To determine position of heavy atom in crystal to

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• To determine position of heavy atom in crystal • To determine phase of heavy atom in crystal • Use 2 different heavy atoms to decrease ambiguity
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• Calculation of the electron density map • Interpretation of the electron density map – Resolution: Quality of crystal (~2 Å) • 5-6 Å: Course of polypeptide chain • ~3 Å: side chains, e.g. ~ 2 Å: side chains • 1-1.5 Å: atoms L I
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Refinement: Model Building R factor: residual disagreement, between a hypothetical crystal containing the model and the experimentally determined crystal – R = 0 perfect agreement – R = 0.59 total disagreement For a resolution of 3Å and R 0.3 ? • For 2Å, R=0.2 OK
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NMR • Atomic Nuclei ( 1 H or 15 N) possess an intrinsic spin angular momentum, resulting in a magnetic moment that can interact with an externally applied magnetic field B • In a B field the spins of H align • Equilibrium alignment can be perturbed by pulses of radiofrequency (RF)
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NMR • Relaxation to equilibrium emits RF that can be measured [with respect to a reference] Chemical shifts “Specific” Nucleus Environment High Low B RF For Unique Assignments: Multi-Dimensional NMR: 2-D, 3-D, 4-D
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NMR 2-D NMR – Diagonal: ~ 1-D spectrum – Peaks off-diagonal : (cross-peaks) Interactions of H atoms that are close to each other in space COSY: (Correlation) Fingerprint of a.a. – Distance between BONDED H atoms ( three chemical bonds, I.e. within the same a.a.) NOESY (NOE, Nuclear Overhausser Effect) – Distance between H atoms close together in space ( 5 Å)
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