It is demonstrated that the spin/orbital magnetic momenta ratio of a BCC iron nano-polycrystalline film is quantitatively measured using ultra-high voltage scanning transmission electron microscopy and electron energy-loss spectroscopy. In the present article we first introduce electron magnetic circular dichroism (EMCD), electron counterpart of X-ray MCD (XMCD) and discuss its principle, experimental setup and technical difficulties. We then propose a new approach to EMCD measurements, taking advantage of a higher accelerating voltage of the incident electrons, which considerably enhances the outreach of the technique and enables us quantitative magnetic information to be routinely obtained using electron beams of only a few nanometres in diameter without imposing any restriction regarding the crystalline order of the specimen.
Heme is an important co-factor for several biological processes such as respiration, electron transfer and enzymatic reaction, whereas free heme is toxic because of the production of reactive oxygen species. Therefore free heme should be decomposed immediately in vivo. Heme oxygenase (HO) is an indispensable player for heme degradation. HO catalyzes the site-specific cleavage of heme porphyrin ring to produce biliverdin, ferrous iron, and carbon monoxide. This reaction requires several oxygen molecules and electrons supplied from NADPH-cytochrome P450 reductase (CPR). We have determined the crystal structure of the redox complex of CPR and HO. In this structure, FMN bound to CPR is close to heme bound to HO, whereas FAD bound to CPR is far from both of FMN and heme. Electron transfer mechanism from CPR to HO with dynamic conformation change of CPR is discussed.
During tRNA maturation tRNAHis guanylyltransferase (Thg1) catalyzes the untemplated 3'-5' addition of a G to the tRNA 5'-end. This additional guanylate provides the major identity element for histidyl-tRNA synthetase to recognize its cognate tRNAHis. Thg1 is a structural homolog of canonical 5'-3' polymerases. Here, we report the structures of Candida albicans Thg1 with its substrates tRNA, or ATP, or GTP. Two tRNAs bind to the tetrameric enzyme complex, but interaction of three subunits is required for tRNA positioning and catalytic activity. The tRNA substrate enters the Thg1 active site from the opposite direction compared to canonical 5'-3' polymerases, indicating that the directionality of nucleotide polymerization is strongly related to the directionality of substrate access. Structural and biochemical data support a reaction model of Thg1 that catalyzes reverse nucleotide addition.
Carbonaceous entities possessing tubular and spherical shapes spontaneously form a host−guest complex. This supramolecular complex, so-called a peapod, is unique among host−guest pairs in that it is assembled solely by van der Waals interactions at the concave−convex interface of sp2-carbon networks. Recently, a molecular version of this supramolecular system revealed the presence of the extremely tight association concomitantly with the dynamic motions of the guest in apolar media. An atomic-level structure of the molecular peapod is revealed by a crystallographic method with synchrotron X-ray source to show the presence of an inflection-free surface inside the tubular molecule. Enjoying rotational freedom at this smooth surface, the guest fullerene molecule rolls dynamically even in the solid state.