Wild-type NikR binds to its promoter DNA in the presence of Ni2+ ions, and a number of other divalent metal ions such as Cu2+, Zn2+, Co2+, Mn2+, and Cd2+ can also induce protein-DNA complex formation. However, NikR does not bind to DNA in the presence of 50 μM UO22+. The <triple> mutant NikR′ binds to DNA neither in the absence of metal ions nor in the presence of Ni2+ ions, but it forms a protein-DNA complex in the presence of UO22+. In comparison to NikR, the metal selectivity of NikR′ has been altered. Experiments with other metal ions show that the mutant protein only forms the protein-DNA complex in the presence of the uranyl cation while Ni2+, Zn2+, Co2+, Cu2+, Cd2+, Mn2+, and Fe2+ ions do not result in any observable complex formation. Attempts to load NikR with uranyl or NikR′ with Ni2+ did not yield any observable metal binding. Thus, this mutant NikR′ shows a uranyl-specific DNA-binding ability.
Wednesday, September 30, 2009
Sunday, September 27, 2009
The Tony Clayton’s website contains a very useful list of metals used in coins and medals. In particular, I have learned that niobium is used as coinage metal. In 2003, Münze Österreich pioneered the use of niobium for coin manufacturing, issuing a bimetallic €25 coin. According to this review,
The colouring <of the niobium insert> is made by a so called anodic oxidation of the material. With this treatment, by electrochemical processing a very thin niobium oxide layer is formed under controlled conditions. By refraction of light in the oxide layer so called interference colours are created which gives the colouring of the niobium. Depending on the processing parameters, the thickness of the oxide layer can be very well controlled, and gives the niobium its noble appearance. Depending on the thickness of the layer different colours are producible.For instance, Latvian bimetallic Coin of Time (struck by Münze Österreich) consists of beautiful blue niobium centre enclosed in an outer silver ring. The obverse of the coin features the heraldic rose and the tiny Gothic script letters ℌ and ℜ standing for Heinrich Rose (1795—1864), discoverer of niobium.
Wednesday, September 23, 2009
Monday, September 21, 2009
Wednesday, September 16, 2009
With our investigations, it was possible for the first time to confirm beyond all doubt the structure suggested by Tschesche and Wulff for digitonin by means of modern NMR techniques, and to assign all proton and carbon resonances.Now I was not able to get to the full text of Tschesche and Wulff, but at least their abstract contains the German name “3[β-D-Glucopyranosyl(I)(1→3Galakt.II)-β-D-galaktopyranosyl(II)(1→2Gluc.III)-β-D-xylopyranosyl(1→3Gluc.III)-β-D-glucopyranosyl(III) (1→4Galakt.IV)-β-D-galaktopyranosyl(IV)(1→3-Digitog.)]5α,20βF,25α Spirostantriol(2α,3β,15β)”, which kind of confirms 20β configuration. (The default configuration of spirostan is 20α.)
I guess this still does not answer what the “correct” structure of digitonin is. All we can say that Muhr et al. reported the structure (a).
Friday, September 11, 2009
Here’s something one doesn’t see in a chemistry textbook. The recent perspective paper in Nature Chemistry deals with a distinct class of electron-pair bonding called “charge-shift” (CS) bonding, which exists alongside classical covalent and ionic bonding. And in not-so exotic molecules.
<A> striking example is the difference between H2 and F2; two homonuclear bonds that by all criteria should be classified as covalent bonds, but exhibit fundamental differences. Consider the energy curves (Fig. 1) of the two bonds calculated recently. Figure 1a shows that the H—H bond is indeed covalent; its covalent structure accounts for most of the bonding energy (relative to the ‘exact’ curve). By contrast, for the F—F bond in Fig. 1b, the covalent structure is entirely repulsive, and what determines the bonding energy and the equilibrium distance is the covalent–ionic mixing. This mixing leads to a resonance energy stabilization, which we have termed the ‘charge-shift resonance energy’ (RECS). Thus, despite their apparent similarity, the two bonds are very different; whereas the H—H bond is a true covalent bond, the F—F bond is a CS bond that is completely determined by the RECS quantity.