Oxygen-free solutions

The most direct and unambiguous way to study the reaction of free radicals with hyaluronic acid is to use radiation-chemical techniques. In dilute aqueous solutions, ionising radiation is absorbed mainly by the water to produce water-derived radicals and molecular products  

The radiolytic yields of these species are now well established and are 0.28, 

05 and 0.27p,mol J 1 respectively [17], In the presence of N2O, hydrated electrons are converted to hydroxyl radicals in a relatively high yield compared to the only other free radical produced in this system, 

the hydrogen atom. The radiolysis of nitrous-oxide-saturated solutions of hyaluronic acid therefore produces a relatively simple system in which the principal reaction is that between hydroxyl radicals and hyaluronic acid. In early radiation-chemical studies, measurements were made of the rate constants for the reaction of hydroxyl radicals and hydrated electrons with hyaluronic acid using pulse radiolysis techniques [85-87], However, very pure samples of hyaluronic acid were unavailable and this has led to the speculation that impurities might have been responsible for some of the observed reactions. Highly pure samples of hyaluronic acid have since become available and in view of its current significance to the degradation of hyaluronic acid in rheumatoid synovial fluid, 

From the figure, the yield of chain breaks is calculated to be 0.3 p,mol J 1, which indicates that about 56% of all hydroxyl radicals produce a chain break. 

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In water pollution studies, the oxygen content can be measured by making the water alkaline and shaking a measured volume with an oxygen-free solution containing Mn- (aq). The solution is acidified with sulphuric acid, potassium iodide added and the liberated iodine titrated with sodium thiosulphate. 

The situation will of course be different if the potential is maintained constant, e.g. in cathodic protection. For example, if the potential was maintained constant at, say, E , in an oxygen-free solution the rate due to the h.e.r. would be rq which would be unaffected if the solution was then aerated. However, since the corrosion rate would then increase, the current required to maintain the potential constant would have to be increased. 

We find that the fluorescence yield of freshly prepared covalent (+)-anti-BaPDE-DNA adducts in oxygen-free solutions is 66+2 lower than the yield of the tetraol 7,8,9,10-tetrahydroxytetrahydro-benzo(a)pyrene (BaPT) in the absence of DNA. Since the fluorescence lifetime of BaPT under these conditions is 200ns, the mean fluorescence lifetime of the adducts (see reference T7) can be estimated to have a lower limit of 3ns, which is close to the mean value of 0.52x1.6 + 0.42x4.0 = 2.7 ns estimated from the two short fluorescence components of Undeman et al (10). 

J. Wang and F. Lu, Oxygen-rich oxidase enzyme electrodes for operation in oxygen-free solutions. J. Am. Chem. Soc. 120, 1048-1050 (1998). 

In oxygen-free solutions, N-methyl-9-cyano-acridanyl radical 81 was detected this stems from a homolytic cleavage of the biacridanyl-9,9 -dicyanide 80. 

We think, however, that this certainly doesn t mean that there aren t any free radicals it means that the detection of these free radicals is going to be very difficult in these metal ion systems. We have to use flow techniques, oxygen-free solutions, and things of this sort in order to do it, and it s rather difficult. 

The photochemical reactions of a-dicarbonyl containing compounds have recently been the subject of a number of investigations. This review will consider reactions that take place in oxygen-free solution and will contrast them with reactions that have been observed in the vapor state. 

The 3,4-dihydrooxocin (147 X = CH2) is obtained in 20% yield by photolytic ring opening of the bicyclic pyran (146) in rigorously oxygen-free solution (78CB3608). The reaction,... 

Dilute oxygen-free solutions of 4 (10 M) are stable up to 70°C, but in concentrated solutions polymerization occurs at and below room temperature. An interesting phenomenon was observed with temperature change whereas solutions are colorless at room temperature, they show a reversible yellow to pink coloration on cooling to dry ice/acetone (in CHCI3 and CCI4). ... 

Mertens R, von Sonntag C (1994b) The reaction of the OH radical with tetrachloroethene and tri-chloroacetaldehyde (hydrate) in oxygen-free solution. J Chem Soc Perkin Trans 2 2181-2185 Miaskiewicz K, Osman R (1994) Theoretical study on the deoxyribose radicals formed by hydrogen abstraction. J Am Chem Soc 116 232-238... 

The kinetic behaviour of strand-break formation in oxygen-containing solutions is quite different to that observed in oxygen-free solutions where... 

Reactions of simple pyridines with bis(trimethylsilyl)mercury results in the formation of l,r-bis(trimethylsilyl)-l,l, 4,4 -tetrahydro-4,4 -bipyridines. These reversibly dissociate on heating in oxygen-free solution to yield 1-trimethylsilyl-l-hydropyridinyl radicals which have been characterized by ESR, e.g., 71 for which splittings are given in gauss.241 If the pyridines bear... 

After mixing the oxygen-free solutions of halocarbons with solutions of pyridinyl radicals, the rate of the reaction may be followed spectroscopically. Reaction of l-isopropyl-4-carbomethoxypyridinyl, (i-Pr)4, with bromochloromethane in acetonitrile gave l-isopropyl-4-carbomethoxypyridinium bromide, (i-Pr) , in 35% yield. The identification of two isomeric, air-sensitive dihydropyridines was facili-tat l by the characteristic patterns (NMR) for the isopropyl groups (Eq. 25),... 

The reaction is very sensitive to metal ion catalysis, particularly by Cu " and Ag", and oxygen inhibits the reaction. Po and Allen studied the uncatalysed reaction in oxygen-free solutions containing 10 M EDTA to ensure that the concentrations of free metal ions were insignificant. Under these conditions the reaction is first order with respect to peroxodisulphate and the rate is essentially independent of oxalate concentration (there is a slight increase in the first-order rate coefficient with increase of oxalate concentration). Allyl acetate inhibits the reaction and reduces the rate to that observed in the absence of oxalate. In the range pH 0.5-10.3 a rate maximum occurs at pH 4.5. The first-order rate coefficient for the reaction using 0.08 M disodium oxalate is expressed by... 

Another example of a mixed potential is encountered in the hypothetical case of a perfectly pure solution of one form of a redox couple. Consider, for instance, a platinum electrode in a solution of Fe(II) containing not even one Fe(III) ion. According to the Nemst equation the potential has a negative value of infinity. Actually, the potential is limited in that a cathodic reaction would occur at some finite potential. In an oxygen-free solution, reduction of hydrogen ions to hydrogen would occur, and the potential would shift to a value such that the rate of reduction would... 

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