[Ifeffit] Beryllium and EXAFS

[Jeff Terry]

To add to what Matt has suggested, If your protein is stable under vacuum, beamline 8-1 at SSRL would be able to do the NEXAFS measurement. You don't need to worry about carbon as it can't reach the carbon edge. 

I left a vacuum flange mounted, low energy fluorescence detector at SSRL many years ago, it may still be around or I can tell you how I made it. It worked very well at the low energy edges or you can use electron yield detection. 

It is fairly easy to get time on 8-1 and worth trying. I would probably try this before exploring x-ray Raman. Of course, I can't say how happy safety people will be with a proposal to put uncontained Beryllium in a vacuum chamber but that is another issue you will have to overcome. 

I'd feel better about x-ray Raman if you could find some standards measured with absorption to compare to the Raman data, just so you could be sure you are not seeing the effects Matt talked about below. 

Jeff

Sent from my iPad

On Dec 18, 2012, at 6:52 AM, Matt Newville <newville at cars.uchicago.edu> wrote:

> Hi Gina,
> 
> On Mon, Dec 17, 2012 at 5:23 PM, Clayton, Gina <ClaytonG at njhealth.org> wrote:
>> Hi there
>> 
>> I am trying to find out more about EXAFS. I am a protein crystallographer.
>> 
>> 
>> I want to identify the environment of a beryllium ion in the protein, I work with,  of which I have a crystal structure.  EXAFS was recommended as a good technique for identification of the beryllium site and species. The beryllium is an acidic pocket of the protein surrounded by oxygens from the protein, likely covalently bound. However I have had  zero result finding a beam line that can tune their setups to the Be edge at 111 eV. Also some EXAFS users say that EXAFS technique would be good for working out the environment of the Be but some say it would not . In particular  I have been advised that  Be EXAFS would not be useful since  the Be  EXAFS signal would have unacceptable interference from the carbon K-edge at 285 eV and the oxygen K edge signal at 535 eV.
>> 
>> Would EXAFS work or something in a similar vein? Or, for instance,  could the acidic pocket be identified using EXAFS, by measuring the oxygens (instead of the Beryllium) using the protein with and without beryllium?
>> 
>> 
>> Thank you so much for any advice.
>> 
>> Gina
> 
> As you and as the person who advised you suspect, you face a number of
> challenges:
> 
> 1.  The extremely low energy (111 eV).  You might be able to find a UV
> source or x-ray beamline that can reach this energy , but often such
> work is done in vacuo, and so may not be suitable for proteins.  Using
> electrons (from a TEM, say) might be possible too -- these can
> sometimes do C and N edges either directly or looking at Electron
> Energy Loss (send in an electron of high energy E1, and look for
> electrons of energy E1 - E, and scan the analyzer for the loss energy
> E through the absorption edge).  Again, electron probes may not be
> what you want for your proteins.
> 
> 2. The C K-edge will limit your signal to "only" 175 eV.  I would say
> to not worry so much about this limitation.  You'll really be
> measuring NEXAFS (near-edge X-ray absorption ....)  but that's OK.
> People measure the B, C, N, and O K edges, all facing the same
> problem, especially for the O K edge: our planet is more or less
> covered with the stuff ;).  These spectra tend to be very rich and
> sensitive to ligands.
> 
> 3. Not much (zero?? I can almost believe that!) literature on Be XAFS.
> This is unfortunate because most NEXAFS is measure and compare to
> known spectra.  I think that might mean you have to spend effort to
> building up a spectral library for Be in organic phases.
> 
> I think trying to measure the O K edge with and without Be is nearly
> hopeless.  All the X-ray spectroscopies average equally over all the
> atoms of the absorbing species.  In addition all the spectra are
> grossly the same (a step function from 0 below the edge to 1 above the
> edge).  These make it difficult to see changes below about 1%.    So I
> think you would just have too high of an O::Be ratio to see anything
> useful.
> 
> One option to consider is using X-ray Raman scattering
> (http://en.wikipedia.org/wiki/X-ray_Raman_scattering).  Here, one hits
> the sample with high-energy X-rays (say, 10 keV), which are highly
> penetrating for reasonably sized protein sample.   One analyzes an
> energy loss (again, E1 - E, where here E1 is around 10 keV) to get the
> edge of interest.  I'm familiar with (been involved with) studies on
> B, C, and O using this technique (all in solids, mostly at high
> pressure).  See, for example,   S. K. Lee, et al "Probing of bonding
> changes in B2O3 glasses at high pressure with inelastic X-ray
> scattering",  Nature Materials 4, pp 851-854 (2005), and also the work
> of Simo Huotari.    I'm not aware of anyone doing Be K-edge this way,
> but it should definitely be possible. For the work I've been involved
> with, Be would be challenging because there are other excitations in
> solids that appear around 100 eV.  I believe that effect would be far
> suppressed in a protein, though I'm not sure.
> 
> That's somewhat more exotic than straightfoward XAFS,, but it might be
> the best chance for measuring Be in a protein.    I'm not sure I know
> any beamlines that do X-ray Raman on proteins, but I'd suggest APS
> beamline 20-ID or ESRF beamline as places to start.    The next
> questions would be "how many Be atoms are in your protein?", and "how
> much material do you have?"....
> 
> Hope that helps,
> 
> --Matt Newville
> 
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