On Tuesday, 30 October 2012, Jrh wrote: > This paper describes use of data either side of the calcium edge:- > > http://dx.doi.org/10.1107/S0907444905002556
I think that counts as "not amenable" (which is not quite the same as "impossible". From the Methods section of that paper: Measurements in the vicinity of the K absorption edge of calcium (3.07 Å) are close to or beyond the physical limit of most beamlines typically used for X-ray crystallography [...] It was not possible to observe interpretable diffraction patterns at λ >= 3 Å with the weakly diffracting furin crystals using the MAR CCD detector and exposure times up to 20 min per degree of rotation. They did soldier on and managed to collect extremely weak data below the Ca edge and stronger but still very weak data above the edge where the Ca f" term was appreciable. But this is far from a routine experiment. Another approach dating back to work in 1972 by Peter Coleman and Brian Matthews http://dx.doi.org/10.1016/0006-291X(72)90750-4 is to replace the Ca with a rare earth having similar chemistry (e.g. La, whose L-1 edge is at 1.98Å). > This next paper describes a case of gallium and zinc mix at > one site with occupancy AND sigmas estimated with different software. > This example is however much better diffraction resolution than > that you may have. But hopefully will still be of interest:- > http://dx.doi.org/10.1107/S0108768110011237 Ga and Zn, sure. That's an easy one. The Ga edge is at 1.196Å and the Zn edge is at 1.284Å, both edges are nicely in range for data collection and they are close enough together that little or no beamline readjustment is needed when jumping from one to the other. Ethan > > Prof John R Helliwell DSc > > > > On 31 Oct 2012, at 04:53, Ethan Merritt <merr...@u.washington.edu> wrote: > > > On Tuesday, October 30, 2012 01:44:43 pm Adrian Goldman wrote: > > > >> The coordination is indicative but not conclusive but, as I responded to > >> the original poster, I think the best approach is to use anomalous > >> scattering. You can measure just below and above the Ca edge, > > > > Actually, you can't. The Ca K-edge is at 3.07Å, which is not a wavelength > > amenable to macromolecular data collection. > > > > cheers, > > > > Ethan > > > > > >> and similarly with the Zn, and those maps will be _highly_ indicative of > >> the relative amounts of metal ion present. In fact, you can deconvolute > >> so that you know the occupancy of the metals at the various sites. > >> > >> Adrian > >> > >> > >> On 30 Oct 2012, at 22:37, Chittaranjan Das wrote: > >> > >>> Veerendra, > >>> > >>> You can rule out if zinc has replaced calcium ions (although I agree with > >>> Nat and others that looking at the coordination sphere should give a big > >>> clue) by taking a few crystals, washing them a couple of times and > >>> subjecting them to ICP-MS analysis, if you have access to this technique. > >>> You can learn how many zinc, if any, have bound per one protein molecule > >>> in the dissolved crystal. > >>> > >>> Best > >>> Chitta > >>> > >>> > >>> > >>> ----- Original Message ----- > >>> From: "Veerendra Kumar" <veerendra.ku...@uconn.edu> > >>> To: CCP4BB@JISCMAIL.AC.UK > >>> Sent: Tuesday, October 30, 2012 2:55:33 PM > >>> Subject: [ccp4bb] Ca or Zn > >>> > >>> Dear CCP4bb users, > >>> > >>> I am working on a Ca2+ binding protein. it has 4-5 ca2+ binding sites. I > >>> purified the protein in presence of Ca2+ and crystallized the Ca2+ bound > >>> protein. I got crystal and solved the structure by SAD phasing at 2.1A > >>> resolution. I can see the clear density in the difference map for metals > >>> at the expected binding sites. However I had ZnCl2 in the crystallization > >>> conditions. Now i am not sure whether the observed density is for Ca or > >>> Zn or how many of them are ca or zn? Since Ca (20 elctron) and Zn (30 > >>> electron), is this value difference can be used to make a guess about > >>> different ions? > >>> is there any way we can find the electron density value at different > >>> peaks? > >>> > >>> Thank you > >>> > >>> Veerendra > >> > > >
