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Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: Holmes on September 28, 2012, 11:33:20 AM

Title: B-factor increases, contribution of atom to scattering decreases, how?
Post by: Holmes on September 28, 2012, 11:33:20 AM

In X-ray crystallography of proteins, the B-factor is given by

Bi = 8π²Ui²

where Ui² is the mean square displacement of atom i. As U increases, the B-factor increases and the contribution of the atom to the scattering is decreased.

(Source: http://proteopedia.org/wiki/index.php/Temperature_value (http://proteopedia.org/wiki/index.php/Temperature_value))

What does it mean when it says that - "contribution of atom to scattering is decreased"?

Title: Re: B-factor increases, contribution of atom to scattering decreases, how?
Post by: Yggdrasil on September 28, 2012, 12:34:24 PM

Atoms in crystals produce diffraction patterns because the x-ray light scattered from the atoms in one unit cell constructively interferes with the light scattered from all the other unit cells in the crystal.  If an atom displays heterogeneity in its position (i.e. it is in different places in different unit cells), the light scattered from these atoms will not constructively interfere as well.  Therefore, the light that would have come from this atom were it well positioned is lessened, and this atom contributes less signal to the intensity of the spots in the diffraction pattern.

Title: Re: B-factor increases, contribution of atom to scattering decreases, how?
Post by: Holmes on October 03, 2012, 02:51:27 PM

Now, I understand it.
Thank you for the explanation.