The determination and refinement of heavy‐atom parameters in protein heavy‐atom derivatives. Some model calculations using acentric reflexions

摘要: Some problems associated with the determination and refinement of heavy-atom parameters in protein derivatives using totally acentric data are critically examined aid calculations on model systems. The difference Patterson (ΔF 2 Patterson) synthesis is found to be a good approximation vector map even when substitution high. A theoretical expansion for coefficients conventional Fourier given. quality decreases rapidly increasing substitution. Theoretical considerations show that background noise closely resembles features map. Statistical errors X-ray likely lead systamatic overestimation occupancy factors refined by least-squares method against contributions estimated from isomorphous anomalous differences. Refined reasonably, close their true values can, however, obtained either empirically evaluated k (the ratio between real imaginary parts form factor) estimating contribution or applying suitable weighting functions terms minimization function. aspects based tbe sum squares lack-of-closure also discussed.