I don't think there is such a rule, but in the old days, when we only had Hampton Screen I and II, the rule was:
- Set up screen 1, look at the drops and you should expect some kind of precipitation in 50% of the drops. If much less than that, increase your protein concentration. If much more than that, decrease protein concentration. - Set up screen 2, look and expect 30% precipitation. I used to cut corners and do the statistics at 1/2 of a screen (one 24-well plate). You can probably use this method to get within a factor of 2 of the optimal concentration. There are probably good statistics in the papers for the screens that you may use. One of the advantages of structural genomics efforts is that these things are known (and hopefully published). Even older trick is to take a drop of protein and look under a microscope, record how much AmSO4 it takes to cause precipitation. Do the same with PEG. Keep adding a little at a time and look immediately. This will give you an idea if you are near a reasonable concentration. I think that this latter method does not tell you much more than "physics-information" - which is how many zeroes there are: whether 1 mg/ml or 10mg/ml or 100mg/ml is reasonable. Mark -----Original Message----- From: james09 pruza <james09x...@gmail.com> To: CCP4BB <CCP4BB@JISCMAIL.AC.UK> Sent: Thu, Jul 19, 2012 1:59 pm Subject: [ccp4bb] Protein concentration vs Molecular wt... Dear Crystallographers, Is there any rule of thumb for Protein concentration and molecular weight for crystallization trials of a soluble protein? Looking for high molecular wt. protein ~50kDa. James.
