Unique aspects of neutron scattering for the study of biological systems

The unique aspects of neutron scattering in biology rely largely on the extraordinary parameters of interaction of neutrons with hydrogen nuclei. The origin of isotopic and spin incoherent scattering and its possible implication for neutron diffraction from protein crystals are described. The impact of a change in the coherent scattering length of hydrogen nuclei with isotopic substitution and spin polarization is illustrated by a number of applications in structural biology, which show clearly the complementarity to x-ray diffraction. A more detailed account is given of polarized neutron scattering from dynamically polarized spin systems. Experimental evidence for the existence of selectively polarized proton spin domains and their use. in structural studies is presented.