The repeating unit of a crystal corresponding to the

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– The repeating unit of a crystal, corresponding ~ to the volume occupied by a single molecule is called a unit cell – A crystal is built by billions of identical unit cell • X-Rays – Electromagnetic radiation of wavelength 1.54 Å – They are produced by a beam of accelerating e- on a copper anode target High energy e- Low energy X-rays
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Detectors a. Radiation destroys crystals i.e. Protection cooling, rotation, brief exposure, large area detectors b. Records of diffracted (scattered) beams are obtained on - film (x-ray): blackening of emulsion - electronic, solid state, detectors - large area detectors (electronic counters) – UCSD c. Comparison of diffraction pattern of native protein crystal with complex of protein and a heavy metal
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Braggs Law: 2dsin θ = d = distance θ = reflection angle = wavelength Dictates the conditions for diffraction
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Braggs Law: 2dsin θ = d = distance θ = reflection angle = wavelength Dictates the conditions for diffraction
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Phase Determination Problem • Each diffracted beam is defined by: – Amplitude = intensity of spot-measure – Wavelength = you know – Phase = lost in experiment • Microscope out of focus/no eyepiece – Solution: Multiple isomorphous replacement • Diffusion of heavy metals into channels – SH groups reactivity – Replacement of light metals Hg 2+ Pt + }
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• Image is formed by applying a mathematical relation • Fourier Transform Spot Wave of e- density Amplitude Phase (Intensity ) Heavy Atom MIR Difference Patterson Map
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