Carbon monoxide as an intrinsic ligand to iron in the active site of the iron-sulfur-cluster-free hydrogenase H2-forming methylenetetrahydromethanopterin dehydrogenase as revealed by infrared spectroscopy

The iron-sulfur-cluster-free hydrogenase Hmd (H(2)-forming methylenetetrahydromethanopterin dehydrogenase) from methanogenic archaea has recently been found to contain one iron associated tightly with an extractable cofactor of yet unknown structure. We report here that Hmd contains intrinsic CO bound to the Fe. Chemical analysis of Hmd revealed the presence of 2.4 +/- 0.2 mol of CO/mol of iron. Fourier transform infrared spectra of the native enzyme showed two bands of almost equal intensity at 2011 and 1944 cm(-)(1), interpreted as the stretching frequencies of two CO molecules bound to the same iron in an angle of 90 degrees . We also report on the effect of extrinsic (12)CO, (13)CO, (12)CN(-), and (13)CN(-) on the IR spectrum of Hmd.     

Radical‐cured block copolymer‐modified thermosets

Poly(ethylene‐alt‐propylene)‐b‐poly(ethylene oxide) (PEP‐PEO) diblock copolymers were synthesized and added at 4 wt % to 2,2‐bis[4‐(2‐hydroxy‐3‐methacryloxypropoxy)phenyl]propane (BisGMA), a monomer that cures using free radical chemistry. In separate experiments, poly(ethylene glycol) dimethacrylate (PEGDMA) was combined as a secondary monomer with BisGMA and the monomers were loaded with 4 wt % PEP‐PEO. The diblock copolymers self‐assembled into well‐dispersed spherical micelles with PEP cores and PEO coronas. No appreciable change in the final extent of cure of the thermosets was caused by the addition of diblock copolymer, except in the case of BisGMA, where the addition of the block copolymer increased extent of cure by 12%. Furthermore, the extent of cure was increased by 29% and 37% with the addition of 25 and 50 wt % PEGDMA, respectively. Elastic modulus and fracture resistance were also determined, and the values indicate that the addition of block copolymers does not significantly toughen the thermoset materials. This finding is surprising when compared with the large increase in fracture resistance seen in block copolymer‐modified epoxies, and an explanation is proposed. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Determination of the Crystal Structure of a New Polymorph of Theophylline

A new approach to crystal structure determination, combining crystal structure prediction and transmission electron microscopy, was used to identify a potential new crystal phase of the pharmaceutical compound theophylline. The crystal structure was determined despite the new polymorph occurring as a minor component in a mixture with Form II of theophylline, at a concentration below the limits of detection of analytical methods routinely used for pharmaceutical characterisation. Detection and characterisation of crystallites of this new form were achieved with transmission electron microscopy, exploiting the combination of high magnification imaging and electron diffraction measurements. A plausible crystal structure was identified by indexing experimental electron‐diffraction patterns from a single crystallite of the new polymorph against a reference set of putative crystal structures of theophylline generated by global lattice energy minimisation calculations.     

The evolution of farnesoid X,vitamin D,and pregnane X receptors: insights from the green-spotted pufferfish (<Emphasis Type="Italic">Tetraodon nigriviridis</Emphasis>) and other non-mammalian species

Background  

The farnesoid X receptor (FXR), pregnane X receptor (PXR), and vitamin D receptor (VDR) are three closely related nuclear hormone receptors in the NR1H and 1I subfamilies that share the property of being activated by bile salts. Bile salts vary significantly in structure across vertebrate species, suggesting that receptors binding these molecules may show adaptive evolutionary changes in response. We have previously shown that FXRs from the sea lamprey (Petromyzon marinus) and zebrafish (Danio rerio) are activated by planar bile alcohols found in these two species. In this report, we characterize FXR, PXR, and VDR from the green-spotted pufferfish (Tetraodon nigriviridis), an actinopterygian fish that unlike the zebrafish has a bile salt profile similar to humans. We utilize homology modelling, docking, and pharmacophore studies to understand the structural features of the Tetraodon receptors.     

Targeted disruption of the CCR5 gene in human hematopoietic stem cells stimulated by peptide nucleic acids

Peptide nucleic acids (PNAs) bind duplex DNA in a sequence-specific manner, creating triplex structures that can provoke DNA repair and produce genome modification. CCR5 encodes a chemokine receptor required for HIV-1 entry into human cells, and individuals carrying mutations in this gene are resistant to HIV-1 infection. Transfection of human cells with PNAs targeted to the CCR5 gene, plus donor DNAs designed to introduce stop codons mimicking the naturally occurring CCR5-delta32 mutation, produced 2.46% targeted gene modification. CCR5 modification was confirmed at the DNA, RNA, and protein levels and was shown to confer resistance to infection with HIV-1. Targeting of CCR5 was achieved in human CD34(+) hematopoietic stem cells (HSCs) with subsequent engraftment into mice and persistence of the gene modification more than four months posttransplantation. This work suggests a therapeutic strategy for CCR5 knockout in HSCs from HIV-1-infected individuals, rendering cells resistant to HIV-1 and preserving immune system function.     

Application of a ligand-based theoretical approach to derive conversion paths and ligand conformations in CYP11B2-mediated aldosterone formation

The biosynthesis of the mineralocorticoid hormone aldosterone involves a multistep hydroxylation of 11-deoxycorticosterone at the 11- and 18-positions, resulting in the formation of corticosterone and 18-hydroxycorticosterone, the final precursor of aldosterone. Two members of the cytochrome P450 11B family, CYP11B1 and CYP11B2, are known to catalyze these 11- and 18-hydroxylations, however, only CYP11B2 can oxidize 18-hydroxycorticosterone to aldosterone. It is unknown what sequence of hydroxylations leads to the formation of 18-hydroxycorticosterone. In this study we have investigated which of the possible conversion paths towards formation of 18-hydroxycorticosterone and aldosterone are most likely from the ligand perspective. Therefore, we combined quantum mechanical investigations on the steroid conformations of 11-deoxycorticosterone and its ensuing reaction intermediates with Fukui indices calculations to predict the reactivity of their carbon atoms for an attack by the iron-oxygen species. Both F(-) and F(0) were calculated to account for different mechanisms of substrate conversion. We show which particular initial conformations of 11-deoxycorticosterone and which conversion paths are likely to result in the successful synthesis of aldosterone, and thereby may be representative for the mechanism of aldosterone biosynthesis by CYP11B2. Moreover, we found that the most likely path for aldosterone synthesis coincides with the substrate conformation proposed in an earlier publication. To summarize, we show that on a theoretical and strictly ligand-directed basis only a limited number of reaction paths in the conversion of 11-deoxycorticosterone to aldosterone is possible. Despite its theoretical nature, this knowledge may help to understand the catalytic function of CYP11B1 and CYP11B2.     

Electric field-induced nucleation and growth of focal-conic and stripe domains in a smectic A liquid crystal

We have studied the electric field-induced first order transition from a homeotropic smectic A structure to a polydomain texture that occurs through the nucleation of toric focal-conic domains (TFCDs). The process involves two steps: first, nucleation of TFCDs of a size larger than a critical radius a*, and then a steady growth of TFCDs to a secondary critical radius a**, when surface anchoring effects become dominant and cause a transition from a circular TFCD to an elongated stripe domain (SD). Studies were performed for pure smectic A materials and for smectic A doped with kunipia nanoparticles. Non-destructive 3D imaging with fluorescence confocal polarizing microscopy showed that field-induced TFCDs can nucleate in the smectic A bulk. Clay particles reduce the energy barrier for nucleation as they distort the smectic A layers and thus increase the ground state energy. Simple elastic models of the TFCD and SD allow us to describe the qualitative features of the observed phenomena.