Optical matching of physical models and electron density maps: early developments.

Publisher Summary This chapter presents the work on various proteins; the maps are displayed as a stack of contoured sections supported on Perspex sheets. Appropriately, scaled coordinate systems are set up for a given model and map. Measurements are transferred back and forth between the model and the map as the construction of the model proceeds. Although faster than the “rod and clip” method, the process is time-consuming and tedious. Ideally, the model is suspended within the contour map so that the “fit” could be evaluated directly. This cannot be done by direct superposition of the two objects, but it can be simulated optically. The device for holding and displaying the map and the model is shown schematically. If the scale of the model is identical with the scale of the map, then corresponding positions in the map and in the model appear superimposed and not move with respect to each other regardless of the position of viewing. When building a model, one manipulates the brass pieces by rotation about single bonds while observing the process in the mirror. The mirror image produces no more difficulty than learning to work under a microscope. The fit to the map can be seen directly and no measurements are required.