Propene Polymerization Promoted by C2-Symmetric Metallocene Catalysts: From Atactic to Isotactic Polypropene in Consequence of an Isotope Effect

We studied the polymerization of propene-2-d promoted by the prototypical isotactic-specific catalyst system rac-ethylenebis(4,5,6,7-tetrahydro-1-indenyl)ZrCl2/MAO. In this communication, we report the results of our investigation, documenting a large isotope effect on the balance between polyinsertion and epimerization and, therefore, on the stereospecificity. The mechanistic implications of such results are also discusse

Photosystem-II D1 protein mutants of Chlamydomonas reinhardtii in relation to metabolic rewiring and remodelling of H-bond network at Q(B) site

Photosystem II (PSII) reaction centre D1 protein of oxygenic phototrophs is pivotal for sustaining photosynthesis. Also, it is targeted by herbicides and herbicide-resistant weeds harbour single amino acid substitutions in D1. Conservation of D1 primary structure is seminal in the photosynthetic performance in many diverse species. In this study, we analysed built-in and environmentally-induced (high temperature and high photon fluency-HT/HL) phenotypes of two D1 mutants of Chlamydomonas reinhardtii with Ala250Arg (A250R) and Ser264Lys (S264K) substitutions. Both mutations differentially affected efficiency of electron transport and oxygen production. In addition, targeted metabolomics revealed that the mutants undergo specific differences in primary and secondary metabolism, namely, amino acids, organic acids, pigments, NAD, xanthophylls and carotenes. Levels of lutein, beta-carotene and zeaxanthin were in sync with their corresponding gene transcripts in response to HT/HL stress treatment in the parental (IL) and A250R strains. D1 structure analysis indicated that, among other effects, remodelling of H-bond network at the Q(B) site might underpin the observed phenotypes. Thus, the D1 protein, in addition to being pivotal for efficient photosynthesis, may have a moonlighting role in rewiring of specific metabolic pathways, possibly involving retrograde signalling

Gas-phase heteroaromatic substitution. Part 4. Electrophilic attack of tert-butyl cation on pyrrole, N-methylpyrrole, furan, and thiophene

The kinetics were studied for the substitution reaction of Me3C+, obtained in the gas phase from γ-radiolysis of Me4C, with pyrrole (I), N-methylpyrrole (II), furan (III) and thiophene (IV) at 50-760 Torr in the presence of variable Me3N concns. The reactivity of simple heteroarom. compds. relative to PhMe in competition expts. depended markedly upon the compn. of the gaseous reaction environment. The apparent ks/k1 ratios decrease slightly with increasing pressure. At atm. pressure, these ratios increase with the Me3N concn., leveling off to values of 1.0, 2.2, 2.6 and 0.6 for I-IV, resp., at the highest base concn. (PMe3N = 10 Torr). Under the same conditions, predominant β substitution occurs in I and II, whereas α-attack is favored with III and IV. The gaseous electrophile appears rather unselective between the n- and π-type centers of III and IV, and displays very limited substitution on the heteroatom of I and II. A mechanism based upon reversible addn. of the Me3C+ to the heteroarom. nucleus, involving preliminary formation of an electrostatic adduct, accounts for the formation of the tert-butylated products and their isomeric compn. Gas-phase tert-Bu alkylation of simple 5-membered heteroarom. compds. is regulated by electrostatic interactions established within the encounter pair

Simplified chemical and radiochemical synthesis of 2-arachidonoyl-glycerol, an endogenus ligand of cannabinoid receptors

Abstract—Endocannabinoids have emerged as a new class of lipid mediators, with manifold roles in the central nervous system and in the periphery. Several studies have identified 2-arachidonoyl-glycerol (2-AG) as a major endogenous agonist of cannabinoid receptors. Here, the chemical synthesis of 2-AG is reported, along with the synthesis of its tritium-labeled derivative. These unlabeled and radiolabeled compounds are suitable tools for unravelling some metabolic routes and biological activities of 2-AG in various cells and tissues

Gas phase ion-molecule reactions of chromium and molybdenum hexacarbonyls with ammonia

The gas phase ion chem. of Cr(CO)6 and Mo(CO)6 with NH3 was studied by high pressure chem. ionization and Fourier transform ion cyclotron resonance mass spectrometry. Reactions in which CO is displaced by NH3 give M(CO)x(NH3)y+ from M(CO)n+ (n = 2-5, M = Cr, Mo) and NH3 mols., through complex pathways identified by double resonance expts. The highest mass ion is M(CO)6+, which is inert to NH3 and its neutral precursor M(CO)6. The formation of NH3-substituted ions involves multiple losses of CO and abundance of Mo(CO)x(NH3)y+ is higher than that of the corresponding Cr(CO)x(NH3)y+ species. Results obtained by these 2 methods are correlated and discussed