Can protein structure be determined by X-Ray Diffraction in a single image?

Question

I'm reading about the use of x-ray crystallography to determine protein structure. According to my book, data is collected at 30-360 angles (dependent on the symmetry of the protein). An illustration is given with concentric rings labelled with distances - the further out the points are, the higher the resolution.

Is the image a composite (where the angle of the point point from the centre is equivalent to the angle of the reading) or is a separate image taken at each angle? Are there any other reasons why more images would be required?

Thanks.

Answer 1

You cannot solve a structure with a single frame, even with perfect diffraction.

The reason you need images over a large swath of angles is because the diffraction pattern is also in three dimensions, in the so-called "reciprocal space". At minimum, a 180° rotation of the crystal is needed to sweep the entire reciprocal space sphere with the plane of detection, though symmetry in the crystal structure can reduce this further (some of my proteins only required 90°, I think hexagonal unit cells with 6-fold symmetry can live with 30°). Because crystals are real things (read: non-ideal, doubly so for protein crystals), the sweep is extended to gain some redundancy.

Answer 2

Likely not. While one can get excellent diffraction data from a high quality crystal, it would be extremely difficult to solve the phase problem. The extra angles will help constraint the solutions.

Answer 3

Not by analysing a single protein. There is work with x-ray lasers.

You have to take a simultaneous image of millions of proteins and use that to get a structure. It's not quite prime time. People are also doing this with electron beams in electron microscopes.

These methods will reconstruct 3D models of the molecules, sometimes in states which cannot be obtained from crystallography. Examples being the structure of the many megadalton nuclear pore complex, and the f-actin fiber. The classic study is 3d model of bacteriorhodopsin, the first membrane protein structure which was at molecular resolution (this was a crystalline sample though).

While in principle, it sounds much simpler - get a pure sample of your protein, or complex and freeze it down and zap it with an Xray or Electron beam, its a lot more work to reconstruct the image and can take as long or longer than getting an x-ray structure. The resolution is also usually poor as the crystal will reinforce coherence, that is all the proteins are aligned in the same way and have close to the same 3d shape in a crystal.