Hydrogen sequences from degraded

Hydrocarbon oxidation may also be considered a free radical chain-type reaction. At elevated temperatures, hydrocarbon free radicals (R) are formed which react with oxygen lo form peroxy radicals (R(X These, in turn, take up a hydrogen atom from the hydrocarbon to form a hydroperoxide (ROOH) and another hydrocarbon free radical. The cycle repeals itself with the addition of oxygen. The unstable hydroperoxides remaining are the major points for degradation and lead to rancidity and color development in oils, fats, and waxes decomposition and gum formation in gasolines sludging in lubricants and breakdown of plastics and rubber products. Antioxidants, such as amines and phenols, are often introduced into hydrocarbon systems in order lo prevent this free radical oxidation sequence. 

Extraction of hemiceUulose is a complex process that alters or degrades hemiceUulose in some manner (11,138). Alkaline reagents that break hydrogen bonds are the most effective solvents but they de-estetify and initiate -elimination reactions. Polar solvents such as DMSO and dimethylformamide are more specific and are used to extract partiaUy acetylated polymers from milled wood or holoceUulose (11,139). Solvent mixtures of increasing solvent power are employed in a sequential manner (138) and advantage is taken of the different behavior of various alkaUes and alkaline complexes under different experimental conditions of extraction, concentration, and temperature (4,140). Some sequences for these elaborate extraction schemes have been summarized (138,139) and an experimenter should optimize them for the material involved and the desired end product (102). 

Differentiation from Decomposition Products. The successive degradation scheme for GA3 in neutral to acid solutions may be crudely portrayed by the sequence I V. With the exception of II, the existence and structures of these compounds have been amply documented [1(3, 5-7), III (4, 8), IV (7), V (7)]. Although II has not been isolated to date, its hydrogenated counterpart with an alicyclic A ring is known (24). 

Pyrolysis of the formed polysilazane at 750° C generates several alkylsilanes such as dimethylsilane, trimethylsilane, ethyldimethylsilane, tetramethyidisiloxane, pentamethyidisiloxane, methylenebisdimethylsilane, hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, etc. The presence of the cyclic compounds similar to those from poly(dimethylsiloxane) was an indication that some polysiloxane sequences may be present in the polymer. Thermal degradation studied between 350° C and 650° C showed the formation of some hydrogen, methane, ethane, and propene. 

Lysine may also present problems because its e-amine couples with phenyl-isothiocyanate to form the e-phenylthiocarbamyl derivative, which is sensitive to any trace of peroxides in the sequencer solvents that may decrease its yield. Its chromatographic elution close to leucine frequently requires special attention to gradient conditions as well. Tryptophan, tyrosine, and methionine are sensitive to severe oxidation, although this is usually an infrequent problem, especially with the latter two. Those residues with sidechains containing only carbon and hydrogen (e.g., Ala, Val, lie. Leu, and Phe) are the most stable and are not known to present any complications from the Edman degradation procedure. 

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