Reversible bleaching

Reversible bleaching experiments, in which the R-band decays to form the V-band, give a ratio of AZ>85o/AZ>65o = 0.92 at —78° C. Assuming that this is a dimer - monomer conversion, we thus obtain R-band = (4.9 X 104) (2) (0.92) = (9.0 it 2) X 104, for the molar extinction coefficient of the dimer at its absorption peak, at — 78 ° C. Table II summarizes these values and corresponding exinction coefficients in ammonia. 

Other reactions include pruritus, which may be intolerable and is common in Africans, headaches, gastrointestinal disturbance, precipitation of acute intermittent porphyria in susceptible individuals, mental disturbances and interference with cardiac rhythm, the latter especially if the drug is given intravenously in high dose (it has a quinidine-like action). Long-term use is associated with reversible bleaching of the hair and pigmentation of the hard palate. 

Hegemann, P., Hegemann, U., and Foster, K. W. (1988) Reversible bleaching of Chlamydomonas reinhardtii rhodopsin in vivo. Photochem. Photobiol. 48, 123-128. 

Heterodimer-containing His - 73 Leu RCs are photochemical active. These RCs exhibit reversible bleaching of their long wavelength absorption band by actinic light. Both the oxidized-minus-reduced difference spectrum and the rate of charge recombination between the oxidized heterodimer and the primary quinone anion are similar to that observed in His OO Leu RCs (data not shown). 

Sample preparation for hole burning spectroscopy generally involves dilution of the RCs (>10 pM RCs 0.05 % (wt/vol) Triton X-100 20 mM Tris/HCl pH 8.0) into glycerol to form a 1 2 aqueous to glycerol glass with an absorbance at 870 of 0.25-0.4. The RCs were reconstituted with UQ2 (2,3-dimethoxy-5-decyl-benzoquinone) at 40 pM to refill any of the sites left vacant by the isolation procedure. The room temperature reversible bleaching of the P870 band is used to indicate the extent of reconstitution. 

The electron can be trapped, for example by an interstitial which is converted to an H atom. The AlO is the hole color center which absorbs light and gives the color to smoky quart2. Bleaching is the result of thermal energy releasing the trapped electron, which then produces the reverse of reaction 1. The amethyst color center in quart2 is exactly like the smoky, except that Fe " replaces. ... 

The perhydroxyl ion, OOH , derived from the dissociation of hydrogen peroxide in alkaline solution, is generally considered the active agent in the bleaching of wood pulp using hydrogen peroxide. This ionization may be expressed by the following reversible reaction ... 

Bleach tower Reverse cleaners Forward cleaners Vtfesher Bleach tower Thickener Dispersion unit Flotation cells Reverse cleaners Vtfesher Bleach tower Vtfesher Bleaching Thickener Dispersion unit... 

Even very small amounts of transition-metal ions like cobalt, nickel, and copper cause rapid decomposition. They form reactive intermediates that can decrease the stabiUty of oxidizable compounds in the bleach solution and increase the damage to substrates. Hypochlorite is also decomposed by uv light (24,25). Acidic solutions also lose available chlorine by the reverse of equations 1 and 2. 

Because they are unstable, hypobromites are usually prepared just before use for such jobs as textile bleaching and desizing. In alkaline solutions at 50—80°C bromine reacts to form bromide and bromate. This reaction is reversed in acidic solutions. 

The reverse reaction, B returning to A, can be driven either by thermal or photochemical energy, or both. When the reversion is photochemicaHy driven, the process is called optical bleaching. Optical bleaching is a general characteristic and is a factor in almost all photochromic systems, even those normally thought of as being thermally reversible. 

The darkening reaction involves the formation of silver metal within the silver haUde particles containing traces of cuprous haUde. With the formation of metallic silver, cuprous ions are oxidized to cupric ions (1,4). The thermal or photochemical (optical bleaching) reversion to the colorless or bleached state corresponds to the reoxidation of silver to silver ion and the reduction of cupric ion to reform cuprous ion. 

Folding box, 5 X 1 X 8 in (125 x 25 x 200 mm), reverse-tuck design, 12-point kraft board with bleached white exterior 0.23 0.028 0.0008... 

Plastics can be used to make erasable printing media by a number of different techniques. Photo changing dyes could be incorporated into the structure of the plastics. The printer could change the dye to the colored form to read, and the material can be bleached with another unit that would reverse the photo coloring process. An ionic type plastic can be incorporated into the plastics and used to color the printed area by the use of an indicator type reaction with an organic acid or base. Another method would be to use a thermal printer in conjunction with liquid crystal type materials that would alter the state of the liquid crystals in the printed areas. Applying heat and electrical fields to the printed sheet would erase the printing. 

Second, reducing bleaches such as lemon juice (in combination with sunlight) or sulfur dioxide convert the double bonds in the chromophore into single bonds, eliminating its ability to absorb visible light. Sometimes the reaction is reversible, where oxygen in the air reacts with the molecule to repair the chromophore, and the stain returns. 

Skin lighteners, freckle and age spot removers, and other remedies for hyperpigmentation are not actually bleaches like the products listed so far. The active ingredient is hydroquinone, which inhibits melanin formation when applied to the skin. Since the effect is easily reversed by exposure to sunlight or ultraviolet light, a sunscreen is usually included in the formula. 

Bleaching is reversed in the dark and the red-purple color of rhodopsin returns. This is thought to occur by the reduction of all-Pms-retinal to vitamin Ai (retinal), which diffuses from the rod into the pigment epithelium, where it is converted enzymatically to the 1 l-c isomer of vitamin At. The enzymatic isomerization is followed by diffusion back into the rod, oxidation to 11 -rfr-retinal, and combination with opsin to form rhodopsin. This process is shown schematically in Figure 12.5.[Pg.289]

---