Neutron protein crystallography: current status and a brighter future.

Hydrogen atoms are rarely seen in X-ray protein crystal structures, but are readily visualized by neutron crystallography, even at typical (1.5-2.5A) resolutions. Recent advances in neutron beamlines and deuterium labeling technologies have dramatically extended the scale and range of structures studied. High-quality neutron data can be collected to near atomic resolution ( approximately 1.5-2.5A) for proteins of 50-175kDa molecular weight, from perdeuterated samples, from crystals with volumes of 0.1mm(3) and at cryogenic temperatures (15K). These structures are providing unique and complementary insights into hydrogen-bonding interactions, protonation states, catalytic mechanisms and hydration states of biological structures that are not available from X-ray analysis alone. The new generation of spallation neutron sources promises further 10-50-fold gains in performance.