SAD phasing using iodide ions in a high-throughput structural genomics environment

The Seattle Structural Genomics Center for Infectious Disease (SSGCID) focuses on the structure elucidation of potential drug targets from class A, B, and C infectious disease organisms. Many SSGCID targets are selected because they have homologs in other organisms that are validated drug targets with known structures. Thus, many SSGCID targets are expected to be solved by molecular replacement (MR), and reflective of this, all proteins are expressed in native form. However, many community request targets do not have homologs with known structures and not all internally selected targets readily solve by MR, necessitating experimental phase determination. We have adopted the use of iodide ion soaks and single wavelength anomalous dispersion (SAD) experiments as our primary method for de novo phasing. This method uses existing native crystals and in house data collection, resulting in rapid, low cost structure determination. Iodide ions are non-toxic and soluble at molar concentrations, facilitating binding at numerous hydrophobic or positively charged sites. We have used this technique across a wide range of crystallization conditions with successful structure determination in 16 of 17 cases within the first year of use (94% success rate). Here we present a general overview of this method as well as several examples including SAD phasing of proteins with novel folds and the combined use of SAD and MR for targets with weak MR solutions. These cases highlight the straightforward and powerful method of iodide ion SAD phasing in a high-throughput structural genomics environment

An analytical electron microscopic study of a pyroxene-amphibole intergrowth

Samples of a garnet granulite from the mafic border units of the Lake Chatuge, Georgia alpine peridotite body were found to contain lamellar intergrowths of a pargastic amphibole in augite having the typical appearance of an exsolution feature. Single crystal X-ray diffraction, optical, electron microprobe and conventional and analytical electron microscopic studies have provided data limiting the compositions and structures of the coexisting phases. Individual lamellae of both materials are from 0.5 to 2.0 μm in width with the lamellar interface parallel to {0 1 0}. The formulae of the minerals, as determined by a combination of electron microprobe and analytical electron microscopy, are (Na 0.1 Ca 1.0 Mg 0.6 Fe 3+ 0.3 )(Si 1.8 Al 0.2 )O 6 for the pyroxene and Na 0.7 Ca 1.9 (Mg 2.1 Fe 2+ 1.4 Fe 3+ 0.5 Ti 0.1 Cr 0.1 Al 0.8 )(Si 5.9 Al 2.1 ) O 22 (OH) 2 for the amphibole. Several other studies have described intergrowths similar to those observed in this work, in general favoring exsolution as the formation mechanism for the intergrowths. In the Lake Chatuge samples however, replacement of pyroxene by amphibole is in part indicated by continuous gradation of amphibole lamellae into amphiboles rimming the clinopyroxenes.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47333/1/410_2004_Article_BF00636515.pd