Dear CCP4BBers,
I'm refining the structure of a Ca-binding protein with several EF-hands
against 2.2A data.
There is clear density for an ion in several EF-hands coordinated by
Asp/Glu, Ser, one backbone O and one water (coordination number 5 and
6). Ca2+ can be excluded as there are no anomalous difference peaks at
these sites when I calculate a map from data collected at 2A wavelength.
I suppose that either Na+ or Mg2+ are bound.
I'd like to ask whether there is a clear way to distinguish between both
ions in a model from data at this resolution.
Thank you in advance for your suggestions,
Florian
P.S. The distances between ion and protein are:
Asp/Glu carboxyl O: 2.04-2.75A
Ser OH: 1.98-2.6A
Backbone O: 2.4-2.65A
