Syntheses of Bile Pigments. Part 16. Synthesis of a vinyl-substituted 2,3-Dihydrobilinedione: Possible role of this new class of bile pigments in phycobilin biosynthesis

The total synthesis of racemic cis-2,3,18¹, 18², -tetrahydroprotobiliverdin IXα dimethyl ester ( 19b ), which is identical with the dimethyl ester of rac- 4 , is described (Scheme 2). Under virtually neutral conditions, in solution, this bile pigment isomerized within a few min to racemic Z-phycocyanobilin dimethyl ester (rac- 5b ). Likewise, acid-catalyzed allyl rearrangement of 3-vinyl-substituted cis- and trans-2,3-dihydrodipyrrin-1(10H)-ones 11c and 13c , respectively, yielded the corresponding ethylidene derivatives. In this case, however, the E-isomer was formed stereo selectively from both substrates. The above results prove that, if protobiliverdin IXα ( 2 ) is transformed enzymatically to its 2,3,18¹, 18²-tetrahydro derivative, the latter would isomerize spontaneously to phycocyanobi-lin. The biosynthesis of bacteriochlorophyll a and b from a common precursor bearing a vinyl group at C(8)may be straightforwardly explained in the same way.     

Mutationsinvariante Unteralgebren von Jordan-Algebren

Ohne Zusammenfassung     

Extreme value theory with operator norming

A new approach to extreme value theory is presented for vector data with heavy tails. The tail index is allowed to vary with direction, where the directions are not necessarily along the coordinate axes. Basic asymptotic theory is developed, using operator regular variation and extremal integrals. A test is proposed to judge whether the tail index varies with direction in any given data set.     

Selective Oxygen and Hydrogen Functionalization of the h-BN/Rh(111) Nanomesh

We present detailed studies on the covalent adsorption of molecular oxygen and atomic hydrogen on the hexagonal boron nitride (h-BN) nanomesh on Rh(111). The functionalization of this two-dimensional (2D) material was investigated under ultra-high vacuum conditions using synchrotron radiation-based in situ high-resolution X-ray photoelectron spectroscopy, temperature-programmed X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy. We are able to provide a deep insight into the adsorption behavior and thermal stability of oxygen and hydrogen on h-BN/Rh(111). Oxygen functionalization was achieved via a supersonic molecular beam while hydrogen functionalization was realized using an atomic hydrogen source. Adsorption of the respective species was observed to occur selectively in the pores of h-BN leading to spatially defined modification of the 2D layer. The adsorption of the observed molecular oxygen species was found to be an activated process that requires high-energy oxygen molecules. Upon heating to 700 K, oxygen functionalization was observed to be almost reversible except for small amounts of boron oxides evolving due to the reaction of oxygen with the 2D material. Hydrogen functionalization of h-BN/Rh(111) was fully reversed upon heating to about 640 K.