In our recent experience, larger PEGs work similarly to smaller PEGs as far as vitrification goes. Additionally, transferring crystals to a solution of increased PEG concentration (as compared to mother liquor) can substantially reduce the amount of cryo needed for freezing and can be more gentle on crystals than just using a high concentration of cryo. Here is a reference that tests cryoprotective ability of PEG 2000 & 20000. J. Appl. Cryst. (2006) 39, 244-251
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