Interpretation of electron diffraction data from organic microcrystals: a theoretical perspective.
The current framework for the quantitative analysis of transmission electron diffraction data from organic microcrystals is discussed. The appropriate formulation depends upon the crystal perfection: solution-grown material tends to be nearly perfect with regular elastic bends whilst the epitaxial samples are usually considerably less perfect with highly irregular bends. It is possible to obtain Bragg intensity data which are near-kinematical and thus amenable to ab initio structure analysis. Thereafter, the further refinement of the model proceeds with a consideration of dynamical and crystal bending effects. If both of these are small it is a good approximation to analyse them independently rather than use the correct description of dynamical scattering from a deformed foil. The analysis of three-dimensional data sets from tilt series is considered. Although the technique has been successfully demonstrated in several instances there are still details of the analysis that need further clarification.
Radiation damage and structural studies: Halogenated phthalocyanines
Abstract Radiation damage makes it more difficult to accomplish structural studies of beam-sensitive materials. Nevertheless, we report here a 500 kV high-resolution study of halogenated phthalocyanines. Epitaxially grown molecular crystals of fully brominated and fully chlorinated phthalocyanines were found to be isomorphous and had only the α-type polymorph structure. Despite the sensitivity of these crystals to beam damage, the characteristics of thin film growth and details of structural defects could be determined, provided that molecular image resolution was available. The effect of electron beam damage was a degradation of crystallinity which varied considerably according to local morphology and depended on the ease with which the displaced halogen could diffuse away from the parent molecule and thereby avoid recombination. Our findings demonstrate the feasibility of similar structural studies of other beam-sensitive materials.
Nuclear sphingolipid metabolism.
Nuclear lipid metabolism is implicated in various processes, including transcription, splicing, and DNA repair. Sphingolipids play roles in numerous cellular functions, and an emerging body of literature has identified roles for these lipid mediators in distinct nuclear processes. Different sphingolipid species are localized in various subnuclear domains, including chromatin, the nuclear matrix, and the nuclear envelope, where sphingolipids exert specific regulatory and structural functions. Sphingomyelin, the most abundant nuclear sphingolipid, plays both structural and regulatory roles in chromatin assembly and dynamics in addition to being an integral component of the nuclear matrix. Sphingosine-1-phosphate modulates histone acetylation, sphingosine is a ligand for steroidogenic factor 1, and nuclear accumulation of ceramide has been implicated in apoptosis. Finally, nuclear membrane-associated ganglioside GM1 plays a pivotal role in Ca(2+) homeostasis. This review highlights research on the factors that control nuclear sphingolipid metabolism and summarizes the roles of these lipids in various nuclear processes.