B-Factors

The B-factor is the standard measure of residue/atom flexibility. It is determined from the oscillations of a residue with respect to its equilibrium position

$B-factor=\frac{8}{3}\pi^2\left<\Delta r^2\right>$

(1)

where $\left<\Delta r^2\right>$ stands for the oscillations of residues around equilibrium positions.

B-factor profiles represent the distribution of residue harmonic oscillations. They can be compared with X-ray data, but caution is needed, since crystal lattice effects tend to rigidify exposed protein residues. Very large-B-factors should be taken with caution since indicate very flexible residues that might display conformational changes along the trajectory, which is difficult to follow within the harmonic approximation implicit to B-factor analysis.