Language
Country/Area
No result found
Why are hydrogen and deuterium so different in SANS? Uncovering the mystery of contrast change technology!
Small-angle neutron scattering (SANS) is an emerging experimental technology specifically used to study the structure of different substances at the mesoscopic scale (about 1-100 nanometers). Compared with small-angle X-ray scattering (SAXS), SANS provides a unique means to analyze the internal structure of disordered systems, especially in samples with randomly arranged density inhomogeneities. The main advantages of using small-angle scattering techniques are its sensitivity to light elements and the possibility of isotopic labeling, especially within the biological sciences.
Small-angle neutron scattering has unique properties that make it superior to other techniques, especially when exploring biological samples.
Principles and Technology
In a SANS experiment, a neutron beam is directed at samples, which can be aqueous solutions, solids, powders or crystals. Neutrons are elastically scattered under the influence of nuclear interactions. This interaction depends on different isotopes. This characteristic makes hydrogen (H) and deuterium (D) show obvious differences in the scattering process. Since hydrogen's scattering length is negative, the phase of neutron scattering from hydrogen atoms is 180 degrees different from that of other elements, allowing SANS technology to effectively exploit these phase differences for contrast changes.
The surprising differences between hydrogen and deuterium allow us to gain insights into complex biological systems through contrastive change techniques.
Related technologies
SANS usually uses collimation of the neutron beam to determine the scattering angle, which results in a low signal-to-noise ratio of the relevant data obtained from the sample. In order to overcome this challenge, many researchers choose to increase the brightness of the light source, such as using ultra-small angle neutron scattering (USANS). An alternative technique, spin-echo small-angle neutron scattering (SESANS), has also recently been introduced to extend the long-scale range that can be studied in neutron scattering by tracking the scattering angle. Some techniques, such as Inclination Small Angle Scattering (GISANS), combine ideas from SANS and neutron reflection techniques, further expanding the scope of research.
Applications in biology
The importance of SANS in biological sciences is closely related to the special behavior between hydrogen and deuterium. In biological systems, the presence of hydrogen can be exchanged for deuterium, which has a minimal effect on the sample but can have a surprising effect on the scattering results. Contrast variation relies on the different scattering properties of hydrogen and deuterium. Biological samples are often dissolved in water, where hydrogen can be exchanged for deuterium in the solvent, making the overall scattering effect of the molecule dependent on the ratio of hydrogen to deuterium.
At certain ratios of hydrogen water to deuterium water, called match points, the scattering of the molecules will match the scattering of the solvent, eliminating interference from the data.
For proteins, for example, the match point is usually at a D2O concentration of about 40%-45%, where the scattering from the sample is nearly indistinguishable from the scattering from the buffer. The technique relies not only on differential scattering of components within the sample, but can also be achieved by differentially labeling components, such as having one protein labeled with heavy deuterium while the rest remains light hydrogen.
Instruments
A variety of SANS instruments are available at neutron facilities around the world, including research reactors and spallation sources. These instruments are designed to deeply explore nanoscale structures and advance research in biology, materials science and other disciplines.
With the advancement of science and technology, the application scope of SANS continues to expand, and many researchers have begun to combine small-angle X-ray scattering, SANS and electron microscopy data to conduct more comprehensive structural modeling. Not long ago, there was a research report that successfully constructed an atomic model of a large multi-subunit enzyme using these technologies, showing the potential of SANS combined with other scattering technologies.
Facing the future, how to further utilize the potential of SANS in various scientific fields, especially its performance in microstructure research, is still an important issue that scientists need to discuss?
