NEW ALGORITHMS FOR INTRACTABLE DIRECT METHODS PROBLEMS

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This project identifies a number of strategies that will be used to increase the power of direct phasing methods for applications to larger molecular structures than are currently determined in a routine fashion. In addition to the time efficient improvements to a number of computational algorithms described in this proposal, insights will be provided with regard to how the availability of more accurate and higher resolution data may or may not be advantageous to strengthening various phasing strategies. Native X-ray data will be recorded and examined for an appropriate base of both solved and unknown macromolecular structures to test these new methods. Computer graphics techniques will play an important role in strengthening the phasing procedures used for these large macromolecular structures and graphics software will be developed to help recognize and extend the structural patterns that exist in marginally phased electron density maps. On the experimental side, preliminary studies indicate that low temperature investigations of single native crystals can provide direct phase information that was previously thought to be unavailable. An analysis using a room temperature and a liquid nitrogen temperature data set, recorded from two different crystals of the orthorhombic gramicidin structure, measured on two different X-ray diffractometers, demonstrates that the precision required of the data to successfully apply this method is not insurmountable. Analysis of several difficult structure determinations, including that of the gramicidin data, moreover, shows that large blocks of phases can be secured with a minimum of effort provided that as few as a dozen well-chosen and identifiable pivotal invariants upon which the structure solution depends can be unambiguously determined. These results are an encouraging indication that complex crystal structures containing as many as 300-500 atoms ban be determined with far less difficulty than is currently experienced for structures containing 100 atoms or less.