Optimization in S-SAD phasing - difference between solved and unsolved structure

Development of X-ray diffraction technologies have made de novo phasing of protein structures by single-wavelength anomalous dispersion by sulphur (S-SAD) more common. As anomalous differences in the sulphur atomic factors are in the order of errors of measurement, careful intensity reading and data processing are crucial. S-SAD was used for de novo phasing of a small 12 kDa protein with 4 sulphur atoms per molecule at 2.3 Å, where the data did not enable a straightforward structure solution. Data processing was performed using XDS [1] and scaling using XSCALE. The sulphur substructure was determined by SHELXD [2] and phases were obtained from SHELXE [2]. Both algorithms strongly depend on input parameters and default values did not lead to the correct phases. Therefore a systematic search of optimal values of several parameters was used to find a solution. This method helped to confirm sulphur substructure and to differentiate the handedness of the solutions. Moreover, a script for comfortable conversion of SHELX outputs to MTZ format was developed, using programmes included in the CCP4 package [3]. The previously unsolvable protein structure was successfully resolved with the described procedure. This work was supported by the Grant Agency of the Czech Technical University in Prague, (SGS13/219/OHK4/3T/14), the Czech Science Foundation (P302/11/0855), project BIOCEV CZ.1.05/1.1.00/02.0109 from the ERDF.

Statistical accuracy test in measurements of circular dichroism

A collection of one hundred circular dichroism measurements (in acetonitrile) of the model cyclohexapeptide cyclo(glycyl-L-phenylalanyl-L-leucyl-glycyl-L-leucyl-L-phenylalanyl) has been evaluated using standard statistical methods. The reproducibility of the experimental intensity values is discussed with respect to the sample concentration and the particular regions of the CD curve. The maximum probable error in the intensity reading varies with wavelength from ±20 to ±10 000 deg cm2 dmol-1 in the spectral range from 274 to 196 nm, i.e. by 5-15% of the observed intensity.

Filter Flame Photometer for Determination of Boron in Plants

A filter photometer employing a manually tilted interference filter provides intensity readings at a maximum and adjacent minimum in the BO2 emission spectrum in the air-hydrogen flame, thus eliminating the need for a separate background intensity reading. Diol chelate-extracted boron of plant samples measured by this flame method is compared with that measured by fluoroborate-methylene blue colorimetry.