Hi Matt, Have you tried changing the pH? Is it possible that at pH 8 you are at the pI of your protein (i.e. where it has zero net charge and is at its least soluble) ?
I have also read a paper (can't find the reference right now) where the authors precipitated their protein by dialysing into water, and then aliquoted the suspended precipitate (suitable amounts to allow concentration measurement by OD280 - if you've got a nanodrop or similar, you only need a few ul). The precipitate was pelleted by centrifugation, and the water was removed. A series of different buffer types / salt types / strengths were added to each aliquot to resolubilise the protein. The tubes were briefly vortexed and left at room temp for 30mins(?) any remaining precipitate was pelleted again, and the OD280 of the supernatant measured. The buffer with the highest OD in solution allowed them to reach higher concentrations of protein for crystal screening. Hope this makes sense - contact me if you need more info, Dave ============================ David C. Briggs PhD Father, Structural Biologist and Sceptic ============================ University of Manchester E-mail: david.c.bri...@manchester.ac.uk ============================ http://manchester.academia.edu/DavidBriggs (v.sensible) http://xtaldave.wordpress.com/ (sensible) http://xtaldave.posterous.com/ (less sensible) Twitter: @xtaldave Skype: DocDCB ============================ On 12 August 2010 19:57, Matthew Bratkowski <mab...@cornell.edu> wrote: > Hi. > > I am working with a protein that has difficulties staying in solution when > concentrated. The buffers that I have been using contain 20 mM Tris pH 8.0, > 5 mM BME (or 2 mM DTT), 10% Glycerol, and NaCl from 50 mM up to 2 M. The > protein seems fine to dialyze into low salt buffer (50 mM NaCl) when dilute, > but seems to precipitate onto an anion exchange column during loading, as > noticed by brown spots on the column and a reduction in yield after the > run. Despite these issues, I have been able to concentrate the protein to > about 18 mg/mL after removing glycerol and using 200 mM NaCl and 2 mM DTT. > However, after sitting at 4 C for a few hours, I again notice precipitate in > the concentrated stock. I figure that I will probably just work with more > diluted protein next time, but was concerned that my protein was not stable > enough in this buffer to use for crystallization. > > I am seeking advice on finding a better buffer for the protein. Can anyone > suggest whether making subtle buffer changes, such as using HEPES instead of > Tris or KCl instead of NaCl would be advantageous? Additionally, I was > interested I find out what concentration of glycerol or NaCl could still be > added for the protein to still be usable for crystallization, as well as any > additives that will prevent precipitation but not interfere with > crystallization. Also, could someone suggest a good way to test protein > stability in various buffers on a small scale before deciding on conditions > for a large batch. > > Thanks, > Matt > > >
