PARTIAL STRUCTURE REFINEMENT COMBINED WITH ELECTRON DENSITY MODIFICATIONS BY USING MAIN. Dusan Turk, dusan.turk@ijs.si, University of Ljubljana, Josef Stefan Institute, Slovenia

In the cases, that is, in most cases, when it is not possible to build almost complete macromolecular models into initial electron density maps, progress of a structure determination is hindered by limitations of a partial model refinement against a full data set and by limitations of electron density modification procedures including phase combination. A phase combination procedure starts to make sense when substantial parts of the model have been built into electron density maps.

MAIN is a program that can be applied in various electron density modification approaches, for interactive model building and structure refinement. Now, tools that allow to combine all these methods into a single task, have been added to the program. The essence of the idea is that accordingly scaled electron density map of a partial model is scaled and merged with a background map. Resulting merged map can be subjected to any electron density modification procedure. This modified map serves then to calculate structure factors and these structure factors are used to calculated derivatives, which are applied during a partial model minimization, and to generate a new background map. This should be repeated in several cycles.

The approach is not limited by the size of a model fraction, although it is evident that small model fractions will not be capable of substantial map improvements. Visual inspection has revealed that the resulting maps are sharper, include more structural details (side chains and loops) and are less model biased than a map based on a phase combination approach.

A MAIN demonstration installation is available via an anonymous FTP server at STEF.IJS.SI.