by Peter Nollert
June 26, 2009 13:53
Thinking ahead and setting some protein aside for optimization screening? Good idea, but how do you store this precious liquid? Keeping it 'on the bench' may be a bad idea. Even a minor contamination of the sample with proteases can wreak havroc with the target protein in a matter of days or weeks. While cooling on ice will slow down proteolytic fragmentation quite a bit it will not prevent it. So you want to freeze the protein for long term storage. And of course the protein sample ought not to be modified by addition of cryoprotectants such as glycerol. So - what's the best practice for storing protein samples?
Freezing as drops dripping in liquid nitrogen and harvesting the 'pearls' is the method of choice in many labs. Drop freezing has its issues though, so Deng et al. (An improved protocol for rapid freezing of protein samples for long-term storage. Acta Crystallogr D Biol Crystallogr. (2004) 60:203-204) researched this topic systematically and came up with these recommendations:
- Use thin-walled PCR tubes for shock-freezing - PCR plates work as well
- Fill with volumes less than 50 ul / tube; for example 20 ul of protein sample in each tube
- Thaw fast: thaw in 'the investigator's hand'
Take note that thawing seems to be just as important to get right as the cooling process. Not *on ice* but in 'the investigator's hand'.
(I don't know why, but the latter somewhat reminds me of that 189X chemistry paper that stated [translated from German] "... the laboratory servant shall stir the mixture until he is exhausted". We've come a long way...)
Cheers,
Peter