Posts Tagged ‘Conversion’

Dynamics of phosphoinositide conversion in clathrin-mediated endocytic traffic

Vesicular carriers transport proteins and lipids from one organelle to another,
recognizing specific identifiers for the donor and acceptor membranes. Two important
identifiers are phosphoinositides and GTP-bound GTPases, which provide well-defined
but mutable labels. Phosphatidylinositol and its phosphorylated derivatives are
present on the cytosolic faces of most cellular membranes.
Reversible phosphorylation of its headgroup produces seven distinct
phosphoinositides. In endocytic traffic, phosphatidylinositol-4,5-biphosphate marks
the plasma membrane, and phosphatidylinositol-3-phosphate and
phosphatidylinositol-4-phosphate mark distinct endosomal compartments. It is unknown what sequence of changes in lipid content confers
on the vesicles their distinct identity at each intermediate step. Here we describe
‘coincidence-detecting’ sensors that selectively report the
phosphoinositide composition of clathrin-associated structures, and the use of these
sensors to follow the dynamics of phosphoinositide conversion during endocytosis.
The membrane of an assembling coated pit, in equilibrium with the surrounding plasma
membrane, contains phosphatidylinositol-4,5-biphosphate and a smaller amount of
phosphatidylinositol-4-phosphate. Closure of the vesicle interrupts free exchange
with the plasma membrane. A substantial burst of phosphatidylinositol-4-phosphate
immediately after budding coincides with a burst of
phosphatidylinositol-3-phosphate, distinct from any later encounter with the
phosphatidylinositol-3-phosphate pool in early endosomes;
phosphatidylinositol-3,4-biphosphate and the GTPase Rab5 then appear and remain as
the uncoating vesicles mature into Rab5-positive endocytic intermediates. Our
observations show that a cascade of molecular conversions, made possible by the
separation of a vesicle from its parent membrane, can label membrane-traffic
intermediates and determine their destinations.

High-spatial-resolution mapping of catalytic reactions on single particles

The critical role in surface reactions and heterogeneous catalysis of metal atoms with low coordination numbers, such as found at atomic steps and surface defects, is firmly established. But despite the growing availability of tools that enable detailed in situ characterization, so far it has not been possible to document this role directly. Surface properties can be mapped with high spatial resolution, and catalytic conversion can be tracked with a clear chemical signature; however, the combination of the two, which would enable high-spatial-resolution detection of reactions on catalytic surfaces, has rarely been achieved. Single-molecule fluorescence spectroscopy has been used to image and characterize single turnover sites at catalytic surfaces, but is restricted to reactions that generate highly fluorescing product molecules. Herein the chemical conversion of N-heterocyclic carbene molecules attached to catalytic particles is mapped using synchrotron-radiation-based infrared nanospectroscopy with a spatial resolution of 25 nanometres, which enabled particle regions that differ in reactivity to be distinguished. These observations demonstrate that, compared to the flat regions on top of the particles, the peripheries of the particles—which contain metal atoms with low coordination numbers—are more active in catalysing oxidation and reduction of chemically active groups in surface-anchored N-heterocyclic carbene molecules.

Cobalt carbide nanoprisms for direct production of lower olefins from syngas

Lower olefins—generally referring to ethylene, propylene and butylene—are basic carbon-based building blocks that are widely used in the chemical industry, and are traditionally produced through thermal or catalytic cracking of a range of hydrocarbon feedstocks, such as naphtha, gas oil, condensates and light alkanes. With the rapid depletion of the limited petroleum reserves that serve as the source of these hydrocarbons, there is an urgent need for processes that can produce lower olefins from alternative feedstocks. The ‘Fischer–Tropsch to olefins’ (FTO) process has long offered a way of producing lower olefins directly from syngas—a mixture of hydrogen and carbon monoxide that is readily derived from coal, biomass and natural gas. But the hydrocarbons obtained with the FTO process typically follow the so-called Anderson–Schulz–Flory distribution, which is characterized by a maximum C2–C4 hydrocarbon fraction of about 56.7 per cent and an undesired methane fraction of about 29.2 per cent (refs 1, 10, 11, 12). Here we show that, under mild reaction conditions, cobalt carbide quadrangular nanoprisms catalyse the FTO conversion of syngas with high selectivity for the production of lower olefins (constituting around 60.8 per cent of the carbon products), while generating little methane (about 5.0 per cent), with the ratio of desired unsaturated hydrocarbons to less valuable saturated hydrocarbons amongst the C2–C4 products being as high as 30. Detailed catalyst characterization during the initial reaction stage and theoretical calculations indicate that preferentially exposed {101} and {020} facets play a pivotal role during syngas conversion, in that they favour olefin production and inhibit methane formation, and thereby render cobalt carbide nanoprisms a promising new catalyst system for directly converting syngas into lower olefins.

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