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Weakly Acidic Conditions (weakly + acidic_condition)
Selected AbstractsThe Fe-only nitrogenase and the Mo nitrogenase from Rhodobacter capsulatusFEBS JOURNAL, Issue 6 2002A comparative study on the redox properties of the metal clusters present in the dinitrogenase components The dinitrogenase component proteins of the conventional Mo nitrogenase (MoFe protein) and of the alternative Fe-only nitrogenase (FeFe protein) were both isolated and purified from Rhodobacter capsulatus, redox-titrated according to the same procedures and subjected to an EPR spectroscopic comparison. In the course of an oxidative titration of the MoFe protein (Rc1Mo) three significant S = 1/2 EPR signals deriving from oxidized states of the P-cluster were detected: (1) a rhombic signal (g = 2.07, 1.96 and 1.83), which showed a bell-shaped redox curve with midpoint potentials (Em) of ,195 mV (appearance) and ,30 mV (disappearance), (2) an axial signal (g|| = 2.00, g, = 1.90) with almost identical redox properties and (3) a second rhombic signal (g = 2.03, 2.00, 1.90) at higher redox potentials (> 100 mV). While the ,low-potential' rhombic signal and the axial signal have been both attributed to the one-electron-oxidized P-cluster (P1+) present in two conformationally different proteins, the ,high-potential' rhombic signal has been suggested rather to derive from the P3+ state. Upon oxidation, the FeFe protein (Rc1Fe) exibited three significant S = 1/2 EPR signals as well. However, the Rc1Fe signals strongly deviated from the MoFe protein signals, suggesting that they cannot simply be assigned to different P-cluster states. (a) The most prominent feature is an unusually broad signal at g = 2.27 and 2.06, which proved to be fully reversible and to correlate with catalytic activity. The cluster giving rise to this signal appears to be involved in the transfer of two electrons. The midpoint potentials determined were: ,80 mV (appearance) and 70 mV (disappearance). (b) Under weakly acidic conditions (pH 6.4) a slightly altered EPR signal occurred. It was characterized by a shift of the g values to 2.22 and 2.05 and by the appearance of an additional negative absorption-shaped peak at g = 1.86. (c) A very narrow rhombic EPR signal at g = 2.00, 1.98 and 1.96 appeared at positive redox potentials (Em = 80 mV, intensity maximum at 160 mV). Another novel S = 1/2 signal at g = 1.96, 1.92 and 1.77 was observed on further, enzymatic reduction of the dithionite-reduced state of Rc1Fe with the dinitrogenase reductase component (Rc2Fe) of the same enzyme system (turnover conditions in the presence of N2 and ATP). When the Rc1Mo protein was treated analogously, neither this ,turnover signal' nor any other S = 1/2 signal were detectable. All Rc1Fe -specific EPR signals detected are discussed and tentatively assigned with special consideration of the reference spectra obtained from Rc1Mo preparations. [source] Self-Assembled Supramolecular Nanocarrier Hosting Two Kinds of Guests in the Site-Isolation StateCHEMISTRY - A EUROPEAN JOURNAL, Issue 43 2009Xing-Long Lou Abstract Hyperbranched polyethylenimine (HPEI) was simply mixed with a solution of amphiphilic calix[4]arene (AC4), which possesses four phenol groups and four aliphatic chains, in chloroform. This resulted in the novel supramolecular complex HPEI,AC4 through the noncovalent interaction of the amino groups of HPEI with the phenol groups of AC4. The formed HPEI,AC4 supramolecular complexes were characterized by 1H,NMR spectroscopy and dynamic light scattering. The cationic water-soluble dye methyl blue (MB) and the anionic water-soluble dye methyl orange (MO) were used as the model guests to test the performance of HPEI,AC4 as a supramolecular nanocarrier. It was found that HPEI,AC4 could accommodate the anionic water-soluble MO guests into the HPEI core. The MO encapsulation capacity of HPEI,AC4 was pH sensitive, which reached maximum loading under weakly acidic conditions. The loaded MO molecules could be totally released when the pH value was reduced to be around 4.5 or raised to be around 9.5, and this process was reversible. HPEI,AC4 could not only accommodate the anionic MO with the HPEI core but could also simultaneously load the cationic MB molecules using the formed AC4 shell, thereby realizing the site isolation of the two kinds of functional units. The amount of MO and MB encapsulated by HPEI,AC4 could be controlled by varying the ratio of hydroxyl groups of AC4 to amino groups of HPEI. [source] The Synthesis and Characterization of all Diastereomers of a Linear Symmetrically Fused Tris-Tröger's Base Analogue: New Chiral Cleft CompoundsCHEMISTRY - A EUROPEAN JOURNAL, Issue 10 2006Josep Artacho Abstract The synthesis and characterization of all diastereomers of a linear symmetrically fused tris-Tröger's base analogue are described. The diastereomers are unambiguously assigned as syn,anti1,a, anti,anti1,b, and syn,syn1,c isomers, by using X-ray diffraction analysis and NMR spectroscopy. For the first time, the anti,anti and the syn,syn diastereomers of a linear symmetrically fused tris-Tröger's base analogue have been synthesized. Molecules 1,a and 1,c are new cleft compounds and analysis of compound 1,a in the solid state shows inclusion of one molecule of CH2Cl2 in the larger aromatic cleft, whereas in isomer 1,c disordered solvent molecules are trapped in the extended aromatic cleft. Furthermore, in the solid state, isomer 1,c forms infinite open channels along one of the crystallographic axes and perpendicular to this axis there are infinitely extending "wedged-ravines". Importantly, each of the diastereomers 1,a,c is resistant to inversion at the stereogenic nitrogen atoms under strongly and weakly acidic conditions in the range from room temperature (RT) to 95,°C. This observed configurational stability at the stereogenic nitrogens of 1,a,c is unique for analogues of Tröger's base in general to date. Finally, the ratio of cleft compounds 1,a and 1,c significantly increased relative to cavity compound 1,b when ammonium chloride was used as an additive in the Tröger's base condensation to 1,a,c suggesting a templating effect of the ammonium ion. [source] CO2 Fixation and Transformation by a Dinuclear Copper Cryptate under Acidic ConditionsCHEMISTRY - AN ASIAN JOURNAL, Issue 6 2007Jia-Mei Chen Abstract CO2 fixation and transformation by metal complexes continuously receive attention from the viewpoint of carbon resources and environmental concerns. We found that the dinuclear copper(II) cryptate [Cu2L1](ClO4)4 (1; L1=N[(CH2)2NHCH2(m -C6H4)CH2NH-(CH2)2]3N) can easily take up atmospheric CO2 even under weakly acidic conditions at room temperature and convert it from bicarbonate into carbonate monoesters in alcohol solution. The compounds [Cu2L1(, - O2COH)](ClO4)3 (2), [Cu2L1(,-O2COR)](ClO4)3 (3: R=CH3; 4: R=C2H5; 5: R=C3H7; 6: R=C4H9; 7: R=C5H11; 8: R=CH2CH2OH), [Cu2L1(, - O2CCH3)](ClO4)3 (9), and [Cu2L1(OH2)(NO3)](NO3)3 (10) were characterized by IR spectroscopy and ESI-MS. The crystal structures of 2,6 and 10 were studied by single-crystal X-ray diffraction analysis. On the basis of the crystal structures, solution studies, and DFT calculations, a possible mechanism for CO2 fixation and transformation is given. [source] | ||||||||||