Biomedical subjects

Qun Shen

Publications and source records attributed to Qun Shen.

4 recordsLinked to original sources

PubMed2003

Anomalous difference signal in protein crystals.

A scattering-angle-dependent formula is obtained for estimation of anomalous diffraction signals in Friedel pairs in protein crystals. In addition to the angle dependences of atomic form factor and temperature factor, a correlation form factor is introduced for the first time to take into account anomalous-scattering clusters such as disulfide bonds. Good agreements with experimental observations suggest that such correlated anomalous difference signals may be visible from measurements before a complete structural solution is obtained.

Crystallization↗

PubMed2003

Improving triplet-phase accuracy by symmetry observations in reference-beam diffraction measurements.

The reference-beam X-ray diffraction technique allows parallel measurements of many three-beam interference profiles in a modified rotating-crystal experiment. Because of the unique symmetry relations that exist in the reference-beam geometry, a complete 360 degrees rotation leads to a fourfold redundancy of any given triplet phase delta(H) involving primary reflection H, reference reflection G and coupling reflection H-G, measured in a data set. Two such redundant measurements correspond to simple duplications with the reciprocal node H going into and coming out of the Ewald sphere, while the other two correspond to the Friedel mate G-H of the coupling reflection H-G passing through the Ewald sphere in a similar fashion. It is shown that substantial differences may exist between the H and the G-H cases, with an enhanced triplet-phase interference effect in one of them. These symmetry-related observations, coupled with inverse-beam triplet measurements, would improve the phase measurement accuracy of the reference-beam diffraction technique.

Crystallography, X-Ray↗

PubMed2003

Recursive direct phasing with reference-beam diffraction.

The reference-beam diffraction technique provides a practical way to measure a large number of triplet phases in a standard oscillating-crystal diffraction experiment for protein crystals. The triplet-phase data set from such reference-beam measurements contains a unique phase-occurrence pattern that leads to a new recursive phasing algorithm for the individual structure-factor phases. Application of the new algorithm is demonstrated for tetragonal lysozyme using 7360 triplet phases measured in a reference-beam experiment with a median phase discrepancy of 45 degrees. An electron-density map obtained using this phasing algorithm and the measured triplet phases shows good agreement with the known protein structure.

Algorithms↗

PubMed2002

[Comparison studies on chemical constituents of essential oil from Ramulus Cinnamomi and Cortex Cinnamomi by GC-MS].

Essential oil from Ramulus Cinnamomi (Cinnamomum cassia Presl) and Cortex Cinnamomi were obtained by steam distillation. The chemical components of the oil were examined and compared by means of gas chromatography-mass spectrometry. 54 of the separated constitutents were identified. 10 components of these were not observed in the oil from Ramulus Cinnamomi, and 16 components of these were not observed in the oil from Cortex Cinnamomi.

Benzaldehydes↗