Hi Patrick -
Just a thought: in a drop that's still equilibrating (i.e. one that's actively nucleating/growing crystals), there will be convection currents, caused by water diffusion out of the drop, protein switching from solution to solid phase, temperature differences, etc. These currents could potentially carry micro-crystals with them until they get too big. Another possibility: crystals are usually denser than their mother liquor, and so tend to end up at the bottom of the drop. The motion seen in the movies could be caused by the micro-crystal nucleating at some random position in the drop, then dropping to the bottom (or sliding down the curved lower face of a hanging drop). I haven't seen this movie myself, so I don't know if this is a reasonable explanation of the motion. - Matt -- Matthew Franklin, Ph. D. Senior Research Scientist New York Structural Biology Center 89 Convent Avenue, New York, NY 10027 (646) 275-7165 _____ From: CCP4 bulletin board [mailto:CCP4BB@JISCMAIL.AC.UK] On Behalf Of Patrick Shaw Stewart Sent: Monday, February 14, 2011 5:18 PM To: CCP4BB@JISCMAIL.AC.UK Subject: [ccp4bb] Crystals that "fly" around a drop until they land and grow. A few years ago someone posted on this bb a link to a time-lapse video of crystals growing. There have been a few of these, but in this particular video you could see very small particles that shot across the screen in straight lines or smooth curves before sticking and growing into obvious crystals. They moved rapidly (a minute or so in real time) right across the field of view. The person who posted it commented that they would love to know what was going on with these particles or words to that effect. Does anyone know of or remember a video that shows this effect? I'm asking because this video always puzzled me and a few days ago a possible mechanism occurred to me. Could it be that due to some instability a mini crystal starts to move a little, leaving a "trail" behind it of depleted protein? Now, is it possible that the higher concentration of protein in front of the crystal causes it to grow faster on that side, so causing it to move forward into the region with higher protein concentration? I have a picture in my mind of a particle being bombarded on all sides by water, precipitant etc, but these particles bounce off again, whereas the protein molecules stick. The momentum picked up by the crystal on each collision is roughly twice the momentum of the particle hitting it in all cases except when a molecule hits and sticks. Then momentum transferred is only one times the momentum of the particle hitting it. The net effect is that the crystal moves in the direction where there is more protein, i.e. forwards. I guess the question is, could the effect be big enough to make a small crystal move more or less in a straight line? I imagine it as a bit like throwing ball bearings at a football on one side, but throwing sticky chewing gum at it (with equal energy) on the other side. The crystal moves around the drop until it becomes so heavy that it stops, a bit like kids rolling snowballs in the park until they can't push them any further. I suggested this mechanism to my colleague here who is a physicist. He was sceptical. Has anyone got any comments? The question has practical implications for crystallization. For example if you add seed crystals to one end of a free interface diffusion experiment (say in a capillary) could micro-crystals buzz up to the far end? Or could a stationary crystal act as a pump, pulling solution past it? Has anything similar been seen with small molecule crystals or crystals in space? Patrick CCP4BB@JISCMAIL.AC.UK - -- patr...@douglas.co.uk Douglas Instruments Ltd. DouglasHouse, EastGarston, Hungerford, Berkshire, RG177HD, UK Directors: Peter Baldock, Patrick Shaw Stewart http://www.douglas.co.uk Tel: 44 (0) 148-864-9090 US toll-free 1-877-225-2034 Regd. England 2177994, VAT Reg. GB 480 7371 36
