Figure 19.1.5.1 

Difference map of Asn34 in the trypsin structure. (a) In a protein X-ray analysis, the difference in scattering intensity between O and NH₂ is much too small to be detected. In contrast, the neutron-scattering magnitudes of oxygen and nitrogen (5.8 f versus 9.4 f) are quite dissimilar, and there is additional scattering at the nitrogen site from the two bound deuterium atoms. The resulting differential is over 350%, quite large enough to be detected for well ordered side chains. The nitrogen and oxygen positions shown are from the X-ray model. The difference density indicates that the orientation of the nitrogen and oxygen atoms is incorrect and should be rotated by 180° around the C_β–C_γ bond. (b) Difference map for Ser139. On well ordered hydroxyl side chains, the orientation of deuterium atoms can sometimes be assigned.