Extent Of Bleaching Research Articles

Bleaching of membrane-bound merocyanine 540 in conjunction with free radical-mediated lipid peroxidation

The lipophilic dye merocyanine 540 (MC540) can photosensitize potentially lethal cell membrane damage as well as its own degradation (bleaching). Photobleaching in a test membrane, the human erythrocyte ghost has been examined. White light irradiation of MC540-sensitized ghosts resulted in lipid hydroperoxide (LOOH) formation, low-level thiobarbituric acid (TBA) reactivity, and dye bleaching (A 568 decay). When the reaction was carried out in the presence of ascorbate (AH −), and added Fe 3+, there was a large enhancement of TBA reactivity (indicative of free radical-mediated lipid peroxidation) and concomitant increase in the rate of photobleaching. Rapid bleaching also occured when MC540 was incubated in the dark with ghosts that had been photoperoxidized with another dye (a phthalocyanine) and then exposed to AH −. The extent of bleaching in this system was found to be proportional to the starting level of LOOH. Like the wave of free radical lipid peroxidation that accompanied it, dye bleaching in AH −-treated, preperoxidized ghosts was stimulated by supplemental Fe 3+, inhibited by desferrioxamine or butylated hydroxytoluene (BHT), but unaffected by catalase or superoxide dismutase. From this and related evidence, we deduce that: (1) in the absence of Fe 3+ AH − , photoperoxidation and photobleaching occur independently and are non radical, singlet oxygen-mediated processes; and (2) in the presence of Fe 3+ AH − , 1-electron reduction of photogenerated LOOHs results in a surge of lipid peroxidation that amplifies dye loss via free radical processes. MC540 bleaching might be exploited as a relatively simple and sensitive indicator of lipid autoxidation in isolated membranes and cells.

Mechanism of manganese toxictty and tolerance of plants VII. effect of light intensity on manganese‐induced chlorosis

Abstract Effect of light intensity on Mn‐induced chlorosiss was investigated with bush bean (Phaseolus vulgaris L.) and corn (Zea mays L.) seedlings. The seedlings were grown in nutrient solutions containing different concentrations of Mn in enclosures which transmitted different percentages of the total solar radiation. At high levels of Mn in nutrient solution, the increase in light intensity increased the Mn uptake by the plant and resulted in a decrease in the chlorophyll content of the leaves. Even at similar levels of Mn concentrations within the leaves, high light intensity increased the severity of Mn‐induced chlorosis. Photobleaching experiments were carried out with isolated chloroplasts suspended in media containing 0, 10‐4 , 10‐3,10‐2 and 10‐1 M Mn2+. Addition of Mn2+ to the medium decreased the extent of photobleacing of chlorophyll with increaing Mn2+ concentration up to 10‐3 M . In concentrations of Mn2+ higher than 10‐3 M, the extent of bleaching was increased again, accompanied by precipi...

Rhodopsin photoenergetics: lumirhodopsin and the complete energy profile

1. Introduction Various intermediate stages in the photolysis of visual pigments have been recognised, but neither their stereochemical form nor their relation to the actual process of visual excitation have been resolved (review [I]). The relative energies of many of the intermedi- ates in the bovine rhodopsin bleaching sequence have been determined in recent years by direct photocalo- rimetric methods [2-I]. Here I report the measure- ment of the enthalpy change associated with the light- induced formation of lumirhodopsin in bovine photo- receptor membranes at -75°C. This completes the sequence of calorimetrically accessible intermediates, and allows construction of an energy profile for the entire process which establishes firm thermodynamic constraints for proposed molecular mechanisms. 2. Materials and methods Bovine rod outer segment membranes (ROS) were prepared by a standard sucrose density gradient tech- nique [5] from the retinas of fresh, dark-adapted cattle eyes, obtained locally. Samples for photocalo- rimetry were prepared by suspension of ROS in a mixture of glycerol (2 vol.) and 0.1 M sodium phos- phate buffer (1 vol.), pH 7.2 at room temperature. Rhodopsin concentrations and the extent of bleaching during calorimetry were determined from room tem- perature absorbance spectra of aliquots of the ROS suspension dispersed in 2% (w/v) CTAB (cetyltrimeth- ylammonium bromide, Sigma Chem. Co.), 0.1 M phosphate buffer, pH 7 .?I, containing 10 mM hydrox- ylamine hydrochloride, assuming a molar extinction coefficient of 41 000 at 500 nm. Unless otherwise stated, all operations were performed in the dark or under dim red light. 324 Instrumental details of the low-temperature photo- calorimeter have appeared in [3,6]. For these experi- ments the entire calorimetric unit was surrounded by a copper circulation coil and suspended in a Dewar vessel filled with methanol. The system was further insulated with expanded polystyrene foam and mounted in a temperature-controlled ( 20 f 1°C) darkroom. The calorimeter was cooled to -75°C by circulation of methanol from a low-temperature bath, modified for proportional temperature control with a Pt resistance probe and precision electronic controller. Sample illumination was via glass fibre optics from a grating monochromator and a stabilised 200 W xenon arc lamp. Electrical calibration gave an instrumental sensitivity of 0.071 pV/pW under these conditions. The amplified signal, integrated over 60 s intervals, was recorded digitally as a function of time through- out an experiment. In a typical experiment the photocalorimeter was loaded with 2 .O ml ROS suspension (Esoo = 1) together with a reference sample of the same material totally bleached at room temperature prior to loading. After thermal equilibration at -75°C for several hours (usually overnight) the sample was irradiated briefly at 480 or 520 nm and the differential thermal response recorded. The sample was then removed, warmed to room temperature, and an aliquot taken for spectral determination of the amount of photoreaction in comparison with starting material. Control experi- ments included trials with bleached material in both calorimeter cells. 3. Results and discussion Lumirhodopsin is the kinetically stable photo- product of bovine rhodopsin at -75°C and the reac- tion only proceeds to subsequent stages on warming

Studies in Wool Yellowing

Variations in the concentrations of aluminum sulfate and sodium formaldehyde sulfoxylate, added with the fluorescent brightening agent to unbleached wool, can affect both the extent of bleaching and the uptake of fluorescent brightening agent. Experi ments with different fluorescent brighteners have confirmed that several of these reagents are suitable for wool fabric if the fabric is subsequently treated with thiourea-formaldehyde. Comparisons made were based primarily on changes in fluorescence and yellowness index on irradiation. Resins may be added with the thiourea-formaldehyde treatment to increase the resistance of the fabric to shrinkage without adversely affecting the protection given against yellowing. Fabrics treated initially with hydrogen peroxide instead of chlorine showed improved shrink-resistance when followed by resin-thiourea-formaldehyde treat ment and appreciably better appearance.

VII2. The Kinetics of the Thionine‐Ferrous Ion Reaction

AbstractThe initial quantum yield of photoreduction of thionine by ferrous ions and the extent of bleaching in the photostationary state are measured. The reaction is interpreted by two mechanisms; in one, a long‐lived, excited species of thionine is reduced by ferrous ions; in the other, an electron transfer occurs on excitation of a complex formed between thionine and ferrous ions and is followed by breakdown to products. It is argued that the main product of photoreduction is leucothionine, produced by dismutation of semithionine.

The use of an oxidation-reduction indicator to compare the oxygen permeabilities of films for rindless cheese

Slices and blocks of cheese were coloured with aqueous methylene blue solution and vacuum-packed or wrapped in the films to be compared. The extent of bleaching of the dye after 14 days gave a good indication of the extent to which oxygen was excluded by the film. Similar results were obtained with vacuum-packed slices and with wrapped rindless cheeses.With polythene-coated and wax-coated MSAT cellophane films there was little or no reduction of methylene blue on the cheese surface. Similar films, including a lamination of foil or MXXT cellophane, allowed complete or almost complete bleaching.