Topotactic Synthesis of Porous Cobalt Ferrite Platelets from a Layered Double Hydroxide Precursor and Their Application in Oxidation Catalysis.

Monocrystalline, yet porous mosaic platelets of cobalt ferrite, CoFe2 O4 , can be synthesized from a layered double hydroxide (LDH) precursor by thermal decomposition. Using an equimolar mixture of Fe2+ , Co2+ , and Fe3+ during co-precipitation, a mixture of LDH, (FeII CoII )2/3 FeIII1/3 (OH)2 (CO3 )1/6 ⋅m H2 O, and the target spinel CoFe2 O4 can be obtained in the precursor. During calcination, the remaining FeII fraction of the LDH is oxidized to FeIII leading to an overall Co2+ :Fe3+ ratio of 1:2 as required for spinel crystallization. This pre-adjustment of the spinel composition in the LDH precursor suggests a topotactic crystallization of cobalt ferrite and yields phase pure spinel in unusual anisotropic platelet morphology. The preferred topotactic relationship in most particles is [111]Spinel ∥[001]LDH . Due to the anion decomposition, holes are formed throughout the quasi monocrystalline platelets. This synthesis approach can be used for different ferrites and the unique microstructure leads to unusual chemical properties as shown by the application of the ex-LDH cobalt ferrite as catalyst in the selective oxidation of 2-propanol. Compared to commercial cobalt ferrite, which mainly catalyzes the oxidative dehydrogenation to acetone, the main reaction over the novel ex-LDH cobalt is dehydration to propene. Moreover, the oxygen evolution reaction (OER) activity of the ex-LDH catalyst was markedly higher compared to the commercial material.

Pyrrolizidine alkaloid biosynthesis in Phalaenopsis orchids: developmental expression of alkaloid-specific homospermidine synthase in root tips and young flower buds.

Pyrrolizidine alkaloids (PAs) are typical compounds of plant secondary metabolism and are believed to be part of the plant's chemical defense. Within the monocotyledonous plants, PAs have been described in only a few genera, mainly orchids, including Phalaenopsis. Because phylogenetic analyses suggest an independent origin of PA biosynthesis within the monocot lineage, we have analyzed the developmentally regulated expression of homospermidine synthase (HSS), the first pathway-specific enzyme of PA biosynthesis, at the cell level. HSS is expressed in the tips of aerial roots exclusively in mitotically active cells. Raphide crystal idioblasts present within the root apical meristem do not show HSS expression. In addition, young flower buds, but not mature flowers, express HSS and have been shown by tracer feeding experiments to be able to catalyze PAs. This second site of PA biosynthesis ensures high concentrations of PAs in the reproductive structures of the Phalaenopsis flower, even after the flower opens. Thus, in spite of its identical function in PA biosynthesis, HSS shows in Phalaenopsis a completely different spatial and developmental expression pattern in comparison to other PA-producing species. These results show that the proverbial diversity of plant secondary metabolism is not just a matter of structural diversity, but is also multifaceted in terms of pathway regulation and expression.

Polyphyletic origin of pyrrolizidine alkaloids within the Asteraceae. Evidence from differential tissue expression of homospermidine synthase.

The evolution of pathways within plant secondary metabolism has been studied by using the pyrrolizidine alkaloids (PAs) as a model system. PAs are constitutively produced by plants as a defense against herbivores. The occurrence of PAs is restricted to certain unrelated families within the angiosperms. Homospermidine synthase (HSS), the first specific enzyme in the biosynthesis of the necine base moiety of PAs, was originally recruited from deoxyhypusine synthase, an enzyme involved in the posttranslational activation of the eukaryotic initiation factor 5A. Recently, this gene recruitment has been shown to have occurred several times independently within the angiosperms and even twice within the Asteraceae. Here, we demonstrate that, within these two PA-producing tribes of the Asteraceae, namely Senecioneae and Eupatorieae, HSS is expressed differently despite catalyzing the same step in PA biosynthesis. Within Eupatorium cannabinum, HSS is expressed uniformly in all cells of the root cortex parenchyma, but not within the endodermis and exodermis. Within Senecio vernalis, HSS expression has been previously identified in groups of specialized cells of the endodermis and the adjacent root cortex parenchyma. This expression pattern was confirmed for Senecio jacobaea as well. Furthermore, the expression of HSS in E. cannabinum is dependent on the development of the plant, suggesting a close linkage to plant growth.

Cell-specific expression of homospermidine synthase, the entry enzyme of the pyrrolizidine alkaloid pathway in Senecio vernalis, in comparison with its ancestor, deoxyhypusine synthase.

Pyrrolizidine alkaloids (PAs) are constitutive plant defense compounds with a sporadic taxonomic occurrence. The first committed step in PA biosynthesis is catalyzed by homospermidine synthase (HSS). Recent evidence confirmed that HSS evolved by gene duplication from deoxyhypusine synthase (DHS), an enzyme involved in the posttranslational activation of the eukaryotic translation initiation factor 5A. To better understand the evolutionary relationship between these two enzymes, which are involved in completely different biological processes, we studied their tissue-specific expression. RNA-blot analysis, reverse transcriptase-PCR, and immunolocalization techniques demonstrated that DHS is constitutively expressed in shoots and roots of Senecio vernalis (Asteraceae), whereas HSS expression is root specific and restricted to distinct groups of endodermis and neighboring cortex cells located opposite to the phloem. All efforts to detect DHS by immunolocalization failed, but studies with promoter-beta-glucuronidase fusions confirmed a general expression pattern, at least in young seedlings of tobacco (Nicotiana tabacum). The expression pattern for HSS differs completely from its ancestor DHS due to the adaptation of HSS to the specific requirements of PA biosynthesis.