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# 092905 - BE.342/442 Tuesday, September 29, 2005 Topic:...

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BE.342/442 Tuesday, September 29, 2005 Topic: Practical Aspects of Single Crystal X-ray Crystallography, Part 1 (Administrative) Next Tuesday: Guest lecturer on NMR. Next Thursday: Guest lecturer on other characterizations methods. Following Tuesday: Institute holiday! Take-home exams handed out next week. The course text has a short but excellent section on NMR. Also consider the text “Crystallography Made Crystal-Clear.” New tools are emerging in X-ray crystallography. In is believed that within 20 years, we will be able to observe single molecules using crystallography! Demonstration of modern crystallography tools: Crystallization: Research Tools . Aliso Viejo, CA: Hampton Research, 2005. Crystallization: Research Tools . Aliso Viejo, CA: Hampton Research, Volume 14, Number 1, 2004. Two pioneers in X-ray crystallography: Lawrence Bragg , Nobel Prize winner at age 25, invented X-ray diffraction for the study of molecules, and is known for the famous Bragg’s Law relation for crystallography. He focuses on small-molecule crystallography of substances like salt and sucrose. Linus Pauling , the only winner of two unshared Novel prizes, observed larger molecules, like peptide, dipeptides, tripeptides, etc. From this data, he proposed the structures of alpha-helices and beta-sheets, which we have seen are ubiquitous patterns in proteins. Ways to observe objects: large: telescope macroscopic: naked eye microscopic: microscope too small to be resolved by visible light: x-ray diffraction The W. L. Bragg’s Law condition for diffraction: n λ = 2 d sin θ Constructive interference between parallel incident beams creates “peaks” of intensirt in the diffraction patterns, indicating reflection off a crystallographic plane. d is the distance between planes in the crystal structure. is the angle of the incident beam relative to the plane. is the wavelength of the light used for the incident beam.

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With a smaller wavelength ( λ ), you can observe smaller objects. For atoms, typical x-ray wavelengths range from 0.1 to 100 angstroms. The larger the crystal, the more intense the diffracted beam, allowing for shorter collection times. Problems with X-ray crystallography include: X-rays cannot be bent with a lens like visible light. Must use a computer as a “pseudo-lens.” X-ray scattering from a single molecule is weak! In order to measure a detectable pattern, the molecules must all be aligned in a crystallographic pattern, so that the sum of their diffraction pattern “amplifies” the signal. What information can X-ray crystallography give about atomic structure:
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## This note was uploaded on 11/11/2011 for the course BIO 20.410j taught by Professor Rogerd.kamm during the Spring '03 term at MIT.

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092905 - BE.342/442 Tuesday, September 29, 2005 Topic:...

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