In the search for macromolecular crystallization conditions, the precipitant is probably the most important variable, such that when problematic crystals are encountered there is always the question of whether an alternative precipitant might resolve the problem. During an effort to obtain high-quality crystals of several problematic
proteins, two new agents,
pentaerythritol propoxylate and
pentaerythritol ethoxylate, yielded well ordered quality crystals where more traditional precipitants were unsuccessful.
Pentaerythritol propoxylate and
pentaerythritol ethoxylate contain a
pentaerythritol backbone to which organic
polymers are bound, forming a branched
polymer. As such, they are larger than small organic precipitants such as low molecular-weight
alcohols or
2-methyl-2,4-pentanediol, but behave differently to
polyethylene glycols. These compounds have been used to crystallize an
enzyme encoded by the Salmonella enterica prpD gene that catalyzes the
dehydration of
2-methylcitrate to form 2-methyl-cis-aconitate. While the PrpD
protein has crystallized readily under a number of conditions, the resultant crystals were unsuitable for a crystal structure determination. The new crystals obtained with 25-40%
pentaerythritol propoxylate belong to the orthorhombic space group C222(1), with unit-cell parameters a = 73.2, b = 216.4, c = 214.3 A, and diffract beyond 2.0 A with
synchrotron radiation. A further benefit of this precipitant for crystallization is its ability to function as a cryoprotectant, allowing the crystals to be transferred directly from the mother liquor to the
nitrogen stream at 113 K.