APPLICATIONS OF THE MINIMAL PRINCIPLE TO PEPTIDE STRUCTURES

A new direct-methods procedure has been devised which consists of phas e refinement via the minimal function, R(phi), alternated with Fourier summation and real space filtering. All phases are initially assigned values by computing structure factors for a randomly positioned set o f atoms. These phases are then refined by using a parameter shift meth od to minimize R(phi). The refined phases are Fourier transformed, and a specified number of the largest peaks in the electron-density funct ion are found and used as a new trial structure. The probability of a trial structure converging to a solution appears to depend on structur al complexity and a number of refinement parameters. This procedure sh ows potential for providing fully automatic routine solutions for stru ctures in the 200-400 atom range.