On Thu, 2011-07-28 at 05:07 +0100, Sean Seaver wrote: > Spoiler - Fabs like ammonium sulfate.
Not really - in my hands the ammonium sulfate was one hit out of 7. While Ivan's question is about Fab complexes with protein antigen, I think it brings up a more general question of protein class-dependent crystallization bias. While some general trends exist for classes of biopolymers (e.g. MPD is number one precipitant for DNA; protein:DNA complexes tend to crystallize in PEG-based conditions), a general idea of assigning a preferred precipitant to a protein class is, imho, pointless. Fabs are a good example - one would think that with half of the protein more or less the same in all instances some general trends should exist. And perhaps they do, as this http://scripts.iucr.org/cgi-bin/paper?S0907444999016224 seems to suggest. But alas, Fab crystallization conditions, once you look into it, appear to be just as diverse as the same for proteins in general. Crystallization conditions may change radically upon point mutation, so why would one expect that a class of proteins sharing some 50% identity will show unusual love for PEG, ammonium sulfate, sodium malonate or any other "miracle precipitant"? Consider this. Thanks to great engineering at the Douglas Instruments, we can routinely set up ~1000 drops for a given protein. If one of them shows a crystalline shower, we celebrate. To me, the fact that we try wrong crystallization conditions 99.9% of the time, proves that any attempt to predict crystallization conditions beyond vague things like "keep pH close to protein pI", "sodium malonate is cool", "PEG and ammonium sulfate are two most successful precipitants in history of protein crystallography", etc., is futile. Time wasted on looking into what is the most common precipitant for a particular class of proteins is better spent on setting up more trays. Cheers, Ed. -- Oh, suddenly throwing a giraffe into a volcano to make water is crazy? Julian, King of Lemurs
