RE: [ccp4bb]: SUMMARY: Selenomethionine oxidation during RP-HPLC

[Phraenquex VD <loretta@scripps.edu>]

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> > gives a stronger MAD signal than reduced selenium. The problem is when you
> > get mixed oxidation states, because then you don't get any absorption peak
> > at all.
> 
> Pardon me but do not get that. The Se has a absorption edge, no matter what.
> The stronger 'oxidized' edge is probably an electronic effect at the XANES
> creating an additional component seen as a white line feature (the
> peak above the edge jump level). A different chemical environment shifts
> the edges (up energy when oxidized, few eV) and may lead to a
> superposition and thus broadening of the edge. The no signal theory I
> do not understand?


Of course selenium still has an edge, but that reply stated "mixed
oxidation states", "you don't get" and "peak", not "there is no" and
"edge".  

In my experience the peak (the anomalous signal) is far more important for
MAD than any dispersive signal (which is very small anyway if the edge is
no good), because it allows you to solve the substructure.  In fact, I go
for the peak and remote, and consider the peak a luxury. 

Yes, the edge gets broadened, and so similarly the peak gets flattened,
especially because the oxidised peak sits on top of the reduced edge.  At
least, it's net effect changes that way, but that's what the anomalous
signal is after all. 

Have a look at: 
 AJ Sharff, E Koronakis, B Luisi and V.Koronakis
 Acta Cryst. (2000). D56, 785-788 
 Oxidation of selenomethionine: some MADness in the method!
and
 Smith, J. L. & Thompson, A. (1998). Reactivity of selenomethionine -
 dents in the magic bullet? Structure 6, 815- 819.




> I also do not quite understand what exact species the 'oxidized Selenium'
> or what oxidized Se-Met actually is. Does anyone have some insight into
> that?

Mono-oxygenated (I believe):

        CH3
        |
        Se=O
        |
        CH2
        |
       ...


Cheers
phx.