Opportunities and challenges with the growth of neutron crystallography.

Neutron crystallography is a newly blossoming field of structural biology. As described in the historical account given by Benno Schoenborn in this issue, its origins can be traced back to the Cambridge MRC tearoom in 1965 when it was discussed as the only plausible experimental method for directly determining the location of H atoms in proteins. Knowing the location of H atoms can provide information on the protonation states of amino-acid residues and ligands, the identity of solvent molecules, and the nature of bonds involving hydrogen. Neutron crystallography can also be used to identify H atoms that are exchanged with their isotope deuterium (deuteration) and the extent of this replacement, thus providing a tool for identifying isotopically labeled features, for studying solvent accessibility and macromolecular dynamics, and for identifying minimal protein folding domains. This unique information, which is often difficult or impossible to obtain using X-ray crystallography, is important for understanding protein function and enzyme mechanism. However, it wasn't until 1968 that neutron crystallography was first used to study a protein at the nuclear reactor run by Brookhaven National Laboratory. With limited equipment and a relatively low flux beam, it took several months to collect a 2 A ˚ data set from the 17 kDa transport protein myoglobin using a crystal that was several tens of mm 3 in volume. Over the years since that beginning, growth has been relatively slow mainly because there have been a limited number of available instruments and their fluxes have been relatively weak compared with X-ray beams, therefore requiring larger crystal sizes. However, the uniqueness of the type of information that can be provided by neutrons, and its complementarity to the information provided by X-rays, has given neutrons a small but important role in biology in the past. Recently, the prospects for this field have changed dramatically and there has been great increase in the application of neutrons in biology. This can be related to improvements in beamline instrumentation, neutron sources, data collection and sample preparation methods, and new approaches to and computational tools for structure determination. These advances are pushing practicable sample sizes down to fractions of a mm 3 , data collection times down to a few days or even hours, and are allowing increasingly complex biological systems to be studied. Growth is bound to continue with the current construction of new and more powerful spallation neutron sources throughout the world and …