Hi Colin
yes, both of those. Plus "freezing out" multiple conformations so you
can model them properly - bear in mind that a small molecule
structure at a resolution worse than 1Å would be challenging to get
past the normal criteria of referees, so you should be able to see
them at least some of the time (depending on how distinct the
multiple conformations are).
It may be easier to distinguish the atom type (C,N,O?) if you have a
lower temperature dataset - at least 50% of the small molecule
structures I did when I was working in that field were _not_ of the
compound the chemist thought they had, so working out the atom type
was rather important. However, I'm not sure this is a strong reason
for doing cryo work on its own - any half decent modern automated
small molecule structure solution program could do it with most room
temperature datasets.
Also, if you have an "air-sensitive" crystal (chemists say this as a
shorthand when they mean more specific things like the chemical is
oxygen or moisture-sensitive) you can slow the reactions down enough
so that your crystal doesn't decompose because of those. But that's
more of a problem with organometallics than organics, except with
some of the more exotic organic species...
And if your crystal loses solvent (e.g. I used to use CH2Cl2
(methylene chloride to the old hacks here) as a solvent, and that
will just evaporate at room temp, leaving you with a powder rather
than a beautiful single crystal.
Although you can get round air-sensitivity and solvent loss by
mounting in a capillary, there are good reasons to avoid that and
cryo-cool with a naked (or semi-naked, dressed only in a gossamer-
like film of perfluoropolyether oil) crystal if you can, as those of
us who have done both regularly know.
I'm sure there are other reasons why it would be substantially
better, but I also know that there is considerable effort going into
high-temperature devices, which will be used to help collect
"substantially better" datasets for the crystals that their
developers are interested in.
does this help?
On 19 Jun 2008, at 10:25, Nave, C (Colin) wrote:
Harry
Can you clarify why you get "a substantially better structure at
cryo temperatures"
e.g higher intensity at high resolution due to reduction in B
factors, reduction in radiation damage, anything else?
Colin
-----Original Message-----
From: CCP4 bulletin board [mailto:[EMAIL PROTECTED] On Behalf
Of harry powell
Sent: 19 June 2008 10:12
To: CCP4BB@JISCMAIL.AC.UK
Subject: Re: [ccp4bb] is it Ok to freeze
Hi
If you mean organic small molecules, then the opinion for the last
15 years at least is probably "yes, unless you know you'll have a
phase change".
Most small molecule crystals don't have the same problems with
needing cryoprotectants as macromolecules, due in large part to not
having a large proportion of water in the lattice, so the process
is somewhat more straightforward. Also, most small molecule
crystals can be handled quite happily in the absence of mother
liquor, and you don't have to worry about them drying out while
transferring to the fibre (rather than loop) which would normally
be used for mounting them. Of course, there are numerous exceptions
to the "most" I'm referring to here.
In most cases you'll get a substantially better structure at cryo
temperatures (of course, what "better" means may be open to debate).
On 19 Jun 2008, at 09:47, Jayashankar wrote:
Dear Scientists and Friends,
I am not sure, whether organic crystals need to be in cryo
stream necessarily during data collection from an in house
xray machine .
How most of the organic crystals have been solved mostly?
--
S.Jayashankar
(A bit confused new generation researcher).
Research Student
Institute for Biophysical Chemistry
Hannover Medical School
Germany
Harry
--
Dr Harry Powell, MRC Laboratory of Molecular Biology, MRC Centre,
Hills Road, Cambridge, CB2 2QH
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