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[ccp4bb]: Ca2+ or Na+ ? at 2.3-2.8A bonding distances



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I think the similar topic has been discussed on this bulletin board. But I
just want to get more comments on how to distinguish between Ca2+ and Na+?

I found a cation binding site in a pentagonal pyramidal arrangement.
A quite weak resdiual positve peak can be seen on the opposite side, which
could be another water molecule forming a pentagonal bipyramidal
coordination. The distances are in a range of 2.3~2.8A with the average
around 2.45A, thereby K+ was excluded due to its coordination distance of
around 2.8A. The only possibility then fall down on Na+ or Ca2+ present in
the crystallization buffer. The concentration of Na+ was much higher than
Ca2+. Thus I think it is more likely to be a Na+. But is there any other
arguments to prove this? I don't think they can be distinguished by bond
distances, because in both cases they are around 2.4A. Or by anomalous
difference map? The data was collected at 13466ev (0.92973A). I don't see
much anomalous difference on both elements at this wavelength.

Thanks in advance.     

regards,
Q. Steven Xu
Dept. of Physiology and Biophysics
Boston Univ. School of Medicine
Tel: (617)638-4083
Fax: (617)638-4041
E-mail: qian@med-biophg.bu.edu