Permeabilization of plant tissues by monopolar pulsed electric fields: effect of frequency

Pulsed electric fields (PEF) nonthermally induce cell membrane permeabilization and thereby improve dehydration and extraction efficiencies in food plant materials. Effects of electrical field strength and number of pulses on plant tissue integrity have been studied extensively. Two previous studies on the effect of pulse frequency, however, did not provide a clear view: one study suggested no effect of frequency, while the other found a greater impact on tissue integrity at lower frequency. This study establishes the effect of pulse frequency on integrity of onion tissues. Changes in electrical characteristics, ion leakage, texture parameters, and percent weight loss were quantified for a wide range of pulse frequencies under conditions of fixed field strength and pulse number. Optical microscopy and viable-cell staining provided direct visualization of effects on individual cells. The key finding is that lower frequencies (f < 1 Hz) cause more damage to tissue integrity than higher frequencies (f = 1 to 5000 Hz). Intriguingly, the optical microscopy observations demonstrate that the speed of intracellular convective motion (that is, cytoplasmic streaming) following PEF application is strongly correlated with PEF frequency. We provide the first in situ visualization of the intracellular consequence of PEF at different frequencies in a plant tissue. We hypothesize that cytoplasmic streaming plays a significant role in moving conductive ionic species from permeabilized cells to the intercellular space between plant cells, making subsequent pulses more efficacious at sufficiently low frequencies. The results suggest that decreasing the pulse frequency in PEF may minimize the number of pulses needed to achieve a desired amount of permeabilization, thus lowering the total energy consumption. Practical Application: PEF cause pores to be formed in plant cell membranes, thereby improve moisture removal and potential extraction of desirable components. This study used in situ microscopic evaluation of onion cells, as they were pulsed with electric fields at different frequencies, to determine whether frequency was an important parameter. We illustrate that membranes were more effectively broken at lower frequencies as compared to higher frequencies. Application of this information will allow for improved design of PEF systems for more energy efficient dehydration or extraction of plant tissues.     

孙筱祥教授：风景园林专业的先驱

风景反映了自然形态、自然过程、自然认知以及风景中所蕴含的文化对这些认知的使用。人们对自然的认知与人们与风景的关系都是形成要素。坚定的观点和价值观伴生于风景,并常常是政治争论的问题。在此情境下,风景园林学科可能需要处理受争议的风景与高涨的价值观之间的关系。风景园林师肩负的任务要求他们要了解人民和文化,     

Biodegradation of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) by Phanerochaete chrysosporium: new insight into the degradation pathway

Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) is a recalcitrant energetic chemical that tends to accumulate in soil, close to the surface. The present study describes the aerobic biodegradability of HMX using Phanerochaete chrysosporium. When added to 7 day old static P. chrysosporium liquid cultures, HMX (600 nmol) degraded within 25 days of incubation. The removal of HMX was concomitant with the formation of transient amounts of its mono-nitroso derivative (1-NO-HMX). The latter apparently degraded via two potential routes: the first involved N-denitration followed by hydrolytic ring cleavage, and the second involved alpha-hydroxylation prior to ring cleavage. The degradation of 1-NO-HMX gave the ring-cleavage product 4-nitro-2,4-diazabutanal (NDAB), nitrite (NO2 -), nitrous oxide (N2O), and formaldehyde (HCHO). Using [14C]-HMX, we obtained 14CO2 (70% in 50 days), representing three C atoms of HMX. Incubation of real soils, contaminated with either HMX (403 micromol kg(-1)) (military base soil) or HMX (3057 micromol kg(-1)), and RDX (342 micromol kg(-1)) (ammunition soil) with the fungus led to 75 and 19.8% mineralization of HMX (liberated 14CO2), respectively, also via the intermediary formation of 1-NO-HMX. Mineralization in the latter soil increased to 35% after the addition of glucose, indicating that a fungus-based remediation process for heavily contaminated soils is promising. The present findings improve our understanding about the degradation pathway of HMX and demonstrate the utility of using the robust and versatile fungus P. chrysosporium to develop effective remediation processes for the removal of HMX.     

Use of a minimum perturbation approach to predict TIM mutant structures

A minimum perturbation conformational search approach is usedto model the structures of the yeast triosephosphate isomerase(TIM) single mutant in which the catalytic base Glul65 is changedto Asp, and the double mutant in which Glul65 is changed toAsp and Ser96 to Pro. In chicken TIM this double mutant is referredto as a pseudo–revertant because some of the catalyticactivity lost due to the first mutation is regained when thesecond mutation occurs. Three minimum energy structures werecalculated for the Asp 165 conformation in the yeast TEM singlemutant and another three for the double mutant One of the calculatedminimum energy conformations for Aspl65 in the E165D structureagrees well with the X–ray structure. However, this conformationis not that of the lowest energy and is not one of the threemost common conformers for Asp found by Ponder and Richards.This suggests that when an amino acid is introduced it may notbe able to conform to the more general rules that apply to proteinstructures of evolutionary origin. While the van der Waals energylargely determines the allowed minima, the relative rankingof the final minima is determined by electrostatic effects andcan therefore be affected by the inclusion of crystal watersin the calculation. When the E165D calculation is repeated withan active–site water molecule fixed in its E165D X–raystructure position, the relative ranking of the minima shiftsand the X–ray conformation for Asp 165 is the lowest interactionenergy conformer. Two of the E165D calculated minimum energystructures are essentially identical to two of the S96P/E165Dminima. All of the calculated minima for both the E165D andS96P/E165D mutants position the Asp side chain such that theanti–orbital, and not the more basic syn–orbital,of the carboxylate would be utilized for proton abstraction.This observation may explain why the chicken TIM S96P/E165Dmutant, for which the X-ray structure indicates that the syn–orbitalis used, is a pseudo–revertant while the yeast TIM doublemutant is not; no X–ray structure is available for thelatter. The multiplicity of minima found in the present analysismakes clear that predicting the exact orientation of a singleside chain is not as simple as might be expected.     

Reinvestigation of the antioxidant properties of conjugated linoleic acid

Despite repeated suggestions that antioxidant activity of conjugated linoleic acid (CLA), a collective of conjugated dienoic isomers of linoleic acid, underlies its reported anticarcinogenic and antiatherosclerotic effects, the antioxidant properties of CLA remain ill-defined. Therefore, this study was undertaken to gain more insight into the mechanism of potential CLA antioxidant activity. It was tested whether CLA could protect membranes composed of 1-palmitoyl-2-linoleoyl phosphatidylcholine (PLPC) from oxidative modification under conditions of metal ion-dependent or-independent oxidative stress. Progress of oxidation was determined by direct spectrophotometric measurement of conjugated diene formation and by gas chromatographic/mass spectrometric analysis of fatty acids. The oxidative susceptibility of CLA was higher than that of linoleic acid, and comparable to arachidonic acid. When oxidation of PLPC (1.0 mM) was initiated using the lipid-soluble 2,2′-azobis(2,4-dimethylvaleronitrile) or the water-soluble 2,2′-azobis(2-amidinopropane) hydrochloride, the radical scavengers vitamin E and butylated hydroxytoluene (BHT) at 0.75 μM efficiently inhibited PLPC oxidation, as evident from a clear lagphase. In contrast, 0.75 μM CLA did not have any significant effect on PLPC oxidation. Inhibition of PLPC oxidation by higher concentrations of CLA appeared due to competition, not to an antioxidant effect. When oxidation of PLPC was initiated by hydrogen peroxide/Fe²⁺ (500 μM/0.05–20 μM), both vitamin E (1 μM) and ethylene glycol-bis(aminoethyl ether) tetraacetic acid (50 μM) efficiently inhibited PLPC oxidation. However, CLA (1–50 μM) did not show a clear protective effect under any of the conditions tested. We conclude that CLA, under these test conditions, does not act as an efficient radical scavenger in any way comparable to vitamin E or BHT. CLA also does not appear to be converted into a metal chelator under metal-ion dependent oxidative stress, as had previously been suggested. On the basis of our observations, a role for CLA as an antioxidant does not seem plausible.     

Effects of systemic atropine sulfate administration on the frequency content of the cat sensorimotor EEG during sleep and waking.

Sensorimotor electroencephalogram (EEG) frequencies in cats were evaluated with power spectral analysis before and after 3 doses of atropine sulfate. All doses of atropine tested caused enhanced EEG slow waves (0–7 Hz) and spindles (8–25 Hz) during waking immobility, and postdrug frequency profiles during slow-wave sleep and waking immobility were identical. With 0.75 mg/kg atropine, movement (head movement, locomotion) resulted in EEG desynchronization and reduced power in all frequencies less than 24 Hz. After 1.5 or 3.0 mg/kg atropine, power in low frequencies remained elevated during movement, but power in spindle frequencies was significantly reduced compared with other states. During active REM sleep after 1.5 mg/kg atropine, power in spindle frequencies was significantly lower than that during quiet REM sleep. These results indicate that the sensorimotor cortical EEG in cats is under the control of multiple systems. At least 1 of these systems is active during movement, and its actions are resistant to muscarinic receptor blockade. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

正确选择合适的熔丝

一般而言,电路保护器件通常在电路设计中扮演着最不起眼的角色:人们通常都是在完成设计之后才想起它们,而且对其琐碎的电气特征不胜其烦.     

Failure of stationary but not of flying honeybees (Apis mellifera) to respond to magnetic field stimuli.

Trained individual honeybees (Apis mellifera) to feed to repletion from a well of sucrose solution so constructed that shock could be delivered when the proboscis was in contact with the solution. If shock was signaled by vibration of the substrate or by an airstream, the animals learned readily to avoid it by breaking contact briefly, but there was no response to change (either constant or time-varying) in the ambient magnetic field. When, however, a magnetic field anomaly in the region of the food well signaled to flying animals that contact would be punished with shock, hesitation to settle was greater in presence than absence of the anomaly. Parallel results were obtained with light, to which flying Ss clearly responded but to which stationary Ss hardly responded at all. We conclude that stationary bees detect magnetic field stimuli but do not process them successfully in the training situation. (PsycINFO Database Record (c) 2010 APA, all rights reserved)