Editing Small-angle neutron scattering (section)

==In biology==
[[Image:Contrast matching.png|400px|right|thumb|'''Figure 1''': The relationship between the scatter of various biological macromolecules as a function of D2O concentration.]]
{{Main|Biological small-angle scattering}}
A crucial feature of SANS that makes it particularly useful for the biological sciences is the special behavior of hydrogen, especially compared to deuterium. In biological systems hydrogen can be exchanged with deuterium which usually has minimal effect on the sample but has dramatic effects on the scattering.

The technique of '''contrast variation''' (or '''contrast matching''') relies on the differential scatter of hydrogen vs. deuterium. Figure 1 shows the scattering length density for [[water]] and various biological macromolecules as a function of the deuterium concentration. (Adapted from.<ref name="Jacrot"/>) Biological samples are usually dissolved in water, so their hydrogens are able to [[Hydrogen-deuterium exchange|exchange]] with any deuteriums in the [[solvent]]. Since the overall scatter of a molecule depends on the scatter of all its components, this will depend on the ratio of hydrogen to deuterium in the molecule. At certain ratios of H<sub>2</sub>O to D<sub>2</sub>O, called match points, the scatter from the molecule will equal that of the solvent, and thus be eliminated when the scatter from the buffer is subtracted from the data. For instance the match point for proteins is typically around 40–45% D<sub>2</sub>O, and at that concentration the scatter from the protein will be indistinguishable from that of the buffer.

To use contrast variation, different components of a system must scatter differently. This can be based on inherent scattering differences, e.g. DNA vs. protein, or arise from differentially labeled components, e.g. having one protein in a complex deuterated while the rest are protonated. In terms of modelling, small-angle X-ray and neutron scattering data can be combined with the program MONSA. An example in which SAXS, SANS and EM data has been used to build an atomic model of a large multi-subunit enzyme has recently been published.<ref>{{cite journal|last=Kennaway|first=Chris|author2=Taylor, James |display-authors=etal |title=Structure and operation of the DNA-translocating type I DNA restriction enzymes|journal=Genes & Development|date=1 Jan 2012|volume=26|issue=4|pages=92–104|doi=10.1101/gad.179085.111|pmid=22215814|pmc=3258970}}</ref> For some examples of this method see.<ref>{{cite journal
 | last = Perkins
 | first = SJ
 | title = Structural studies of proteins by high-flux X-ray and neutron solution scattering
 | journal = Biochemical Journal
 | volume = 254
 | issue = 2
 | pages = 313–27
 | date =  January 1, 1988 | doi = 10.1042/bj2540313
 | pmid = 3052433
 | pmc = 1135080
 }}</ref>

For the study of large scales of matter (e.g. Soft matter) and slow dynamics, very cold neutrons (VCN) should be used. However, because of the weak neutron flux and lack of optical components in this range, most scientists use neutrons of shorter wavelengths. Efforts are being made to remedy this lack.<ref>{{Cite book |last1=Hadden |first1=Elhoucine |last2=Iso |first2=Yuko |last3=Kume |first3=Atsushi |last4=Umemoto |first4=Koichi |last5=Jenke |first5=Tobias |last6=Fally |first6=Martin |last7=Klepp |first7=Jürgen |last8=Tomita |first8=Yasuo |title=Photosensitive Materials and their Applications II |chapter=Nanodiamond-based nanoparticle-polymer composite gratings with extremely large neutron refractive index modulation |editor-first1=Robert R. |editor-first2=Yasuo |editor-first3=John T. |editor-first4=Inmaculada |editor-last1=McLeod |editor-last2=Tomita |editor-last3=Sheridan |editor-last4=Pascual Villalobos |date=2022-05-24 |chapter-url=https://www.spiedigitallibrary.org/conference-proceedings-of-spie/12151/1215109/Nanodiamond-based-nanoparticle-polymer-composite-gratings-with-extremely-large-neutron/10.1117/12.2623661.full |publisher=SPIE |volume=12151 |pages=70–76 |doi=10.1117/12.2623661|isbn=9781510651784 |s2cid=249056691 }}</ref>