electron-density maps

A method for the identification of -helices in electron-density maps at low resolution followed by interpretation at moderate to high resolution is presented. Rapid identification is achieved at low resolution, where -helices appear as tubes of density. The positioning and direction of the -helices is obtained at moderate to high resolution, where the positions of side chains can be seen. The method was tested on a set of 42 experimental electron-density maps at resolutions ranging from 1.5 to 3.8 Å. An average of 63 % of the -helical residues in these proteins were built and an average of 76 % of the residues built matched helical residues in the refined models of the proteins. The overall average r.m.s.d. between main-chain atoms in the modeled -helices and the nearest atom with the same name in the refined models of the proteins wa