[10] Electron-density map interpretation

Abstract

Any errors that occur in a crystallographic project usually will be found before publication. Often, if an error has been made, the project will stall and there will be no publication. Introducing a serious error in a model can be different. This chapter discusses the kinds of error that might be made and why these errors are made. It discusses some of the features of the crystallographic model-building program O. Real errors in models occur with frequencies that are, fortunately, inversely proportional to the seriousness of the error. Building a molecular model from electron density is a complicated process. During the interpretation of an electron-density map, the basic function of the molecular graphics program is to assist the scientist in imagining, and then remembering, the three-dimensional folding and features of the structure. Thus, it is important to be able to change the model quickly and not to be interrupted by the details of operating a computer program. To facilitate the rapid building and rebuilding of molecular models, O incorporates autobuild options, allowing the user to create a molecular structure quickly from a rough three-dimensional sketch. This has the drawback of possibly making it even easier to build a wrong structure.