Asymmetric Photolysis with Elliptically Polarized Light

Previous investigations have shown the efficacyof right-(RCPL) and left-(LCPL) circularly polarized light inpromoting the asymmetric photolysis of racemic organic substratesand producing measurable enantiomeric excesses (e.e.s) whenphotolysis is incomplete. Synchrotron radiation, polychromaticand having out-of-plane components which are elliptically andultimately circularly polarized, has been suggested as auniversal source of RCPL and LCPL on a cosmic scale. The moreprevalent right-(REPL) and left-(LEPL) elliptically polarizedcomponents have never been investigated for similar capabilities.The present study, using a 212.8 nm laser beam to mimic thesynchtrotron radiation, explores the potential of REPL and LEPLin this context and finds a qualitative trend indicating thateach induces asymmetric photolysis in the same sense as RCPL andLCPL, but to a lesser degree.     

Hydrolytic resolution of (R,S)-naproxen 2,2,2-trifluoroethyl thioester by Carica papaya lipase in water-saturated organic solvents

For the first time, the Carica papaya lipase (CPL) stored in crude papain is explored as a potential enantioselective biocatalyst for obtaining chiral acids from their racemic thioesters. Hydrolytic resolution of (R,S)-naproxen 2,2,2-trifluoroethyl thioester in water-saturated organic solvents is employed as a model system for studying the effects of temperature and solvents on lipase activity and enantioselectivity. An optimal temperature of 60 degrees C, based on the initial rate of (S)-thioester and a high enantiomeric ratio (i.e., E-value defined as the ratio of initial rates for both substrates) of >100 at 45 degrees C in isooctane, is obtained. Kinetic analysis, considering product inhibition and enzyme deactivation, is also performed, showing agreement between the experimental and best-fit conversions for (S)-thioester. A comparison of the kinetic and thermodynamic behaviors of CPL and Candida rugosa lipase (CRL) in isooctane and cyclohexane indicates that both lipases are very similar in terms of thermodynamic parameters DeltaDeltaH and DeltaDeltaS, initial rate of (S)-substrate, and E-value when (R,S)-naproxen 2,2,2-trifluoroethyl thioester or ester is employed as substrate.     

Stereoselective effects of (R)- and (S)-carvedilol in humans

Carvedilol is currently used as the racemic mixture, (R,S)-carvedilol, consisting of equal amounts of (R)-carvedilol, an alpha-blocker, and (S)-carvedilol, an alpha- and beta-blocker, which have never been tested in their optically pure forms in human subjects. We performed a randomized, double-blind, placebo-controlled, crossover study in 12 healthy male volunteers. Subjects received single oral doses of 25 mg (R,S)-carvedilol, 12.5 mg (R)-carvedilol, 12.5 mg (S)-carvedilol, and placebo at 8 AM as well as at 8 PM. Exercise was performed at 11 AM, and heart rate and blood pressure were measured at rest and after 10 min of exercise. Urine was collected between 10 AM and 6 PM, as well as between 10 PM and 6 AM, and the amounts of urinary 6-hydroxy-melatonin sulfate (aMT6s) were determined by RIA. Compared to placebo, (R)-carvedilol increased heart rate during exercise (+4%, P < 0.05) and recovery (+10%, P < 0.05); (S)-carvedilol decreased heart rate during exercise (-14%, P < 0.05) and recovery (-6%, P < 0.05), and systolic blood pressure during exercise (-12%, P < 0.05); (R,S)-carvedilol decreased heart rate during exercise (-11%, P < 0.05), and systolic blood pressure at rest (-7%, P < 0.05) and during exercise (-10%, P < 0.05). None of the agents had any significant effect on the release of aMT6s. Our results indicate that only (S)-carvedilol causes beta-blockade, whereas (R)-carvedilol appears to increase sympathetic tone, presumably as a physiological reaction to the decrease of blood pressure caused by alpha-blockade. None of the drugs had any influence on melatonin release. The weak clinical net effect of beta-blockade of (R,S)-carvedilol at rest might be one reason why this drug causes fewer side effects than other beta-blockers, such as a reduction of nocturnal melatonin release.     

Left- and right-handed LHC II macroaggregates revealed by circularly polarized chlorophyll luminescence

Circularly polarized chlorophyll luminescence (CPL) may serve as a measure of chiral macroaggregates of the light-harvesting chlorophyll-protein complexes (LHC II) in both isolated chloroplasts and intact leaves (Gussakovsky et al (2000) Photosynth Res 65: 83–92). In the present work, we applied the CPL approach to study the effect of fast (1–2 min) thermal impacts on LHC II macroaggregates. The results revealed unexpected temperature-response kinetics, composed of initial bell-shaped changes in the CPL signal, followed by degradation down to a steady state (equilibrium). The bell-shape effect was dependent upon illumination, and vanished in the dark. A mathematical analysis of the temperature-response kinetics uniquely indicated that LHC II chiral macroaggregates may persist in both left- and right-handed forms. These forms differ in their response to high temperatures. Both forms are more thermostable in leaves than in isolated chloroplasts. The cooperative degradation of LHC II macroaggregates, which is induced by the thermal impact, is irreversible. It is therefore suggested that the native LHC II macroaggregates are stable, stationary, non-equilibrium, spatially heterogeneous (dissipative) structures. The dissipative properties probably allow the interconversion between left- and right-handed forms under perturbation by certain factors. Illumination probably serves as one such perturbation factor, initiating the interconversion of dark-adapted, left-handed to light-dependent, right-handed LHC II macroaggregates. The chiral heterogeneity of the LHC II macroaggregates is a newly revealed aspect which needs to be taken into consideration in future circular dichroism or CPL studies.     

Time-of-day effects of exposure to solar radiation on thermoregulation during outdoor exercise in the heat

High solar radiation has been recognised as a contributing factor to exertional heat-related illness in individuals exercising outdoors in the heat. Although solar radiation intensity has been known to have similar time-of-day variation as body temperature, the relationship between fluctuations in solar radiation associated with diurnal change in the angle of sunlight and thermoregulatory responses in individuals exercising outdoors in a hot environment remains largely unknown. The present study therefore investigated the time-of-day effects of variations in solar radiation associated with changing solar elevation angle on thermoregulatory responses during moderate-intensity outdoor exercise in the heat of summer. Eight healthy, high school baseball players, heat-acclimatised male volunteers completed a 3-h outdoor baseball trainings under the clear sky in the heat. The trainings were commenced at 0900 h in AM trial and at 1600 h in PM trial each on a separate day. Solar radiation and solar elevation angle during exercise continued to increase in AM (672–1107 W/m² and 44–69°) and decrease in PM (717–0 W/m² and 34–0°) and were higher on AM than on PM (both P < 0.001). Although ambient temperature (AM 32–36°C, PM 36–30°C) and wet-bulb globe temperature (AM 31–33°C, PM 34–27°C) also continued to increase in AM and decrease in PM, there were no differences between trials in these (both P > 0.05). Tympanic temperature measured by an infrared tympanic thermometer and mean skin temperature were higher in AM than PM at 120 and 180 min (P < 0.05). Skin temperature was higher in AM than PM at the upper arm and thigh at 120 min (P < 0.05) and at the calf at 120 and 180 min (both P < 0.05). Body heat gain from the sun was greater during exercise in AM than PM (P < 0.0001), at 0–60 min in PM than AM (P < 0.0001) and at 120–180 min in AM than PM (P < 0.0001). Dry heat loss during exercise was greater at 0–60 min (P < 0.0001), and lower at 60–120 min (P < 0.05) and 120–180 min (P < 0.0001) in AM than PM. Evaporative heat loss during exercise was greater in PM than AM at 120–180 min (P < 0.0001). Total (dry + evaporation) heat loss at the skin was greater during exercise in PM than AM (P < 0.0001), at 0–60 min in AM than PM (P < 0.0001) and at 60–120 and 120–180 min in PM than AM (P < 0.05 and 0.0001). Heart rate at 120–150 min was also higher in AM than PM (P < 0.05). Neither perceived thermal sensation nor rating of perceived exertion was different between trials (both P > 0.05). The current study demonstrates a greater thermoregulatory strain in the morning than in the afternoon resulting from a higher body temperature and heart rate in relation to an increase in environmental heat stress with rising solar radiation and solar elevation angle during moderate-intensity outdoor exercise in the heat. This response is associated with a lesser net heat loss at the skin and a greater body heat gain from the sun in the morning compared with the afternoon.     

Growth and adaptation to phototrophic conditions of Rhodospirillum rubrum and Rhodopseudomonas sphaeroides at different temperatures

The influence of temperature on yields of cell protein and bacteriochlorophyll as well as on the rates of growth and bacteriochlorophyll synthesis was studied with Rhodospirillum rubrum and Rhodopseudomonas sphaeroides. Under chemotrophic conditions net cell-protein production increased in cultures of both species along with temperature from 14°C up to the optimum at 33°C. Under phototrophic conditions cell-protein yields were largely constant within the range from 21°C to 33°C. At temperatures below 21°C and above 33°C yields decreased. These results are interpreted in terms of coupling between energy yielding or redox equivalent providing metabolisms and cell biosynthesis. Upon adaptation from chemotrophic to phototrophic conditions a direct relationship between temperature increase and bacteriochlorophyll level was observed. Arrhenius plots of both, specific growth rates and rates of bacteriochlorophyll synthesis, revealed discontinuities at about 20°C. Temperature coefficients either above or below those discontinuities were similar in both species. In R. rubrum temperature coefficients of the synthesis of total bacteriochlorophyll were also representative of the synthesis of photochemical reaction center and light harvesting bacteriochlorophylls. But in R. sphaeroides significant differences were observed between temperature coefficients of the syntheses of bacteriochlorophylls of the costantly composed reaction centerlight harvesting complex on one hand and of both, total and the quantitatively variable light harvesting bacteriochlorophylls on the other. The results are interpreted in light of hypotheses on the regulation (a) of cellular bacteriochlorophyll levels as well as (b) of the ratio of functionally different bacteriochlorophylls in the photosynthetic apparatus.Abbreviation Bchl bacteriochlorophyll     

Sensitive, rapid and validated gas chromatography/negative ion chemical ionization-mass spectrometry assay including derivatisation with a novel chiral agent for the enantioselective quantification of amphetamine-type stimulants in hair

A novel chiral derivatisation agent, (2S,4R)-N-heptafluorobutyryl-4-heptafluorobutoyloxy-prolyl chloride, was used for the indirect resolution of amphetamine (AM), methamphetamine (MA), 3,4-methylenedioxyamphetamine (MDA), 3,4-methylenedioxymethamphetamine (MDMA) and 3,4-methylenedioxyethylamphetamine (MDEA) enantiomers using gas chromatography coupled to mass spectrometry operating in the negative-ion chemical ionization mode (GC/MS-NICI). This new chiral derivatisation reagent was readily obtained in optically pure form after a simple two-step synthesis. Optimal derivatisation was accomplished in 15 min at room temperature in a carbonate buffer and the resulting diastereoisomers were base line separated by GC in 12 min only. No racemization was observed during the derivatisation. The method was applied and fully validated for the enantiomeric quantification of amphetamines and methylenedioxylated amphetamines in hair. The analyses of 24 hair specimens from suspected ATS abusers showed that 24 cases were positive for MA and/or AM enantiomers and that in most cases the concentrations of (S)-MA and (S)-AM exceeded those of the corresponding (R)-enantiomers. One hair specimen was tested positive for both enantiomers of MDMA and MDA.     

Relation between the parameters of lipid peroxidation and the constant potential of the rat brain

Studies were carried out of the relationship between the constant potential level (CPL) and the intensity of lipid peroxidation (LP) of the brain membranes in rats with different motor asymmetry. Strong correlation was found between CPL and the content of LP products (diene conjugates, hydroperoxides and Schiff bases) in the hemispheres of the brain (R = 0.7-0.8).     

Orientation of the tryptophans responsible for the photoinactivation of nerve sodium channels

UV irradiation of squid giant azons at wavelengths of 280 or 290 nm produces nearly the same rate of irreversible decrease of sodium currents. The rate of photodeactivation is unaffected by extensive removal of axoplasm with pronase, and it is independent of temperature in the range 5° to 20°C. The photochemical effect appears to be all or nothing. It does not alter the time course and the voltage dependence for activation and inactivation of the residual currents. Similar deactivation rates were produced by irradiations of the same intensity, but linearly polarized either parallel or perpendicular to the axon. The efficiency of the deactivation process is close to that expected if it was caused by the photooxidation of a single tryptophan residue per sodium channel. Owing to the geometry of the preparation the lack of polarization asymmetry suggests that this residue assumes nearly random (or pseudo-random) orientation in the three-dimensional structure of the sodium channel corresponding to the closed state.     

Functioning of photosystems I and II in pea leaves exposed to heat stress in the presence or absence of light

Fluorimetric, photoacoustic, polarographic and absorbance techniques were used to measure in situ various functional aspects of the photochemical apparatus of photosynthesis in intact pea leaves (Pisum sativum L.) after short exposures to a high temperature of 40 ° C. The results indicated (i) that the in-vivo responses of the two photosystems to high-temperature pretreatments were markedly different and in some respects opposite, with photosystem (PS) II activity being inhibited (or down-regulated) and PSI function being stimulated; and (ii) that light strongly interacts with the response of the photosystems, acting as an efficient protector of the photochemical activity against its inactivation by heat. When imposed in the dark, heat provoked a drastic inhibition of photosynthetic oxygen evolution and photochemical energy storage, correlated with a marked loss of variable PSII-chlorophyll fluorescence emission. None of the above changes were observed in leaves which were illuminated during heating. This photoprotection was saturated at rather low light fluence rates (around 10 W · m^–2). Heat stress in darkness appeared to increase the capacity for cyclic electron flow around PSI, as indicated by the enhanced photochemical energy storage in far-red light and the faster decay of P ₇₀₀ ⁺ (oxidized reaction center of PSI) monitored upon sudded interruption of the far-red light. The presence of light during heat stress reduced somewhat this PSI-driven cyclic electron transport. It was also observed that heat stress in darkness resulted in the progressive closure of the PSI reaction centers in leaves under steady illumination whereas PSII traps remained largely open, possibly reflecting the adjustment of the photochemical efficiency of undamaged PSI to the reduced rate of photochemistry in PSII.Abbreviations B₁ and B₂ fraction of closed PSI and PSII reaction centers, respectively - ES photoacoustically measured energy storage - F_o, F_m and F_s initial, maximal and steady-state levels of chlorophyll fluorescence - P₇₀₀ reaction center of PSI - PS (I, II) photosystem (I, II) - V = (F_s – F_o)/(F_m – F_o) relative variable chlorophyll fluorescence We wish to thank Professor R. Lannoye (ULB, Brussels) for the use of this photoacoustic spectrometer and Mrs. M. Eyletters for her help.     

An Extended Model for the Evolution of Prebiotic Homochirality: A Bottom-Up Approach to the Origin of Life

A generalized autocatalytic model for chiral polymerization is investigated in detail. Apart from enantiomeric cross-inhibition, the model allows for the autogenic (non-catalytic) formation of left and right-handed monomers from a substrate with reaction rates epsilon L and epsilon R, respectively. The spatiotemporal evolution of the net chiral asymmetry is studied for models with several values of the maximum polymer length, N. For N = 2, we study the validity of the adiabatic approximation often cited in the literature. We show that the approximation obtains the correct equilibrium values of the net chirality, but fails to reproduce the short time behavior. We show also that the autogenic term in the full N = 2 model behaves as a control parameter in a chiral symmetry-breaking phase transition leading to full homochirality from racemic initial conditions. We study the dynamics of the N--> infinity model with symmetric (epsilon L = epsilon R) autogenic formation, showing that it only achieves homochirality for epsilon > epsilon c, where epsilon c is an N-dependent critical value. For epsilon 相似文献   

Alterations in photochemical efficiency of photosystem II in wheat plant on hot summer day

In this study the effect of increasing temperature on photochemical efficiency of PS II in wheat plants has been studied on a hot summer day (9:00 AM (Control)–7:00 PM) by measuring Chl a fluorescence. Increasing temperature for a short period of time (2–4 h), in nature affects the efficiency of PS II complex reversibly and does not cause permanent damage to any of the components of photosystem II. A scheme has been provided to demonstrate the sequence and severity of events which get affected maximum by temperature stress.     

Supernovae, neutron stars and biomolecular chirality

Recent theoretical and experimental investigations of the origin of biomolecular chirality are reviewed briefly. Biotic and abiotic theories are evaluated critically with the conclusion that asymmetric photochemical processes with circulary polarized light (CPL), particularly asymmetric photolyses, constitute the most viable mechanisms. Solar CPL sources appear too weak and random to be effective. We suggest an alternative CPL source, namely, the synchrotron radiation from the neutron star remnants of supernova explosions. This could asymmetrically process racemic compounds in the organic mantles of the dust grains in interstellar clouds, and the resulting chiral molecules could be transferred to Earth by cold accretion as the solar system periodically traverses these interstellar clouds.     

Chiral inversion of 2-phenylpropionic acid by Cordyceps militaris

Y.-F. HUNG, M.J. THOMASON, W. RHYS-WILLIAMS, A.W. LLOYD AND G.W. HANLON. 1996. Fifty-nine microbial cultures were screened for their ability to carry out the chiral inversion of (R,S) and (R)-2-phenylpropionic acid under both growing and non-growing conditions. While a number of these micro-organisms metabolized the substrate, only Verticillium lecanii IMI 68689 and Cordyceps militaris CBS 128.25 were able to invert the (R)-enantiomer to its (S)-antipode with no other biotransformation occurring. The rate of inversion was faster for C. militaris but in both cases the reaction was independent of growth and reached an equilibrium after 5 d incubation. Evidence suggests that the (R) to (S) inversion is inhibited in the presence of excess (S)-enantiomer which may account for the equilibrium enantiomeric ratios (S:R) of 90:10 and 70:30 for C. militaris and V. lecanii , respectively. Optimization studies revealed that the fastest rates of reaction for C. militaris were found at lower substrate concentrations (100 μg ml^-1) whilst the greatest yield of (S)-2-PPA was obtained at 250 μg ml^-1. Optimum inversion was found to occur using Sørensen's phosphate buffer at pH 5.5. These results may have implications for the use of micro-organisms in the production of pure enantiomers from racemic mixtures and, in addition, provide an in vitro model system for the study of a unique and interesting biotransformation.     

Optimization of furfural production from hemicellulose extracted from delignified palm pressed fiber using a two-stage process

This study aims to optimize the conditions for furfural production from hemicellulose extracted from delignified palm pressed fiber (dPPF) via two-stage process: acid hydrolysis followed by dehydration, using response surface methodology (RSM). The extracted hemicellulose contained 80.8% xylose. In order to convert hemicellulose to xylose in the acid hydrolysis step, there were four important parameters consisting of reaction temperature (100–150 °C), sulfuric acid concentration (1–10% v/v), ratio of sulfuric acid to hemicellulose (L/S ratio) (10, 9, and 8 v/w), and reaction time (30–120 min). The maximum xylose production (12.58 g/L) was achieved at 125 °C, 5.5% sulfuric acid, L/S ratio of 9 mL/g for 30 min with the determination coefficient (R²) value of 0.90. For the dehydration process, two parameters; reaction temperature (120–160 °C) and reaction time (30–150 min), were optimized. The maximum furfural production (8.67 g/L) was achieved at a reaction temperature of 140 °C for 90 min with the determination coefficient (R²) value of 0.93.     

Stereoselective bioaccumulation,transformation, and toxicity of triadimefon in Scenedesmus obliquus

In this study the stereoselective bioaccumulation and transformation of triadimefon and the toxicity of triadimefon and its metabolite triadimenol to the green algae Scenedesmus obliquus were studied. In growth inhibition experiments, triadimenol was more toxic than triadimefon, and (1S,2R)‐triadimenol, which has the largest fungicidal activity, presented the highest toxicity to the algae. In bioaccumulation experiments, triadimefon was rapidly taken up by algae cells, and the decrease in the concentration of triadimefon was accompanied by an increase in triadimenol. The transformation of S‐(+)‐ triadimefon was faster than that of the R‐(−)‐enantiomer, resulting in four triadimenol stereoisomers at different forming rates: B2 (1S, 2S) > B1 (1R, 2R) > A2 (1S, 2R) > A1 (1R, 2S). Thus, it is necessary to explore the enantioselective toxicology and ecological fate of these chiral pesticides in an environmental risk assessment. Also, their metabolites should be paid specific attention to since they may pose higher ecological risks.     

Cutaneous vasoconstrictor response to whole body skin cooling is altered by time of day.

To test for a diurnal difference in the vasoconstrictor control of the cutaneous circulation, we performed whole body skin cooling (water-perfused suits) at 0600 (AM) and 1600 (PM). After whole body skin temperature (T(sk)) was controlled at 35 degrees C for 10 min, it was progressively lowered to 32 degrees C over 18-20 min. Skin blood flow (SkBF) was monitored by laser-Doppler flowmetry at three control sites and at a site that had been pretreated with bretylium by iontophoresis to block noradrenergic vasoconstriction. After whole body skin cooling, maximal cutaneous vascular conductance (CVC) was measured by locally warming the sites of SkBF measurement to 42 degrees C for 30 min. Before whole body skin cooling, sublingual temperature (T(or)) in the PM was significantly higher than that in the AM (P < 0.05), but CVC, expressed as a percentage of maximal CVC (%CVC(max)), was not statistically different between AM and PM. During whole body skin cooling, %CVC(max) levels at bretylium-treated sites in AM or PM were not significantly reduced from baseline. In the PM, %CVC(max) at control sites fell significantly at T(sk) of 34.3 +/- 0.01 degrees C and lower (P < 0.05). In contrast, in the AM %CVC(max) at control sites was not significantly reduced from baseline until T(sk) reached 32.3 +/- 0.01 degrees C and lower (P < 0.05). Furthermore, the decrease in %CVC(max) in the PM was significantly greater than that in AM at T(sk) of 33.3 +/- 0.01 degrees C and lower (P < 0.05). Integrative analysis of the CVC response with respect to both T(or) and T(sk) showed that the cutaneous vasoconstrictor response was shifted to higher internal temperatures in the PM. These findings suggest that during whole body skin cooling the reflex control of the cutaneous vasoconstrictor system is shifted to a higher internal temperature in the PM. Furthermore, the slope of the relationship between CVC and T(sk) is steeper in the PM compared with that in the AM.     

Characterization of Diastereo‐ and Enantioselectivity in Degradation of Synthetic Pyrethroids in Soils

Permethrin (PM), cypermethrin (CP), and cyfluthrin (CF) are three important synthetic pyrethroids, which contain two, four, and four enantiomeric pairs (diastereomers) and thus have four, eight, and eight stereoisomers, respectively. In this study, the stereo‐ and enantioselective degradation of PM, CP, and CF in a Shijiazhuang alkaline yellow soil and a Wuhan acidic red soil were studied in detail by a combination of achiral and chiral high‐performance liquid chromatography (HPLC). The results showed that PM, CP, and CF degraded faster in Shijiazhuang soil than in Wuhan soil, and the dissipation rate followed an order of PM > CF > CP in both soils. The three pyrethroids exhibited similar diastereomer selectivity, while CP and CF showed higher enantioselectivity than PM. Moreover, the trans‐diastereomers degraded faster, and showed higher enantioselectivity than the corresponding cis‐diastereomers. For PM, the enantiomer 1S‐trans‐PM degraded most rapidly in both soils. As for CP and CF, the highest enantioselectivity was observed for diastereomer trans‐3, and the insecticidally active enantiomer 1R‐trans‐αS degraded fastest among the 8 CP or CF stereoisomers in both soils. In addition, the Wuhan acidic soil displayed higher diastereomer and enantiomer selectivity than the Shijiazhuang alkaline soil for the three pyrethroids. Further incubation of CF in an alkaline‐treated Wuhan soil showed that the dissipation rate greatly increased and the diastereo‐ and enantioselectivity significantly decreased after the alkaline treatment process. Chirality 28:72–77, 2016. © 2015 Wiley Periodicals, Inc.     

Photosystem II quantum yield as a measure of radical scavengers in chilling injury in cucumber fruits and bell peppers

Cucumber fruits (Cucumis sativus L. cv. Jessica) and green bell peppers (Capsicum annuum L. cvs. Lokas and Medeo) were stored at different temperatures ranging from 2 to 12°C. After three different storage periods, fruits from each temperature were transferred to 20°C for 7 d to allow for the development of visual symptoms of chilling injury (CI). During storage, the photochemical quantum yield of photosystem II (PSII) in peel tissue adapted to darkness, was calculated from measurements of pulse-amplitude-modulated chlorophyll fluorescence. The decrease in PSII quantum yield during storage at low temperatures in darkness can be described as a temperature-dependent inhibition of an enzyme according to Arrhenius, assuming a negative activation energy. By comparison with the radical-scavenger measurements of Hariyadi and Parkin (1991, Postharvest Biol. Techn.1, 33–45) it is postulated that the time and temperature dependence of the quantum yield parallels the diminution of radical-scavenging activity at lower temperatures in cucumber and capsicum fruits. This is combined with an equation for the process of radical scavenging itself and an equation for the auto-catalytic radical-producing lipid peroxidation reaction. These three basic processes lead to both a static and a dynamic model for the occurrence of chilling injury in low-temperature-sensitive plant tissue. A statistical fit of the measured data using the static model leads to the estimates of the different activation energies and reaction rates with a high degree of accuracy. The estimated values are in accord with what one would expect on the basis of knowledge of the processes leading to chilling injury, and directly point to meaningful physico-chemical parameters.Abbreviations C Symmetry point of logistic curve - CI intensity of chilling injury - e base of the natural logarithm system - E activation energy - Eff efficiency - F_v/F_m quantum yield of PSII - k reaction rate constant - R amount of free radicals - S amount of substrate for chilling injury (double bonds in fatty-acid chains) - t time - T temperature - Z amount of radical-scavenging enzyme Indices - CI chilling injury - d denaturation - i any index - max maximum value - min minimum value - r radical scavenging - ref reference (temperature) - s substrate - 0 initial amount This study was conducted in the framework of a research program on fruits and vegetables, partly financed by the Dutch Commodity Board for Vegetables and Fruits.     

A critical comparison of methyl and ethyl esters production from soybean and rice bran oil in the presence of microwaves

Transesterification of vegetable oils (from soybeans and rice bran) into methyl and ethyl esters using a batch microwave system was investigated in this study. A critical comparison between the two alcohols was performed in terms of yields, quality, and reaction kinetics. Parameters tested were temperature (60, 70 and 80 °C) and time (5, 10, 15 and 20 min). At all tested conditions, more than 96% conversion rates were obtained for both ethanol and methanol. Use of microwave technology to assist the transesterification process resulted in faster reaction times and reduced catalyst requirement (about ten-fold decrease). Methanol required lower alcohol:oil ratios than normally used in conventional heating, whereas ethanol required higher molar ratios. All esters produced using this method met ASTM biodiesel quality specifications. Methanol performed better in terms of performance and costs, while ethanol may have some environmental and safety benefits.