Your mother liquor is already very close to being a cryoprotectant. I think adding 5% ethylene glycol, glycerol, etc. is enough. Too much may damage your crystals. In addition, you can also try:
1. Grow new crystals in your current crystallization condition + 5% cryoprotectant. 2. Increase the PEG in your reservoir to 40% and allow vapor diffusion to gently dehydrate your crystals to allow direct freezing. ho UC Berkeley
