Enantioselective separations by capillary GC on derivatized cyclodextrins. Part 2: Determination of enantiomeric excess of some racemic 2,3-isopropylidene-1,2,3-cyclohexanetriol derivatives on permethyl α-, β-, and γ-cyclodextrins

Capillary GC on permethyl α-, β-, and γ-cyclodextrins has been applied to separate and quantify the enantiomers of some 2,3-iso-propylidene-1,2,3-cyclohexanetriol derivatives. Quantitative CGC data are compared to those obtained with chiral shift ¹H NMR.     

Reversible Full‐Range Color Control of a Cholesteric Liquid‐Crystalline Film by using a Molecular Motor

By using a chiral molecular motor as a dopant in a cholesteric liquid‐crystalline film, fully reversible control of the reflection color of this film across the entire visible spectrum is possible. The large difference in helical twisting power between the two isomeric forms of the motor allows efficient light‐ and heat‐induced switching of the helicity of the cholesteric liquid‐crystal superstructure.     

Characterization of low-energy chlorophylls in the PSI-LHCI supercomplex from Chlamydomonas reinhardtii. A site-selective fluorescence study

Almost all photosystem I (PSI) complexes from oxygenic photosynthetic organisms contain chlorophylls that absorb at longer wavelength than that of the primary electron donor P700. We demonstrate here that the low-energy pool of chlorophylls in the PSI-LHCI complex from the green alga Chlamydomonas reinhardtii, containing five to six pigments, is significantly blue-shifted (A(max) at 700 nm at 4 K) compared to that in the PSI core preparations from several species of cyanobacteria and in PSI-LHCI particles from higher plants. This makes them almost isoenergetic with the primary donor. However, they keep the other characteristic features of "red" chlorophylls: clear spectral separation from the bulk chlorophylls, big Stokes shift revealing pronounced electron-phonon coupling, and large homogeneous and inhomogeneous broadening of approximately 170 and approximately 310 cm(-1), respectively.     

Macroscopic expression of the chirality of amino alcohols by a double amplification mechanism in liquid crystalline media

The central chirality of simple amino alcohols was amplified by binding to a dynamically axially chiral biphenol receptor and expressed as supramolecular chirality by effecting a change from a nematic to a cholesteric liquid crystalline phase.     

TEMPERATURE DEPENDENCE OF CHLOROPHYLL FLUORESCENCE FROM THE LIGHT HARVESTING COMPLEX II OF HIGHER PLANTS

Abstract— Chlorophyll fluorescence spectra of LCHII, the light harvesting complex of photosystem II, have been recorded in the aggregated and trimeric forms for a range of temperatures from 293 to 4 K. At least five long-wavelength emitters in the 682–702 nm region with different temperature dependencies were found in the spectra of the aggregates. At 293 K the yield of LCHII trimers was higher than aggregates by a factor of 4, but, upon lowering the temperature, a fluorescence rise was observed which was much stronger for LCHII aggregates than for LCHII trimers, so that at 4 K their yields were comparable. The implications of these data in terms of the function of LCHII are discussed.     

The LOV2 domain of phototropin: a reversible photochromic switch

Light, oxygen, or voltage (LOV) domains constitute a new class of photoreceptor proteins that are sensitive to blue light through a noncovalently bound flavin chromophore. Blue-light absorption by the LOV2 domain initiates a photochemical reaction that results in formation of a long-lived covalent adduct between a cysteine and the flavin cofactor. We have applied ultrafast spectroscopy on the photoaccumulated covalent adduct state of LOV2 and find that, upon absorption of a near-UV photon by the adduct state, the covalent bond between the flavin and the cysteine is broken and the blue-light-sensitive ground state is regained on an ultrafast time scale of 100 ps. We thus demonstrate that the LOV2 domain is a reversible photochromic switch, which can be activated by blue light and deactivated by near-UV light.     

Excitation decay pathways of Lhca proteins: a time-resolved fluorescence study

Light-harvesting complex I (LHCI), which serves as a peripheral antenna for photosystem I (PSI) in green plants, consists mainly of four polypeptides, Lhca1-4. We report room temperature emission properties of individual reconstituted monomeric Lhca proteins (Lhca1, -2, -3, and -4) and dimeric Lhca1/4, performed by steady-state and time-resolved fluorescence techniques. The emission quantum yields of the samples are approximately 0.12, 0.085, 0.081, 0.041, and 0.063 for Lhca1, -2, -3, -4, and the -1/4 dimer, respectively, which is considerably lower than the value of 0.22 found for light-harvesting complex II (LHCII), the main peripheral antenna complex of photosystem II in green plants. The decay components of LHCI proteins can be divided in two categories: Lhca1 and Lhca3 have decay times of 1.1-1.6 ns and 3.3-3.6 ns, and Lhca2 and Lhca4 have decay times of 0.7-0.9 ns and 3.1-3.2 ns. These categories seem to correlate with the pigment composition of the samples. All decay times are faster than that observed previously for LHCII. When the absolute emission yields and the lifetimes of the Lhca samples are combined, the overall emission properties of the individual Lhca proteins are expressed in terms of their emitting dipole moment strength. In the samples without extreme red states, that is, Lhca1 and Lhca2, the emitting dipole moment has a value close to unity (relative to monomeric chlorophyll in acetone), which is similar to that for LHCII, whereas, in the samples with the red-most state (F-730), that is, Lhca3, -4, and the -1/4 dimer, the emitting dipole moment has a value less than unity (0.6-0.8), which can be explained by mixing the red-most (exciton) state with a dark charge-transfer state, as suggested in previous PSI red pigment studies. In addition, we find a lifetime component of approximately 50-150 ps in all red-pigment-containing samples, which cannot be due to "slow" energy transfer, but is instead assigned to an unrelaxed state of the pigment-protein, which, on this time-scale, is converted into the final emitting state.     

Mediation of ultrafast light-harvesting by a central dimer in phycoerythrin 545 studied by transient absorption and global analysis

We report ultrafast femtosecond transient absorption measurements of energy-transfer dynamics for the antenna protein phycoerythrin 545, PE545, isolated from a unicellular cryptophyte Rhodomonas CS24. The phycoerythrobilins are excited at both 485 and 530 nm, and the spectral response is probed between 400 and 700 nm. Room-temperature measurements are contrasted with measurements at 77 K. An evolution-associated difference spectra (EADS) analysis is combined with estimations of bilin spectral positions and energy-transfer rates to obtain a detailed kinetic model for PE545. It is found that sub pulse-width dynamics include relaxation between the exciton states of a chromophore dimer (beta 50/60) located in the core of the protein. Energy transfer from the lowest exciton state of the phycoerythrobilin (PEB) dimer to one of the periphery 15,16-dihydrobiliverdin (DBV) bilins is found to occur on a time scale of 250 fs at room temperature and 960 fs at 77 K. A host of energy-transfer dynamics involving the beta 158, beta 82, and alpha 19 bilins occur on a time scale of 2 ps at room temperature and 3 ps at 77 K. A final energy transfer occurs between the red-most DBV bilins with a time scale estimated to be approximately 30 ps. The role of the centrally located phycoerythrobilin dimer is seen as crucial-spectrally as it expands the cross-section of absorption of the protein; structurally as it sits in the middle of the protein acting as an intermediary trap; and kinetically, as the internal conversion and subsequent red-shift of the excitation is extremely fast.     

Automated precolumn derivatization device to determine neurotransmitter and other amino acids by reversed-phase high-performance liquid chromatography

A device to derivatize amino acids with o-phthaldialdehyde, which is directly connected to high-performance liquid chromatographic equipment is described. Its principle is that a sample (10-500 microliters) is mixed with a reagent (containing o-phthaldialdehyde, 2-mercaptoethanol and sodium hydrogen carbonate buffer), using a peristaltic pump. This mixture is pumped into a loop of a pneumatically controlled injection valve at atmospheric pressure. When the derivatization is complete the valve switches, so that the sample is applied to a column and the amino acid derivatives are separated with a gradient of methanol-phosphate buffers. The reproducibility is such that brain perfusates or tissue extracts can be analyzed for the amino acid transmitter content and no internal standard is necessary. The major advantages of the present device are that it produces thorough mixing of reagent and sample, so that a high and constant degree of derivatization occurs (thus producing high sensitivity; less than 0.1 pmol can be detected) and its low cost.     

A more reliable detergent for constant load experiments on polyethylene

Testing polyethylene materials for stress cracking resistance at elevated temperatures in the presence of a detergent solution is relatively popular. The detergents used for these tests are—to our knowledge—only of the ethoxylated nonylphenol type (trade names: Igepal, Antharox, Arkopal). Extensive investigations of solutions of these nonionic detergents used at 80 °C have shown that they are very susceptible to oxidation. Oxidation has already started after about one day at 80 °C and leads to erroneous failure times of polyethylene test bars exposed to the detergent and to an unacceptably large scatter in the data.

An alternative anionic alkylbenzene sulphonate (ABS) detergent has been tested which, when dissolved in drinking water, produces reproducible failure times of welded polyethylene samples, if the age of the detergent solution does not exceed about 2000 h at 80 °C.