Dear Anna, I know that you already have gotten replies from some top experts, but your intriguing problem brought up some issues I have run across in the past.
First, from you experience with single crystal diffraction, your results are not that much different from those seen in virus structures where the nucleic acid structure is averaged out. As the nucleic acid doesn't (and mostly can't) adopt the symmetry of the protein shell, the crystallization process alone does the "averaging." Just because that ferritin and magnetite have cubic symmetry elements, if they don't line up, the magnetite structure can be "averaged out" upon crystallization. So, working at lower symmetry may not help, unless there is some directional correlation of the magnetite symmetry and position with the crystal axes. But try P1 and see what happens. A second comment is why not try neutron scattering (SANS or single crystal neutron diffraction), particularly as you can match out the protein with D2O and see just the magnetite. While the same concerns apply for single crystal neutron diffraction, you see more clearly regions of higher average density inside the protein shell. And lastly, have you tried crystallizing your ferritin/nanoparticle complexes in the presence of a magnetic field? It would be a neat trick, and people have tried such things in the past, such as for orienting biomolecules. Some even used old NMR magnets. Would be wild, if it worked. Good luck, Michael **************************************************************** R. Michael Garavito, Ph.D. Professor of Biochemistry & Molecular Biology 603 Wilson Rd., Rm. 513 Michigan State University East Lansing, MI 48824-1319 Office: (517) 355-9724 Lab: (517) 353-9125 FAX: (517) 353-9334 Email: rmgarav...@gmail.com **************************************************************** On May 7, 2012, at 12:30 PM, anna anna wrote: > Dear all, > I'd like some suggestions/opinions about the sense of an experiment proposed > by a collaborator expert in saxs. > In few words, he wants to collect SAXS data on a suspension of protein xtals > to investigate "low resolution periodicity" of the xtal (more details below). > The experiment requires a very huge number of xtals to obtain the circles > typical of saxs and it is very time-consuming to me (I know nothing about > saxs, I have only to prepare the sample). I proposed to measure a single > rotating xtal (like in XRD) but he told they don't have a goniometer on saxs > beamline. > Here is my concern: does it make sense to measure many xtals together? Don't > we lose information with respect to single xtal? And, most of all, what can I > see by saxs that I can't see by waxs?? > Sorry for the almost off-topic question but I think that only someone who > knows both the techniques can help me!! > > > Some detail for who is intrigued by my story: > we prepared doped magnetite nanoparticles using ferritin as bioreactor. I > crystallized this spheres filled with metal and solved the structure at 3.7A > but I can see only the protein shell while there is no density inside, even > if I know that the nanoparticles are there. A simple explanation is that the > particles are free to move in the cavity(note that the diameter of the > nanoparticle is shorter then the inner diameter of the protein shell), ie are > disordered, and do not contribute to diffraction, in fact, to my knowledge, > nobody have ever seen the metal core inside ferritin or dps proteins. > However, since they are magnetic particles they must "see" each other through > the protein wall, ie they can't be completely free to move in the cavity. > Maybe, but this is just my opinion, I don't see the particle because the > "period of the particle" in the xtal is different/longer than the period of > the protein shell. > Anyway, we are interested in the relative distance between the magnetic > particles in the xtal to study the effects of magnetostatic interactions in > nanoparticles 3D arrays. We are going to do this by saxs since, they told me, > lower resolution is useful in studying this long range periodicity (the > diameter of ferritin is about 120A) but it seems fool to me using a > suspension of so many xtals to obtain a scattering curve while I could > collect diffraction images from a single xtal!!! I know that saxs is used > when you don't have xtals but if you have xtals, ie your system is ordered, > xtallography is much more powerful!! > > Another question: how can I handle my diffraction data at 3.7A resolution to > "look for" nanoparticles? Should I try a lower symmetry? Maybe the anomalous > signal? Have you any reference for a similar case? > > Thank you very much!! > > anna > > > > >
