NH4+ vs. H2O

[Adrian Goldman, BTK x8029]

> 
> 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

Adrian Goldman                         |  Internet:  Goldman at Ala.BTK.UTu.Fi
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