Dear Yang Li, Cryoprotection of crystals is not an exact predictive science - you have to try a number of things, and in the worst case scenario, none of the stuff you try may work out.
Assuming that your crystals are OK to start with (capillary mounted room temp. pattern would confirm that), you can explore a whole wide range of cryoprotectants. I am slightly surprised that you need one at all - at 20% isopropanol together with 20% PEG you have 40% organic content which is usually enough for glassification. You may want to explore the actual cryo-mounting method - there are several options for flash freezing and they're not all the same in terms of results. Assuming that you do really need cryoprotectant, you may want to experiment with addition of EG or glycerol, etc. right into the mother liquor when you set things up - this way, if the crystals grow they're already protected and ready to go. If you can somehow get rid of isopropanol, your life would be easier as well because this alcohol is very volatile which can cause no end of trouble when working with crystals in the open air (among other things, they tend to 'zoom' around the drop as the surface of the drop is pulled around by fluxes of evaporating iPrOH. There are numerous other things you can try, including the one mentioned here: http://www.xtals.org/ under 'stuff'. Regards, Artem > Hi, > l have a crystal grow at condition screen l 40: > 0.1M tri-sodium citrate dihydrate pH5.6 isoproponal 20%PEG4k 20% > and the crystal need a cryoprotectant, we have used the 30% glycerol but > it > is not good, the mosaicity of > the diffraction pattern is a little high, so anyone knows which is the > best > cryoprotectant for this crystal? > Thanks! >
