Chiral polymerization: symmetry breaking and entropy production in closed systems

We solve numerically a kinetic model of chiral polymerization in systems closed to matter and energy flow, paying special attention to its ability to amplify the small initial enantiomeric excesses due to the internal and unavoidable statistical fluctuations. The reaction steps are assumed to be reversible, implying a thermodynamic constraint among some of the rate constants. Absolute asymmetric synthesis is achieved in this scheme. The system can persist for long times in quasi-stationary chiral asymmetric states before racemizing. Strong inhibition leads to long-period chiral oscillations in the enantiomeric excesses of the longest homopolymer chains. We also calculate the entropy production σ per unit volume and show that σ increases to a peak value either before or in the vicinity of the chiral symmetry breaking transition.     

Asymmetric additive-free aryl addition to aldehydes using perhydrobenzoxazines as ligands and boroxins as aryl source

A highly efficient enantioselective aryl addition to aldehydes using boroxins as aryl source and conformationally restricted perhydro-1,3-benzoxazines as ligands is reported. Both enantiomeric forms of chiral arylphenylmethanols and 1,1'-disubstituted diarylmethanols are afforded with excellent yields and enantioselectivities using the same ligand by means of an appropriate combination of boroxin and aromatic aldehyde. The enantiocontrol is not significantly influenced by electronic effects or steric hindrance, even with substituted boroxins. Very homogeneous ee's are reached when substituted arylboroxins are employed, without the use of any class of additive or pre-treatment.     

Multipumping Nitrite Determination in Exhaled Breath Condensate

Abstract

A flow system based on multicommutation is proposed for the rapid, clean, and inexpensive determination of nitrites in small volumes of breath condensates. The procedure exploits the colorimetric detection of nitrite with the Griess reagent [0.03% naphthylethylene diamine dihydrochloride (NED), 0.5% sulpfhanilamide, and 3.0% phosphoric acid] in acidic medium at 540 nm correcting the variations of the baseline with measurements at 424 nm. The flow system was designed with a set of solenoid micropumps to minimize sample and reagent consumption and waste generation. The detection limit was estimated as 3.8 ng mL^?1 (99.7% confidence level) with a linear response ranging up to 500 ng mL^?1. The coefficient of variation was estimated as 0.7% for a solution containing 300 ng mL^?1 nitrite (n=9). Approximately 144 determinations can be carried out per hour, consuming only 678.4 µg Griess reagent and generating 1.184 mL of effluent per determination, thus providing an environmentally friendly alternative and a nonexpensive method. The procedure was successfully applied to determine nitrite in breath condensates.     

Development and validation of a direct competitive monoclonal antibody-based immunoassay for the sensitive and selective analysis of the phytoregulator forchlorfenuron

Forchlorfenuron is a synthetic phytohormone with cytokinin-like activity used worldwide as a plant growth regulator to increase fruit size in a number of crops, mostly in kiwifruit and grape vines. A monoclonal antibody-based enzyme-linked immunosorbent assay (ELISA) for the determination of forchlorfenuron has been characterized and optimized. The selected immunoreagents afforded a highly selective assay with a limit of detection of 10 ng L(-1) in buffer. This direct competitive ELISA was validated in terms of trueness, precision, and robustness using both commercial juice and whole fruit samples. Recoveries from fortified kiwifruit juices and white and red musts were between 97 % and 131 %, with relative standard deviations below 16 %. When homogenized whole fruits were analysed after acetonitrile extraction, recoveries between 96 % and 113 % were found, with a limit of quantification of 5 μg kg(-1). The proposed immunoassay was validated by comparison with a reference chromatographic method using fruits from in-field treated grape and kiwifruit vines. Linear regression analysis of ELISA and HPLC-UV determinations showed an excellent correlation (r(2)=0.998), whereas analysis of the slope (0.99±0.01) and of the intercept (-1±3) clearly proved that the developed competitive immunoassay provided results that were statistically comparable to those obtained by the instrumental method for the analysis of forchlorfenuron in fruits at trace levels.     

New growth mode through decorated twin boundaries

Scanning tunneling microscopy and low energy electron diffraction were used to investigate the growth of partly twinned Ir thin films on Ir(111). A transition from the expected layer-by-layer to a defect dominated growth mode with a fixed lateral length scale and increasing roughness is observed. During growth, the majority of the film is stably transformed to twinned stacking. This transition is initiated by the energetic avoidance of the formation of intrinsic stacking faults compared to two independent twin faults. The atomistic details of the defect kinetics are outlined.     

Asymmetric Total Synthesis of (−)‐Erogorgiaene and Its C‐11 Epimer and Investigation of Their Antimycobacterial Activity

A short, nine‐step, highly enantioselective synthesis of (?)‐erogorgiaene and its C‐11 epimer is reported. The key stereochemistry controlling steps involve catalytic asymmetric crotylation, anionic oxy‐Cope rearrangement and cationic cyclisation. (?)‐Erogorgiaene exhibited promising antitubercular activity against multidrug‐resistant strains of Mycobacterium tuberculosis.