J. Konnert et al., Isomorphous replacement combined with anomalous dispersion in the linear equations: application to a crystal containing four nonapeptide conformers, ACT CRYST D, 55, 1999, pp. 448-457
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.