TIME-AVERAGING CRYSTALLOGRAPHIC REFINEMENT - POSSIBILITIES AND LIMITATIONS USING ALPHA-CYCLODEXTRIN AS A TEST SYSTEM

The method of time-averaging crystallographic refinement is assessed u sing a small molecule, alpha-cyclodextrin, as a test system. A total o f 16 refinements are performed on simulated data. Three resolution ran ges of the data are used, the memory relaxation time of the averaging is varied, and several overall temperature factors are used. The most critical factor in the reliable application of time-averaging is the r esolution of the data. The ratio of data to molecular degrees of freed om should be large enough to avoid overfitting of the data by the time -averaging procedure. The use of a free R factor can aid in determinin g whether time-averaging can be reliably applied. Good ensembles of st ructures are obtained using data up to 1.0 or 2.0 Angstrom resolution. Comparison of electron-density maps from time-averaging refinement an d anisotropic temperature-factor refinement indicates that the former technique yields a better representation of the exact data than the la tter.