Approximate Ratio (approximate + ratio)

Distribution by Scientific Domains


Selected Abstracts


Pirouetting in chiral [2]catenanes,

ISRAEL JOURNAL OF CHEMISTRY, Issue 2 2007
Seogshin Kang
One of the best known classes of mechanically interlocked molecules is the category of [2]catenanes, which exhibit donor-acceptor interactions between the 1,5-dioxynaphthalene (DNP) units in a crown ether and the bipyridinium units in the tetracationic cyclophane, cyclobis(paraquat- p -phenylene) (CBPQT4+). In order to gain an in-depth understanding and appreciation of the stereochemistry and dynamic behavior of these [2]catenanes, chiral analogues,having both the DNP ring, which is capable of displaying planar chirality, and the axially chiral binaphthol (BINAP) moiety (as both enantiomers and as the racemic modification), in a crown ether, in addition to the CBPQT4+ cyclophane,have been synthesized using a template-directed protocol. Dynamic 1H NMR spectroscopy shows that (i) the presence of immutable axial chirality, arising from the BINAP moiety in the crown ether component, leads to no induction of diastereoselectivity,the chiral catenanes exist as a mixture of diastereoisomers in solution at low temperatures in the approximate ratio of 1:1, (ii) the barrier (,GcD,) to the interconversion between these two diastereoisomers is 7.9 ± 0.1 kcal mol,1 at 171 K, and (iii) no induction of diastereoselectivity is observed upon the addition of a chiral solvating agent to the chrial catenanes. The pattern of behavior in the variable temperature 1H NMR spectra and the low ,TGcD, value indicates that the dynamic process involving the interconversion between these two diastereoisomers is one of a pirouetting nature. Of the four possible diastereoisomers, only two, (R)-(pR/pS) or (S)-(pR/pS), are shown to exist in solution. [source]


KINETICS OF SOYBEAN LIPOXYGENASES ARE RELATED TO pH, SUBSTRATE AVAILABILITY AND EXTRACTION PROCEDURES

JOURNAL OF FOOD BIOCHEMISTRY, Issue 2 2008
VERONICA S. CHEDEA
ABSTRACT The kinetic patterns of pure soy lipoxygenase LOX-1 and crude or defatted soybean extracts containing LOX isoenzymes (LOX-1, LOX-2 and LOX-3) were studied by UV spectrometry at 234 and 280 nm, depending on their extraction and measurement conditions. Different pHs (from 6.0 to 9.0), corresponding to specific activation of LOX isoenzymes and the ratios of enzyme protein per substrate were used in order to evaluate the enzyme rates, as indicators of its affinity for substrate in different environments. The crude soy extract contained mainly LOX-1 activity (measured at 234 nm, at pH 9.0) and LOX-3, in an approximate ratio of 3:1. The LOX-2 activity was very low. The defatted extracts buffered at pH 6.8 and 7.1 showed a low LOX-1 and LOX 2 activity, but mostly LOX-3 activity (measured at 280 nm, at pH 7.1), with a mirror-type relation between the enzyme/substrate ratio and their enzymatic specific activity. The results suggest that defatting inhibits specifically the LOX-1 activity and indicate the possibility to modulate LOX activity by modifications of enzyme/substrate ratios and modifications of pH in the enzyme environment. PRACTICAL APPLICATIONS Because of the specific kinetic behaviors of the three different LOXs found in crude soy extracts involved in off-flavor generation, one can modulate the inhibition of these isoenzymes during soybean processing. Our experiments showed that pH variation could be a simple solution to inhibit the LOX isoenzymes, and therefore, the off-flavor generation. From the analytical point of view, the techniques described in this article are designed to be as simple as possible, and easy to use at large-scale level in food industry (food chain control). The idea is to minimize the number of separate chemical manipulations and, thereby, minimize errors. These studies can offer the background of further inhibition experiments in vitro using natural extracts. The LOX inhibition by natural antioxidants is related as well to pH and other factors influencing the enzyme's activity; this idea can be also valorized practically in the future. [source]


Separation and quantification of the cellular thiol pool of pea plants treated with heat, salt and atrazine

PHYTOCHEMICAL ANALYSIS, Issue 4 2007
Sergei Veselinov Ivanov
Abstract A novel procedure for the separation of the cellular thiol pool according to the molecular weight and localization of compounds with sulphydryl groups is presented. This simple and rapid method allows the differentiation of thiols into three major fractions,low molecular weight (LMT, primarily glutathione and free cysteine), protein-bound (TPT) and pellet-bound (PBT, associated with cell walls and broken organelles). Moreover, determination of the ratio between surface (readily reactive) thiols (ATG) and those that are more or less buried in the protein structure (BTG) can be achieved. In intact pea leaves, the amounts of the total thiols (LMT + PBT + TPT) varies from 2.5 to 4.8 µmol/g of fresh material. The data for LMT, PBT and TPT were related to each other in the approximate ratio 1:2:7. Treatments of pea plants with high temperature, salinity and low amounts of atrazine affect these sulphydryl types differently. For a greater understanding of the applicability of this method to physiological research, the main mechanisms leading to alterations in the cellular thiol pool are discussed. Furthermore, it is suggested that the proportion of available to buried thiols (ATG/BTG) in proteins could be used as a convenient marker for stress impacts. Copyright © 2007 John Wiley & Sons, Ltd. [source]


Kinetic Study of the Conversion of Different Substrates to Lactic Acid Using Lactobacillus bulgaricus

BIOTECHNOLOGY PROGRESS, Issue 3 2000
Concepción N. Burgos-Rubio
Lactic acid fermentation includes several reactions in association with the microorganism growth. A kinetic study was performed of the conversion of multiple substrates to lactic acid using Lactobacillusbulgaricus. Batch experiments were performed to study the effect of different substrates (lactose, glucose, and galactose) on the overall bioreaction rate. During the first hours of fermentation, glucose and galactose accumulated in the medium and the rate of hydrolysis of lactose to glucose and galactose was faster than the convesion of these substrates. Once the microorganism built the necessary enzymes for the substrate conversion to lactic acid, the conversion rate was higher for glucose than for galactose. The inoculum preparation was performed in such a way that healthy young cells were obtained. By using this inoculum, shorter fermentation times with very little lag phase were observed. The consumption patterns of the different substrates converted to lactic acid were studied to determine which substrate controls the overall reaction for lactic acid production. A mathematical model (unstructured Monod type) was developed to describe microorganism growth and lactic acid production. A good fit with a simple equation was obtained. It was found experimentally that the approximate ratio of cell to substrate was 1 to 10, the growth yield coefficient (YXS) was 0.10 g cell/g substrate, the product yield (YPS) was 0.90 g lactic acid/g substrate, and the , parameter in the Luedeking-Piret equation was 9. The Monod kinetic parameters were obtained. The saturation constant (KS) was 3.36 g/L, and the specific growth rate (,m ) was 1.14 l/h. [source]