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| K is for Kosmotrope |
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| Kosmotropes (order-making) or lyotropes are ions that display strong interactions with water and generally stabilize proteins. This stabilization effect is brought about by increasing the order of water and increasing its surface tension.
Examples of kosmotropes include, but are not limited to, sodium citrate, sodium sulfate, trimethylamine N-oxide, proline, ectoine, trehalose, glycine betaine, 3-dimethylsulfoniopropionate, ammonium sulfate, lithium sulfate, sodium phosphate, and magnesium chloride. |
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| K is for Krypton |
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| Named from the Greek kryptos or “hidden”, krypton is neither green, nor a solid material that can defeat Superman. Rather it is another noble gas discovered in 1898 by Ramsay and Travers. It ranks sixth in abundance in the atmosphere. Krypton gas is used in various kinds of lights, from small, bright, flashlight bulbs to special strobe lights for airport runways. As with the other noble gases, krypton is isolated from the air by liquefaction.
Krypton and crystals. The use of xenon and krypton at high pressure is becoming a popular method for the phasing of proteins. There are several homemade as well as commercially available pressure cells for creating xenon and krypton derivatives.
In essence, the crystal is exposed to the krypton or xenon gas at high pressures for a short period of time in order to allow the krypton or xenon to bind to the protein within the crystal. Following incubation and depressurization, the crystal is flash cooled in liquid nitrogen and mounted onto a goniometer in a cryostream for X-ray diffraction analysis. Freezing of the crystal is necessary to prevent the diffusion, release and loss of gaseous krypton or xenon. As a side note, methods of generating and analyzing xenon derivatives in glass or quartz capillaries have been described (see references below).
Krypton and xenon binding sites are generally different from those for other heavy atom sites so the screening of krypton and xenon can be used as a follow up when the initial soaks in heavy atoms are not successful.
When the specialized hardware is available for screening krypton and xenon under high pressure, the method is a convenient and rapid way for screening successful derivatives for phasing.
References:
• High-pressure krypton gas and statistical heavy-atom refinement: a successful combination of tools for macromolecular structure determination. Schiltz, M., Shepard, W., Fourme, R., Prange, T., de La Fortelle, E. and Bricogne, G. Acta Cryst. D53: 78-92, 1997
• Exploring hydrophobic cavities in proteins using xenon and krypton noble gas. Prange,T., Schiltz, M., Pernot, L., Colloc’h, N., Longhi, S., Bourguet, W. and Fourme, R. Protein Struct. Funct. and Genetics 30(1): 61-73, 1998.
• Solubility of krypton and xenon in blood, protein solutions, and tissue homogenates. Yeh, S.Y., Peterson, R.E. J Appl Physiol 5: 1041-1047, 1965.
• Flash freezing isomorphous xenon or krypton derivatives of protein crystals. Sauer, O., Dutzler, R and Kratky, C. ECM 17, Seventeenth European Crystallographic Meeting (Lisboa, Portugal 24/28 aug.1997). Book of Abstracts p 18 (ref. MS1.6-4)
• Xenon and Krypton at LURE http://www.lure.u-psud.fr/sections/Xenon/XENON.HTM
• Tutorial for Krypton-Elastase SIRAS refinement http://utica.med.jhmi.edu/sharp/tutorials/KrEl.html |
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