2FO-FC maps

Fig. 18 The solution of epothilone bound to tubulin by electron crystallography, a 2Fo-Fc map and model of epothilone A bound to tubulin at la (ltvk) [3], b Energy optimized model derived from ltvk through MAID protocol used as template for analysis of SAR and design of new analogs...

Fig. 26.11 The rationale behind the 2Fq - Fc maps. Shown is a carbohydrate attachment site at Asn109 in human leukocyte elastase. The carbohydrate chain has not been added to the model. The Fq map in (a) mainly shows the electron density of the model, owing to the model bias. Some electron density is present for the carbohydrate, but it is very weak and not connected to the Asn residue. The Fo - Fc map in (b) shows only features which have not been accounted for in the model, in this case the carbohydrate moiety. The 2Fo - Fg map in (c) is a combination of the two and shows both density belonging to the model and strong connected density for the carbohydrate. From these figures, it is clear why crystallographers prefer a 2Fo - Fc map over an Fq map.

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