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RE: [ccp4bb]: Re: Xenon at high pressure
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Hi There,
The Xe cylinder is supplied at around 50 atmosphere, whatever that
translates to in terms of psi. The regulator allows a maximum of 25
atmosphere, but one does not have to use all that pressure.
In practice, if you are using quartz capillaries at room temperature, it is
not clever to go any higher up than 20 bar. It depends on how sturdy the
capillary is, and that is why you need to use quartz, not the the usual
X-ray glass capillaries. They explode under a slight rise in pressure.
The Xcell or equivalent commercial products would allow the use of pressure
as far as the cylinder/regulator let through. In this case, the limitation
is how quickly can the experimentalist take the loop, with the crystal in,
out of the pressure cell and into the cryostream. People quote a diffusion
time of a few minutes for Xe through the crystal lattice, i.e. residence
time in the pressure cell makes no difference beyond 15 minutes. The
important thing is that in the few seconds it takes to transfer the loop,
the pressure will have been released. There is no way one can tell how much
Xe is left in the lattice (unless you do some fancy spectroscopy). It
obviously depends on the lattice in hand. Proteins that bind non-polars are
naturally prediposed to trapping Xe, like the crustacyanin. Those with no
such binding sites, can still trap Xe at non-obvious places. So one can
discover where all the hydrophobic patches are on the protein surface. If
these patches are genuinely absent, then maintaining the pressure at RT
might be the answer.
As for the suitability of Xe as a derivative, it has a massive 7e anomalous
signal at a wavelength around 1.5 A. The low affinity and natural disorder
in a poor vdw interaction may reduce the usable signal, but it could still
be as significant as the 3.7e signal from Se at its correct edge. And with
synchrotrons, one can use 2.0 A wavelength, where the signal becomes 11e. I
have used Xe with 3 different proteins so far, at wavelengths of 1.488 or
2.0 A. Only crustacyanin had an unknown structure, and now it is solved. The
other 2 gave easily interpretable pattersons, and they will be assessed
properly in the fullness of time. So I claim 100% hit rate, but I'll
probably fail with the next one. Such is PX!
I hope I have answered all your questions, but please come back if there are
more points to discuss. Cheers, and Happy Pressurising.
Pierre Rizkallah
-----Original Message-----
From: ernst@UTBC08.CM.UTEXAS.EDU [mailto:ernst@UTBC08.CM.UTEXAS.EDU]
Sent: 22 May 2001 20:26
To: CCP4BB@dl.ac.uk
Subject: [ccp4bb]: Re: Xenon at high pressure
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how deep is the ocean, how high is high?
we get to about 400-420 psi with the regulator on our lab cylinder and
have had one fair derivative out of about 6 trials (3 different proteins).
pressurized time 10-45 minutes.
gordon webster reported:
Pierre Rizkallah said that he and his colleagues had found Xenon at high
pressure to be an excellent
choice, their results for this work on the structure of crustacyanin will
appear very soon in Acta D.
Steve Ernst