AGGREGATION OF MONOVINYL- and DIVINYL-PROTOCHLOROPHYLLIDE IN ORGANICSOLVENTS*

Abstract— Two different protochlorophyllides (PChlide), PChlide 629/433 (absorption data in methanol) and PChlide 630/441 (the monovinyl (MV) and divinyl (DV) forms) were isolated from the pigment mutant C-2A'of Scenedesmus obliquus. Their spectroscopic behaviour in several organic solvents and their aggregation in toluene was investigated. In polar solvents such as ether, acetonitril or acetone, absorption maxima similar to those in methanol were observed, while in solvents such as tetrahydrofuran and pyridine a bathochromic shift of the blue absorption band compared to the spectra in methanol occurred. The absorption maxima of MV-PChlide shifted from 629 nm and 433 nm in methanol (monomeric form), to 631 nm and 443 nm in toluene (aggregated form). The absorption maxima of DV-PChlide shifted from 630 nm and 441 nm in methanol to 655 nm and 483 nm in toluene (aggregated form). The fluorescence excitation and emission spectra of the two protochlorophyllides yielded the according results. The aggregation process was faster for DV-PChlide than for MV-PChlide and was reversible upon addition of small amounts of polar solvents. The similarity of the spectral characteristics of the aggregated forms of the different protochlorophyllides after toluene treatment with those reported for “active”-PChlide in vivo are discussed.     

Properties of high-energy pions emitted from heavy-ion collisions at 1 GeV/nucleon

Positively charged pions and protons from collisions of Ne+NaF and Au+Au at 1 GeV/nucleon incident energy were measured near midrapidity. The center-of-mass pion spectra deviate from a Maxwell-Boltzmann distribution. The slope of the high-energy part of the pion spectra varies significantly with the system mass and little with the size of the reaction zone. While the total pion yield rises linearly with the number of participant nucleons, the highenergy component increases more than linearly.     

Probing the plateau-insulator quantum phase transition in the quantum hall regime

We report quantum Hall experiments on the plateau-insulator transition in a low mobility In(0.53)Ga(0.47)As/InP heterostructure. The data for the longitudinal resistance rho(xx) follow an exponential law and we extract a critical exponent kappa = 0.55+/-0. 05 which is slightly different from the established value kappa = 0. 42+/-0.04 for the plateau transitions. Upon correction for inhomogeneity effects, which cause the critical conductance sigma(*)(xx) to depend marginally on temperature, our data indicate that the plateau-plateau and plateau-insulator transitions are in the same universality class.     

Calculated molecular double-differential cross-sections for ionisation under proton impact

This paper describes an approach (DDCS-MT), based on the plane wave Born approximation, intended to calculate double-differential ionisation cross-sections (DDCS) under heavy charged particle impact. A procedure is proposed in order to adapt this formalism to the treatment of molecules, which consists essentially to break up each molecular orbital into its atomic components and to sum up the partial DDCS. The increase of the binding energies in respect to the ion energy, as well as the post-collisional interaction between the passing ion and the ejected electron were taken into account. The method proposed led generally to a satisfactory agreement with measurements from the literature for electron ejection energies larger than 10 to 20 eV, in the case of a variety of low-Z molecules (NH₃, CH₄, CH₃NH₂, N₂, O₂, CO₂) bombarded by 0.25-to 2.0-MeV protons. The whole procedure is free from any fitting parameter and requires no experimental ionisation cross-sections as input data.     

Altered pterin patterns in photobehavioral mutants of Phycomyces blakesleeanus.

Pterins were extracted with methanol from sporangiophores of the lower fungus Phycomyces blakesleeanus and separated and identified by high performance liquid chromatography (HPLC) with fluorescence detection. The following pterins were found and identified for the wild-type strain NRRL1555: carboxypterin (6.7 x 10(-6) M), neopterin (4.2 x 10(-7) M), xanthopterin (5.3 x 10(-6) M), biopterin (3.9 x 10(-7) M), pterin (9.1 x 10(-7) M), and 6,7-dimethylpterin (1.2 x 10(-6) M). The HPLC elution profiles of the wild type were compared to a set of phototropism mutants (genotype mad) with specific defects in the light-transduction pathway. The mutant profiles were qualitatively similar to those of the wild type. Quantitative differences were, however, discerned for madA, madC, and madH mutants. The madA mutation was associated with increased amounts of biopterin and 6,7-dimethylpterin and a reduction of neopterin, pterin, xanthopterin, and unidentified pterins eluting at 14-18 min. The stimulatory effect of the madA mutation on biopterin and 6,7-dimethylpterin appears to be compensated by a secondary mutation (pde) which is responsible for the loss of 75% of adenosine 3',5'-cyclic monophosphate (cAMP)-phosphodiesterase activity. In a madA pde double mutant the amounts of biopterin and 6,7-dimethylpterin fell below the wild-type level. These results suggest that an increased level of endogenous cAMP represses the biosynthesis of these pterins. The madC mutation increased the amounts of biopterin and xanthopterin and that of the unidentified pterins which could be derivatized to carboxypterin. Single madB mutations had, compared to the wild type, two times higher amounts of biopterin and two times lower amounts of neopterin.(ABSTRACT TRUNCATED AT 250 WORDS)     

Coulomb dissociation of 8B and the low-energy cross section of the 7Be(p,gamma)8B solar fusion reaction

An exclusive measurement of the Coulomb breakup of 8B into 7Be+p at 254A MeV allowed the study of the angular correlations of the breakup particles. These correlations demonstrate clearly that E1 multipolarity dominates and that E2 multipolarity can be neglected. By using a simple single-particle model for 8B and treating the breakup in first-order perturbation theory, we extract a zero-energy S factor of S17(0)=18.6+/-1.2+/-1.0 eV b, where the first error is experimental and the second one reflects the theoretical uncertainty in the extrapolation.     

Ring opening of 1-methylcyclopropene and cyclopropene: matrix infrared spectroscopic identification of 2-butene-1,3-diyl and propene-1,3-diyl

Triplet 2-butene-1,3-diyl (T-11) was generated on irradiation of 1-methylcyclopropene (10) in a bromine-doped xenon matrix and was characterized by means of IR spectroscopy for the first time. Experimental results suggest that triplet propene-1,3-diyl (T-3) is formed from cyclopropene (1) under similar conditions. In accordance with theoretical calculations, the experimental data indicate that the reactions 1-->3 and 10-->11 are the lowest energy ground-state pathways for the ring opening of 1 and 10, respectively.