Invited Paper NT-TuM1
Cold and Thermal Neutrons: Suitable Probes for Biomaterials
Tuesday, October 16, 2007, 8:00 am, Room 618
Neutrons are electrically neutral, subatomic elementary particles whose existence had been postulated by Ernest Rutherford and discovered by James Chadwick in 1932. With the exception of hydrogen, neutrons are found in all atomic nuclei, and free neutrons are unstable with a mean lifetime of ~ 900 s. Thermal and cold neutrons, used for the study of materials ranging from engineering to biomedical applications, are typically produced by reactor- or accelerator-based sources and are the result of interacting with a given temperature moderator (e.g., graphite, water, liquid deuterium). Compared to thermal neutrons, cold neutrons possess 5 - 20 times longer wavelengths and are preferred for the study of biological materials with inherently large unit cells. Unlike x-rays, neutrons interact strongly with nuclei and the strength of their interactions varies dramatically and non-monotonically from element-to-element across the periodic table. This statement applies equally to isotopes of the same element and has been used to great advantage by biologists and polymer scientists who typically study materials rich in hydrogen. The classic example is the isotopic substitution of 1H for 2H (deuterium), where one can accentuate, or nullify, the scattering from particular parts of a macromolecular complex by selective deuteration. Recently, we have used a number of neutron scattering techniques to study for example, the location of cholesterol in polyunsaturated fatty acid membranes (discovered that cholesterol can reside in unexpected places within the membrane!) and the distribution of water in aligned, self-assembled lipopolysaccharide bilayers isolated from the Gram-negative bacterium, Pseudomonas aeruginosa. These results provide insights as to how molecules penetrate and assemble in biologically relevant membranes, and will be the topic of discussion. Web Site: http://neutron.nrc-cnrc.gc.ca/people/katsaras/index.html.