Refinement

Theory

For now, see http://www.usm.maine.edu/~rhodes/ModQual/index.html#RefineXray

Programs

what can go wrong in refinement?

R-factor does not go down

If this happens in the R-factor range of 30-40, here are a couple of possible reasons:

wrong spacegroup (usually the true symmetry is lower; see below)
twinning (this happens more often than you'd like, see A. A. Lebedev, A. A. Vagin and G. N. Murshudov (2006) Intensity statistics in twinned crystals with examples from the PDB. Acta Cryst. D62, 83-95)
bad data - check the statistics of the data reduction program
model incomplete or wrong
other refinement options should be exploited, e.g. try TLS refinement

help, my protein has high B-factors!

This is also a FAQ on CCP4BB. The answer is: there's probably nothing wrong with it. If your crystals diffract to 3 A at a synchrotron, then the average B-factor should most likely be on the order of 100 A^2. If your crystals diffract to 2 A, then the average B-factor is most likely on the order of 40 A^2 or so. Use B. Rupp's calculator ([1]) to find out the dependance of scattering power on B-factor.

R_free much higher than R

how large should the difference between R_free and R be?

For now, see Kleywegt GJ, Jones TA."Homo crystallographicus--quo vadis?" Structure. 2002 Apr;10(4):465-72.

Wrong space group

Sometimes crystal symmetry combines with non-crystallographic symmetry (NCS) and produces a diffraction pattern resembling higher symmetry space group than what you really have. NCS in this case closely resembles crystallographic symmetry. If resolution is not high enough, the difference in spot positions may be too small to give any detectable problems with indexing, integration and scaling. Even phasing (e.g. molecular replacement) may be successful. But if your R-factor hangs fairly high and you have problems building parts of your structure, it is worth trying to check other space groups. The most straightforward approach is to try processing data in P1, because if that does not bring R-factor down significantly, other space group choices will not solve the problem either.

This occurs most often at moderate resolution. However, the structure of the ketosteroid isomerase had to be refined in P1 at atomic resolution, although it refines well in C2221 at lower resolution such as 1.5A.

Refining low resolution structures

Maintaining the secondary structure of your model when refining against weak data can be really challenging. There are some options, but in the end you might have to accept a fairly large number of Ramachandran plot outliers.

Try phenix.refine with the keyword "discard_psi_phi=False". Then the psi and phi dihedral angles should be restrained according to the CCP4 monomer library definitions. There was a discussion of in in the phenixbb in juli 2007. Also see the discussion in the ccp4bb from december 2006.

Remember that phi-psi angels are excellent for validation purposes, but only when they are unrestrained, so if you restrain them you lose this option!

You can also try restraining alpha-helices hydrogen bonding, and beta-sheet cross-strand hydrogen bonds. This can be done in REFMAC, phenix.refine and CNS (its documented for all of them).

If you are really desperate, another option could be to use harmonic restraints in CNS to keep your backbone fairly fixed in parts of the map where you believe the secondary structure is correct (most likely alpha-helices). You could also fix main chain elements completely (in any refinement program), but it is definitely preferable to leave some room for change in the xyz-positions, and harmonic restrains are a nice way of doing exactly that.