NH4+ vs. H2O

Adrian Goldman, BTK x8029 GOLDMAN at ALA.BTK.UTU.FI
Mon Apr 18 01:24:04 EST 1994

> Imagine you are interpreting an electron density map for a 
> new protein crystallized from ammonium sulfate.  And you're
> at the stage of adding water molecules.  How do you decide
> whether you have a water, or an ammonium ion close to a
> carboxylate on the protein?  

In my opinion, you don't, unless you can make some positive chemical
argument about what it is an NH4, rather than an H2O (such as 3 or 4
tetrahedrally-arranged hydrogen bond ACCEPTORS).  You'll certainly
never be able to tell from the number of electrons -- unless the
resolution & quality of your maps is a _lot_ better than mine!  (What
are you working on?  Crambin?)

I would be interested to know what others think.


Adrian Goldman                         |  Internet:  Goldman at Ala.BTK.UTu.Fi
Centre for Biotechnology,              |  Bitnet:    AGoldman at Finabo
P. O. Box 123,                         |  Phone:     358-21-6338029   
Turku, SF-20521                        |  Fax:       358-21-6338000   
Finland.                               |                              

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