Stereoselective Ring Opening of Electronically Excited Cyclohexa-2, 4-dienones: Cause and effect

The two conformers of a cyclohexa-2, 4-dienone with different substituents at C(6) on irradiation are believed to undergo ring opening stereospecifically affording a mixture of two configurationally isomeric diene-ketenes (and descendents thereof)- Exceptions are generally found for those dienones with one C and one O substituent or even with two C substituents, if one of them carries a polar group at a site able to interact through space with the ring C?O group. In these cases, only one of the two anticipated diene-ketenes (and descendents thereof) is produced. A thorough investigation of the photochemistry of a series of structurally different cyclohexa-2, 4-dienones on analytical as well as on preparative scale extends our mechanistic knowledge of the various routes from diene-ketenes into a variety of compound classes. Novel compound classes accessible to diene-ketenes are seven-membered carbocycles (by intramolecular aldolization of the zwitterion of appropriately substituted, transiently formed diene-(N, O)-ketene acetals) and β-lactams (by Staudinger reaction).     

Femtosecond Spectroscopic Studies on the Red Light-Absorbing Form of Oat Phytochrome and 2,3-Dihydrobiliverdin

Abstract— The excited state behavior of the red light-absorbing form of phytochrome (Pr) was studied on the femtosecond time scale. After excitation of Pr with 75 fs laser pulses at 616 nm the kinetics of the transient absorption changes was recorded at selected wavelengths probing mainly the bleaching of the Pr ground-state absorption and the stimulated emission. The kinetic data obtained indicate the population of an excited state with a 3 ps lifetime immediately after excitation. This state precedes the formation of another excited state with a 32 ps lifetime. The decay of the latter state is followed by the appearance of a first product state that is assumed to represent lunii-R. In addition, 2,3-dihydrobiliverdin, which is considered to be an adequate model of the Pr chro-mophore, was included in the femtosecond studies. The absorption difference spectra recorded at various delay times show an immediate bleaching of the ground-state absorption. Simultaneously with bleaching a broad transient absorption appears between 410 and 525 nm. The data analysis yields similar kinetic components as they were observed in the decay of Pr. It is suggested from this finding that within the first tens of picoseconds after excitation the excited-state properties of Pr are mainly determined by the properties of the chromophore itself.     

Reaction Engineering Aspects of α-l,4-D-Glucan Phosphorylase Catalysis

Some important process properties of α-l,4-D-ghican phosphorylases isolated from the bacteriumCorynebacterium callunae and potato tubers (Solatium tuberosum) were compared. Apart from minor differences in their stability and specificity (represented by the maximum degree of maltodextrin conversion) and a 10-fold higher affinity of the plant phosphorylase for maltodextrin (K _M of 1.3 g/L at 300 mM of orthophosphate), the performances of both enzymes in a continuous ultrafiltration membrane reactor were almost identical. Product synthesis was carried out over a time course of 300–400 h in the presence or absence of auxiliary pullulanase (increasing the accessibility of the glucan substrate for phosphorolytic attack up to 15–20%). The effect of varied dilution rate and reaction temperature on the resulting productivities was quantitated, and a maximum operational temperature of 40°C was identified.     

Mesophase structure and alignment under different fields of liquid crystalline main-chain/side-group block copolymers

Liquid crystalline block copolymers are new materials in which multiple molecular interactions can provide the driving force for complex phase behaviors and states of order. Block copolymers containing both liquid crystalline main-chain polyester and side-group polymethacrylate blocks were investigated. They phase separated in the liquid crystalline state and their individual mesophases coexisted. The copolymers responded very differently when either a mechanical or a magnetic field was used for alignment. In the fibers the orientations of the side-group and main-chain smectic planes with respect to the fiber axis depended critically on the block lengths and on their distinct tendencies to align, whereas under a magnetic field the mesogens aligned collectively with their long molecular axis parallel to the field, independent of the copolymer structure.