William,
it is not clear from your message why you think this is a Mg++, not Mn++
which has been observed in this site before? Mn++ would be visible in
anomalous data from a home source, but not Mg++. The only way to
distinguish Mg++ and water is the number of coordination which should be
very clearly organized around a Mg++. You could propably also tell by
the bond lengts as the Mg-O coordination should be 2.1-2.2, water
hydrohgen bonding around 2.7 Å
Could the atom be Zn++ or Na+ ?
Pirkko Heikinheimo
Martin Hallberg wrote:
Hi Bill,
On Apr 2, 2008, at 10:58 PM, William G. Scott wrote:
I've got what appears to be an inner-sphere interaction between Mg++
and the N7 of a G. The mode of binding is the same as what is
observed at this site for Mn++, confirmed with anomalous data. Our
resolution is 1.6 Å, so I am reasonably confident this is right.
However, my chemist's viewpoint is that Mg++ is too hard and N is too
soft for this to happen.
I would also be hesitant.
Since Mg++ has the same number of electrons as water and no useful
absorbance, it seems assigning them based on hydration geometry and
bond distances is the only hope.
One can often see magnesium ions in anomalous-difference fouriers with
good home-source data. Since you appear to have very good diffraction
and your magnesium is most definitely relatively well ordered (if it
is a magnesium..) I think it is worth a try to carefully collect a
data set using a wavelength around 1.5-1.6 Å (or simply your home
source depending on how good it is).
Cheers,
Martin
.
B. Martin Hallberg, PhD
Assistant professor
Department of Cell and Molecular Biology
Medical Nobel Institute
Karolinska Institutet
Nobels väg 3
SE-171 77 Stockholm
Sweden
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Pirkko Heikinheimo
Structural Biology and Biophysics,
Institute of Biotechnology,
P. O. Box 65,
FIN-00014 University of Helsinki, Finland
Visit address:
Biocenter 3, room 4320
Viikinkaari 3, 00790 Helsinki, Finland
http://www.biocenter.helsinki.fi/bi/xray/pirkko
e-mail: [EMAIL PROTECTED]
phone: 358-(0)9-191 58957
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