Bfaktorer

Bfaktorer, or B-factors, are parameters used in X-ray crystallography to describe how much an atom deviates from its average position. They reflect both thermal motion of atoms and static disorder within the crystal. The term Bfaktorer is commonly used in Scandinavian literature and is equivalent to the English term temperature factors or isotropic displacement parameters.

In the simplest case, the isotropic B-factor (Biso) relates to the mean square displacement through B = 8π^2 ⟨u^2⟩. In models that distinguish direction, anisotropic displacement parameters (ADP) are represented by a 3x3 matrix Uij. The scattering intensity is damped by a factor exp(-B sin^2 θ / λ^2) for isotropic B, or by the corresponding anisotropic form.

Interpreting Bfaktorer: low B values indicate tightly bound, rigid regions; high B values point to flexible side chains, solvent exposure, or poorly defined regions. Bfaktorer are refined alongside atomic coordinates during structure refinement and are often influenced by data resolution, model quality, and refinement strategy. Tools such as Wilson plots, refinement with TLS groups, and map validation are used to interpret and improve Bfaktorer.

Limitations: Bfaktorer do not separate thermal motion from static disorder and can be affected by resolution and model bias. They are best understood as indicators of positional uncertainty in the final model rather than direct physical motion measurements. In modern practice, Bfaktorer are routinely reported for all atoms in PDB entries and can be used to guide further refinement and interpretation.