Dear Samy, You mentioned that your enzyme crystallizes in 0.1M citric acid pH3.5 plus 25% PEG 3,350. I wonder if you have tried to systematically scan for other carboxylic acids as buffer/co-precipitating salts. I would suggest you to try the serie of carboxylic acids, citrate, acetate, formate and also the magic carboxylate malonate; a little bit like the Hofmeister series. You maybe able to find a surrogate to citrate that will able you to either soak your crystals or co-crystallize successfully with your substrate(s).
Hope this helps Cheers, Pascal Egea University of California San Francisco Department of Biochemistry and Biophysics
