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As a basis for direct-methods phasing at very low resolution for macromolecular crystal structures, normalized structure-factor algebra is presented for few-atoms structure models with N = 1, 2, 3, ... equal atoms or polyatomic globs per unit cell. Main results include: For ,
For ,
For , is obtained in an exact closed form as a rational function of nine magnitudes , , , , , , , , . For ,
Triplet discriminant and triplet weight parameters, and , respectively, were determined empirically in numerical error analyses. Tests with phases calculated for few-atoms `super-glob' models of the protein apo-D-glyceraldehyde-3-phosphate dehydrogenase (~10000 non-H atoms) showed that low-resolution phases from the new few-atoms tangent formula were much better than conventional tangent formula phases for and 3; phases from the two formulae were essentially the same for .