Isomorphous replacement combined with anomalous dispersion in the linear equations: application to a crystal containing four nonapeptide conformers

The investigation of the structure of the four conformers of the nonapeptid e described here has an additional purpose: to illustrate a method for comb ining isomorphous replacement information with anomalous dispersion informa tion within the linear equations that have found use in the analysis of mul tiple-wavelength anomalous dispersion data. In the present application, iso morphous replacement data were obtained from the replacement of naturally o ccurring S atoms in the nonapeptide with Se atoms. Only one wavelength was used for the analysis: Cu K alpha radiation. Details of the analysis are pr esented, as well as the structural results obtained. It was found that the four independent molecules in the structure have similar, but not identical , conformations. The backbones fold into predominantly alpha-helices with o ne or two 3(10)-type hydrogen bonds and have extended side chains. Three to four water molecules are associated with each of the four head-to-tail reg ions between the peptides. Optimal packing between hydrophobic surfaces may account for the existence of four molecules in an asymmetric unit.