Biaxial modeling of the structure of the chevron interface in smectic liquid crystals

We have included the inherent molecular biaxiality of the smectic C phase in a model of the chevron structure. This molecular biaxiality is related to a hindered rotation about the molecular long axis which for chiral, polar molecules induces a spontaneous polarization. Through the coupling between biaxiality and the smectic cone angle, continuity of the molecular distribution at the chevron interface leads to changes in the cone angle. Under certain approximations we are able to find analytic expressions for the chevron structure and consequently estimate the width of the chevron interface. There are in fact two correlation lengths which govern variations in the cone angle and the biaxiality.     

Intracellular targets of cyclin-dependent kinase inhibitors: identification by affinity chromatography using immobilised inhibitors

BACKGROUND: Chemical inhibitors of cyclin-dependent kinases (CDKs) have great therapeutic potential against various proliferative and neurodegenerative disorders. Olomoucine, a 2,6,9-trisubstituted purine, has been optimized for activity against CDK1/cyclin B by combinatorial and medicinal chemistry efforts to yield the purvalanol inhibitors. Although many studies support the action of purvalanols against CDKs, the actual intracellular targets of 2,6, 9-trisubstituted purines remain unverified. RESULTS: To address this issue, purvalanol B (95. ) and an N6-methylated, CDK-inactive derivative (95M. ) were immobilized on an agarose matrix. Extracts from a diverse collection of cell types and organisms were screened for proteins binding purvalanol B. In addition to validating CDKs as intracellular targets, a variety of unexpected protein kinases were recovered from the 95. matrix. Casein kinase 1 (CK1) was identified as a principal 95. matrix binding protein in Plasmodium falciparum, Leishmania mexicana, Toxoplasma gondii and Trypanosoma cruzi. Purvalanol compounds also inhibit the proliferation of these parasites, suggesting that CK1 is a valuable target for further screening with 2,6,9-trisubstituted purine libraries. CONCLUSIONS: That a simple batchwise affinity chromatography approach using two purine derivatives facilitated isolation of a small set of highly purified kinases suggests that this could be a general method for identifying intracellular targets relevant to a particular class of ligands. This method allows a close correlation to be established between the pattern of proteins bound to a small family of related compounds and the pattern of cellular responses to these compounds.     

Shearing of planar smectic C chevrons

We have theoretically investigated chevron formation in smectic C materials and the transformation of this chevron structure to a tilted layer structure as the cell is sheared. We find a series of transition temperatures at which the behaviour of the cell critically changes. As the cell is cooled from the smectic A phase past the first critical temperature there is a second order transition which forms two tilted layer states with lower energy than the smectic A bookshelf structure. Although these low energy tilted structures exist the bookshelf structure is the stable state for zero shear. However, upon further cooling this bookshelf structure becomes unstable to the formation of a chevron state. Now when the cell is sheared the chevron structure smoothly transforms into the tilted layer structure. As each further critical temperature is passed an additional multiple chevron solution is formed which although a high energy, unstable state may be observed transiently. For sufficiently low temperatures the transition from chevron to tilted layer becomes first order. This first order transition occurs as the chevron interface merges with the surface alignment region to form the tilted layer structure. Received 28 December 1998 and Received in final form 8 April 1999     

镝氢化物掺杂钕铁硼稀土永磁体的研究

发现了采用减量化重稀土镝添加高效制备块状超高矫顽力稀土永磁体新型方法。在钕铁硼(Nd-Fe-B)系稀土永磁材料中以DyH3的形式掺入稀土镝(Dy)元素,在不同烧结温度1020,1040,1050,1060,1080℃下进行烧结,制备成一系列DyH3掺杂(PrNd)30Fe69B磁体和未掺杂的(PrNd)30Fe69B磁体。研究剩磁、矫顽力与烧结温度及显微结构之间的关系。结果显示:随着温度的升高,剩磁不断上升;矫顽力在烧结温度1020~1050℃时达到最大。重稀土Dy添加可有效增加Nd-Fe-B磁体的矫顽力。采用扫描电镜和能谱仪研究了不同烧结温度对磁体晶粒尺寸、Dy在晶粒内与晶界处成分分布的影响,发现温度超过1060℃时,晶粒会过度长大,使其矫顽力随之下降。稍低的烧结温度1020~1050℃可以获得Dy分布于晶粒呈"壳"状结构并使得Dy尽量分布于晶界处,获得了制备高矫顽力块体稀土永磁较满意的Dy减量化方法。     

Preparation and characterization of MgB2 superconductor

The MgB₂ superconductor, synthesized using solid-state and liquid-phase sintering methods, have been characterized for various properties. The upper critical field, irreversibility line and critical current density have been determined using magnetization data. The current-voltage characteristics recorded under an applied magnetic field revealed the existence of vortex glass transition. The surface analysis using X-ray photoelectron spectroscopy shows that MgB₂ is sensitive to atmospheric degradation.     

Regiospecific characterisation of the triacylglycerols in animal fats using high performance liquid chromatography-atmospheric pressure chemical ionisation mass spectrometry

High performance liquid chromatography-atmospheric pressure chemical ionisation mass spectrometry (HPLC-APCI MS) was applied to the characterisation of triacylglycerols (TAGs) in animal fats. The major TAGs in four fats (beef, chicken, lamb and pork) were identified and positional isomers assigned according to their APCI mass spectra. Beef and lamb fat TAGs were confirmed as containing higher proportions of saturated fatty acids compared with those of chicken and pork. HPLC-APCI MS was also shown to be of value in providing regiospecific information for the fatty acids in individual TAG species. For example, beef and lamb fat were shown to contain both cis- and trans-isomers of the 18:1 fatty acid, whilst chicken and pork contained only the cis-isomer. When the position of fatty acid substitution was determined from the APCI spectra, whilst the cis- 18:1 was predominantly found in the 2-position of the TAG, the trans-18:1 showed a preference for the 1/3-position. Similarly, it was confirmed that although the 2-position of beef, chicken and lamb fat TAGs was dominated by unsaturated fatty acids, in pork fat, a characteristically high proportion of palmitic acid was seen in this position. The TAGs identified compared well with those reported previously. The distributions of 2-position fatty acids seen in lamb and pork fat compared favourably with those obtained by the more traditional method of lipase degradation. Although the distributions for chicken and beef showed some discrepancies, these can be attributed to weaknesses in the quantification procedure or the specificity of the lipase. Overall, the technique of HPLC-APCI MS has been shown to be very powerful for the regiospecific analysis of animal fats.