The 3.7 angstrom projection map of the glycerol facilitator GlpF: a variant of the aquaporin tetramer

GlpF, the glycerol facilitator protein of Escherichia coli, is an archetypal member of the aquaporin superfamily. To assess its structure, recombinant histidine-tagged protein was overexpressed, solubilized in octylglucoside and purified to homogeneity. Negative stain electron microscopy of solubilized GlpF protein revealed a tetrameric structure of similar to 80 Angstrom side length. Scanning transmission electron microscopy yielded a mass of 170 kDa, corroborating the tetrameric nature of GlpF. Reconstitution of GlpF in the presence of lipids produced highly ordered two-dimensional crystals, which diffracted electrons to 3.6 Angstrom resolution. Cryoelectron microscopy provided a 3.7 Angstrom projection map exhibiting a unit cell comprised of two tetramers. In projection, GlpF is similar to AQP1, the erythrocyte water channel. However, the major density minimum within each monomer is distinctly larger in GlpF than in AQP1.

Electron probing of oxygen-evolving oxide catalysts

Tzu-Hsien Shen

Water splitting is one of the cleanest ways to store energy. The production of hydrogen and oxygen gases can be utilized in fuel cells to generate electricity, power, and heat. In the water splitting process, the oxygen evolution reaction (OER), taking place at the anode, is the sluggish reaction limiting the efficiency. Electrocatalysts are used to reduce the kinetic barrier of OER. Development of high-performance and stable OER catalysts requires fundamental understanding of their electrocatalytic properties and, hence, their holistic characterization is indispensable. The objective of this thesis is to probe Co-based oxide catalysts for OER in an alkaline medium using advanced (scanning) transmission electron microscopy ((S)TEM) techniques. Starting with the surface characterization of a highly active OER oxide catalyst, Ba0.5Sr0.5Co0.8Fe0.2O3-d (BSCF), it is shown that the surfaces of the as-synthesized BSCF particles are Co/Fe rich and adopt a spinel-like structure with a reduced valence of Co ions. The Co/Fe spinel structure is further linked to the formation of the highly active Co(Fe)OOH at the BSCF surface. Further real-time probing of BSCF oxide catalysts using electrochemical liquid-cell TEM shows a switchable wetting behavior of Co-based oxide catalysts by analyzing the liquid movement around oxide particles. The interfacial interactions at the surface-electrolyte under potential cycling are correlated to electrowetting and hydrophilic surface transformation. Molecular oxygen evolution is detected qualitatively under OER conditions by identifying the O2 feature in electron energy-loss spectra. Quantification of the O2 using electron energy-loss spectroscopy is also demonstrated, pending further improvements.The work herein offers new insights into the characterization framework of oxide catalysts for OER using (S)TEM. The bulk to surface structure of the catalysts can be analyzed, and the interfacial interactions of single-particle catalysts can be probed in real-time. The findings aid in understanding the performance and stability of oxygen-evolving oxide catalysts with further prospects envisioned towards a complete quantitative probing of electrocatalytic reactions in real-time. Ultimately, quantitative characterization in electrochemical liquid-phase (S)TEM can provide understanding of surface-active sites and reaction kinetics of oxygen evolution catalysts.

EPFL2022

Are the light-harvesting I complexes from Rhodospirillum rubrum arranged around the reaction center in a square geometry?

Jacques Dubochet, Henning Paul-Julius Stahlberg, Horst Vogel

The basic photosynthetic unit contg. the reaction center and the light-harvesting I complex (RC-LHI) of the purple non-sulfur bacterium Rhodospirillum rubrum was purified and reconstituted into two-dimensional (2D) membrane crystals. Transmission electron microscopy using conventional techniques and cryoelectron microscopy of the purified single particles and of 2D crystals yielded a projection of the RC-LHI complex at a resoln. of at least 1.6 nm. In this projection the LHI ring appears to have a square symmetry and packs in a square crystal lattice. The square geometry of the LHI ring was obsd. also in images of single isolated particles of the RC-LHI complex. However, although the LHI units are packed identically within the crystal lattice, a new rotational anal. developed here showed that the reaction centers take up one of four possible orientations within the ring. This fourfold disorder supports our interpretation of a square ring symmetry and suggests that a hitherto undetected component may be present within the photosynthetic unit. (c) 1998 Academic Press. [on SciFinder (R)]

1998

Electron probing of oxygen-evolving oxide catalysts

Tzu-Hsien Shen

EPFL2022