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a and b) see figure 16-16 in Eisenbergand Crowthers Physical Chemistry With Applications to the LifeSciences Benjamin-Cummings 1979Ĥ. 3-Dimensional Lattice Types: 14 BravaisLattices characterized by 3 lattice vectors (a, b, c) and 3angles, a (between b and c), b (bet. Not all point groups are compatible with every latticetype there are 17 possible combinations, five are compatible withbiological structures which cannot have a mirror plane or inversioncenter.ģ. This is the basis of all regular2-dimensional patternsī. Plane Groups: 2-dimensional lattice type + motifpoint group symmetryĪ. Centered Rectangle => C - a = b g = 90°Ģ. 2-dimensional - 5-types characterized by latticevectors a and b and the angle between them, g.ī. Asymmetric Unit - the smallest part of an objectfrom which the whole can be generated by symmetry operations.ġ. Basic icosahedron contains 60 subunits viruses contain multiples of 60 subunits.Į. This is the point group symmetry of spherical viruses. axes of an icosahedron, a regular 12-sided solid (6 5-fold 10 3-fold 15 2-fold) => icosahedral, symbol = I = 532. many transition state metal ions form ligand complexes having octahedral symmetry.axes of cube (4-, 3-, and 2-fold) => Octahedral, symbol = O = 432 three 2-fold axes connecting opposite edges => tetrahedral, symbol = T = 23.four 3-fold axes along body diagonals of a cube plus.examples: D2 = 222 D3 = 322 D4 = 422 D6 = 622 (glutamine synthetase from E.may mave mirror planes and inversion centers (but not for chiral biomolecules).Dihedral Point Groups (the first two are alwayspresent) coli GlutamineSynthetase has 622 symmetry or D6Ĭ.
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1 or more planes of reflections or mirrors (not allowed in chiral biomolecules).single axis of rotation - n-fold -> rotation by 360º/n.Crystal Composition: typically 30 - 60% liquid whichfills in the spaces between protein molecules=> very fragile withthe consistency of jello. Uses very smallvolumes of protein (10 - 20 µl) and permits organization andeasy observation.Ĭ. hanging drop method - drop hanging from glasscoverslip sealed over well of tissue culture plate. Ifprecipitant is not volatile then must include it at lowerconcentration in protein solution will become more concentrated aswater is drawn out.Ī. Vapor Diffusion - seal droplet of protein solution inchamber with large volume of solution containing precipitant. Dialysis - dialyze in small glass tubes with 1 endcovered with dialysis membraneģ. Requires large amounts of protein and very goodeyesight.Ģ. Batch - add precipitant dropwise to small beaker ofstirred protein solution when it becomes faintly turbid, set asidefor several weeks. polyethylene glycol (PEG) - very frequently usedġ. alcohols such as 2,4-methyl-pentanediolī. Isoelectric pH - minimal solubility because net chargeson protein molecules is 0 => no repulsive electrostatic forces=> salt in or salt out at pI.Ī. Salting Out - most proteins become insoluble atvery high salt concentrations (ions bind most H 2Omolecules) dialyze protein solution against high commonlyuse (NH 4)SO 4ģ. Salting In - many proteins are not very solublein pure water dissolve protein at moderate salt concentration andthen dialyze against distilled waterĢ. Protein Solubility - howto make protein less soluble -> crystal growth make asupersaturated protein solution in which crystal nuclei will form andgrow into crystals until the solution is no longer supersaturatedġ. Resolution - Higher resolution allowsmore accurate positioning of atomsĪ. Refine structure-modify atomic model until it fits diffraction patternsĬ.Collect diffraction patterns-intensities only.Diffraction Patterns / FourierTransforms of Crystals-See "Diffraction and Fourier Transforms" fromMarch 12, 1998) Bio750: X-Ray Diffraction/Crystallography X-Ray Crystallography I.