A protein can crystallize in different ways with different crystallization buffers, yielding crystals of different shapes.

Dear Zaiddodine Pashandi,

Yes, it can happen, because the shape of the crystals depend of some factors, such as:

- What is the solvent type, time of the solvent evaporation (slowly or fast), the surface where the crystals going to grow, the temperature and pressure, among other.

But, you must have in mind that these macro different shapes do not mean, necessarily, different internal patterns. For instance, you can obtain plate like, rod or triangular star like, but all these can be crystallized as monoclinic system.

Best regards

I assume that your "several" screening buffers amounts to potentially 100's of crystallization experiments. This is not impossible, and is often a nice indication that the crystal are in fact protein. How long have the experiments been incubating? Do each of the conditions that yielded xtals have similar reagents?

If you have access to a UV microscope you can see if there are any tryptophans present in the crystal. If you have a home source and the in situ plate adapter you can directly shoot the crystal in the plate and look for reflections...

The first thing that I do when I find crystal in a crystallization set-up (and don't have the aforementioned tools) is check the reservoir solution (in hanging-drop vapour diffusion) to see if the same crystal morphology is present. IF the same crystal morphology is present in the reservoir solution then you are probably seeing salts. Next, I check the crystallization buffer recipe. Often screens contain some conditions that have 1-4 M phosphates, sulfates, NaCl, etc. These guys are very much likely to crystallize over time. You can also check the crystallization condition stock tube at RT or 4C to see if the same crystal morphology is present.

Are the crystals distributed throughout the drop? Was the experiment microbatch under oil, sitting-drop, hanging-drop? If you have loads of crystals the next step is to try and crush a crystal with a cat hair or the hampton microtool set (I do not work for Hampton, but that tool kit is a life saver). A protein crystal is fairly easy to crush and usually is quiet when it breaks. A salt crystal is much more resistant to the crushing force and will often make an audible sound once crushed. You can also transfer a crystal or two to a watch glass and add some dye to see if the crystal has water channels that can accept bulky dyes. If the crystal takes up dye it is probably a protein crystal.

Next step, you have a crushable crystal that can take up dye, transfer a crystal or two into the mother liquor supplemented with 1-5% 2r-3r-butanediol (only keep the xtal in the butanediol solution for a max of 30 sec), or stepwise increase of glycerol (10%, 15%, 20%, 25%) for cryopreservation and loop the crystal. Mount on your home source. Collect a couple frames once centered in the beam and check for low resolution reflection (a salt crystal is not going to have low resolution reflections).

In short, yes. A protein can crystallize in different conditions with crystals of varying shapes & sizes with same or different space groups. 

Carter's suggestions are all excellent for trying to determine if your xtal is protein or salt, especially in the absence of extra equipment (e.g. home source or UV microscope).

I will also add that if the crystals are large enough or easily extracted, you can sequentially 'wash' them in mother liquor drops (to remove protein still in solution) and then dissolve them in a drop of water.  Run this sample on SDS-PAGE.  You should see a band of similar size that you started with.  Coomassie stain should be sufficient if your xtals are large enough, but silver stain would also be an option for shards, small needles, etc.