ABSTRACT
Hydrogen atoms comprise 5-10% of a protein’s mass and approximately 50% of the atoms by number, and the position of certain hydrogen atoms can play a major role in protein function. Whilst atomic positions can be determined using either X-ray or neutron diffraction, X-rays will predominantly give information about electron-rich, non-hydrogen atoms, whereas neutrons allow the position of hydrogen atoms to be determined. This is due to the fact that X-rays are scattered proportionally to the number of electrons; neutrons, however, are scattered by the atomic nucleus and as such, hydrogen and deuterium scatter with the same order of magnitude as carbon or oxygen (1). For example Kossiakoff and Spencer (2) used neutron protein crystallography to identify that His57 in trypsin was protonated (determined as deuterium as required by the method). Neutron protein crystallography typically between 1.5 and 2.7 Å resolution and ultra-high resolution X-ray structure analysis, typically around 0.9 Å, together allow the determination of the positions of deuterium/ hydrogen atoms with very high condence.
