Single-wavelength anomalous

The thiazole biosynthetic enzyme (THIl), produced using heterologous expression in bacteria, is unexpectedly bound to 2-carboxylate-4-methyl-5-/3-(ethyladenosine 5-diphosphate)thiazole, a potential intermediate of thiazole biosynthesis in eukaryotes. The 3-D structure of this complex was determined by single-wavelength anomalous X-ray diffraction to l.bA resolution <2006JBC30957>. 

The tunability of synchrotron radiation allows for data collection at or near the x-ray absorption edge of anomalous scatterers present in the protein or crystal to provide experimental phase information. Using techniques such as multi-wavelength anomalous dispersion (MAD) and single-wavelength anomalous diffraction (SAD) researchers are now able to solve macromolecular structures in a matter of days or weeks, a process that required months, or even years, a decade ago. 

Chen, L., Rose, J. R, Breslow, E., Yang, D., Chang, W. R., Furey, W. R, Sax, M., and Wang, B.C. (1991). Crystal structure of a bovine neurophysin II dipeptide complex at 2.8 A determined from the single-wavelength anomalous scattering signal of an incorporated iodine atom., Proc. Natl. Acad. Sci. U.S.A., 88, 4240-4244. 

OAD scattering One or single wavelength anomalous dispersion experiment... 

Karle, J. Triplet phase invariants from single isomorphous replacement or one-wavelength anomalous dispersion data, given heavy-atom information. Acta Cryst. A42, 246-253 (1986). 

Currently, use of selenium methionine, recombinant protein crystals, or crystals of wild type protein into which have been introduced heavy atom anomalous scatterers, has made single and multiple wavelength anomalous scattering the method of choice for phase determination. In addition, the ways it can be used, and the increasing opportunities for its application are dramatically expanding its popularity. 

Crystals from selenium-methionine- and/ or selenium-cysteine-labeled proteins can be studied by multi-wavelength anomalous dispersion (MAD) phasing techniques that can facilitate the solution of an X-ray crystal structure from a single crystal form [11]. However, if in vivo expression systems are used to prepare selenium-labeled proteins, amino acid metabolism and the toxicity of Se-methionine can result in low protein yields and low incorporation rates. 

Liu Z-J, Vysotski E, Rose J, Lee J and Wang B. De novo structure determination of the photoprotein obelin at 1.7 angstrom resolution using single wavelength sulfur anomalous scattering data. Protein Sci 2000 9 2085-93. 

X-Ray diffraction from single crystals is the most direct and powerful experimental tool available to determine molecular structures and intermolecular interactions at atomic resolution. Monochromatic CuKa radiation of wavelength (X) 1.5418 A is commonly used to collect the X-ray intensities diffracted by the electrons in the crystal. The structure amplitudes, whose squares are the intensities of the reflections, coupled with their appropriate phases, are the basic ingredients to locate atomic positions. Because phases cannot be experimentally recorded, the phase problem has to be resolved by one of the well-known techniques the heavy-atom method, the direct method, anomalous dispersion, and isomorphous replacement.1 Once approximate phases of some strong reflections are obtained, the electron-density maps computed by Fourier summation, which requires both amplitudes and phases, lead to a partial solution of the crystal structure. Phases based on this initial structure can be used to include previously omitted reflections so that in a couple of trials, the entire structure is traced at a high resolution. Difference Fourier maps at this stage are helpful to locate ions and solvent molecules. Subsequent refinement of the crystal structure by well-known least-squares methods ensures reliable atomic coordinates and thermal parameters. 

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