[Ifeffit] Problems with EXAFS Fitting of Metalloprotein Zinc Samples

[Carolyn Carr]

Hi,

I have a general question rather than specific. I have only ever fit  
XAS data on metalloproteins and one feature that I see is that when  
fitting Zinc samples in R-space there is no local minima. For Cobalt  
and Nickel samples I typically get one or a few fits that are  
obviously significantly better in R-factor (with reasonable distances,  
sigma^2, etc) but for Zinc this is not the case.

I can get many good fits and Zinc likes to increase the coordination  
up to 8 for all data sets I have ever fit, although there is obviously  
no physical basis in this. This is true of not just my own proteins  
but Zinc samples made by collaborators, and after looking through  
previous group members fit tables, they had similar issues.

My understanding is that one of the benefits of fitting in R-space is  
that there is a local minima, whereas in k-space there are many  
minima. I was wondering if there was a physical basis for this feature  
in Zinc samples, or if perhaps my group is not aware of some  
experimental setup that we should be doing for Zinc that would resolve  
this problem.

Thank you for any help regarding this matter,

Carolyn Carr