>> in principle, you could use 'floating chirality' (i.e., unrestrained improper
>> of the chiral carbon) but this is likely to lead to distorted geometry (i.e.,
>> an improper that lies somewhere in between +35 and -35 degrees).
>
> That would be true were it not for the cleverness of whoever set up
> the sugar restraint system for refmac. The syntax allows you to set the
> chirality restraint on the anomeric sugar to "both". This restrains to the
> correct absolute value of the chiral volume, thus accepting either anomer.
the question didn't include any hint as to which software is used (other than
that it was asked on ccp4bb), so the solution i suggested will work in any
program (provided you can make the two molecules 'not see each other' in terms
of van der waals and any other distance-dependent interactions)
moreover, afaik, upon deposition you'll have to generate the two isomers
explicitly anyway, since they are chemically different entities (as in the
example of alpha and beta glucose)
also, depending on the make-up of the binding site, i would suspect that quite
often two stereo-isomers are "bound" to be bound in different orientations
(since stereo-selectivity is a hallmark of many biological molecules and
processes), making the option with two different molecules a better
representation of what's in your crystal
finally, although admittedly a debatable issue, i consider it preferable, both
philosophically and educationally, to treat different chemicals as different
entities
--dvd
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Gerard J. Kleywegt
[Research Fellow of the Royal Swedish Academy of Sciences]
Dept. of Cell & Molecular Biology University of Uppsala
Biomedical Centre Box 596
SE-751 24 Uppsala SWEDEN
http://xray.bmc.uu.se/gerard/ mailto:[log in to unmask]
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The opinions in this message are fictional. Any similarity
to actual opinions, living or dead, is purely coincidental.
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