were to use comparable concentration of a salt additive i.e. ammonium sulfate, you would have real dehydratation and protein would precipitate out.

    Also, they used 60% solution of 3.5kDa PEG. At this Mw and concentration, the solution becomes viscous. Using high viscosity media is a trick for slowing down the crystallization rate – you get bigger crystals that way

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