Metaphosphates, determination phosphates

Scheme 2. Conformational changes of the y-phosphate in a) phosphoryl-transfer reaction transition state K), and various species of AlFx b) AlF41, c) A1C13. Dotted lines indicate that the degree of bond making and bond breaking determines whether the transition is more dissociative, with a metaphosphate-like intermediate, or associative, with a pentavalent intermediate. Charges have been omitted for clarity. N = adenosine or guanosine. According to [16, 17, 21]...

Hydrolysis of Phosphates. The conclusion that phosphoryl transfer between amines is concerted does not exclude the formation of a metaphosphate intermediate in reactions with weaker nucleophiles, including water. Pyridine is a much stronger nucleophile than water and it is possible that phosphoryl transfer between pyridines is concerted because the metaphosphate ion does not have a significant lifetime in the presence of pyridine the metaphosphate ion might exist for a short time in water. However, it is much more difficult to determine whether or not the mechanism is concerted for a reaction with water than for a reaction with pyridine. 

The stereochemical consequences of the methanolyses of the monoanion of phenyl [ 0, 0, 0]phosphate, the dianion of 4-nitrophenyl [" 0, 0, 0] phosphate, and [ 0, 0, 0]phosphocreatine have been determined by Knowles and co-workers (35). The monoanion of phenyl phosphate behaves as a typical phosphate monoester in that its rate of hydrolysis is maximal at pH 4, where an intramolecular proton transfer is possible. The dianion of 4-nitrophenyl phosphate is highly reactive since protonation of the leaving group is not necessary. Finally, A-phosphoguanidines have been reported to be the most reactive phosphoryl compound (the chiral phosphocreatine can be enzymically synthesized from [ y- 0, 0, 0]ATP). Thus, the solvolyses of all three of these compounds are believed to involve the participation of metaphosphate anion. The meth-anolysis of each of these compounds proceeds with quantitative inversion of configuration. 

Structure of the substrate and the reaction conditions determine the transition state for reaction with a particular nucleophile 104, 105). The extreme cases are generally described as the dissociative and associative substitution mechanisms. The fully dissociative mechanism entails the formation of monomeric metaphosphate monoanion as a discrete intermediate and was first formulated by F. H. Westheimer, who pioneered the physical organic chemistry of the hydrolysis of phosphate esters 106, 107). This mechanism is depicted in Eq. (40) and is possible only for phosphomonoesters with good leaving groups, examples of which are shown. 

Quantitative estimation of the compounds of simple mixtures of phosphates, polyphosphates, and polymetaphosphates can be effected without too much difiSculty. Determinations of orthophosphate, pyrophosphate, triphosphate, and so-called hexametaphosphate" are usually made on mixtures of soluble phosphates. Total phosphorus content is also determined by hydrolyzing all other phosphates to orthophosphoric acid. No distinctive analytical methods are, however, available for the direct quantitative determination of the cyclic metaphosphates. 

Of the esters, starch phosphate is produced by reaction with phosphorus oxychloride, polyphosphates, or metaphosphates a cross-bonded product results. Total degree of substitution is determined by measuring the phosphorus content, and the mono- to disubstitution ratio can be calculated by potentio-metric titration. Allowance is made for the natural phosphorus content of the starch. Treatment of starch with acetic anhydride produces starch acetate, which has improved paste stability over native starch. The acetyl group is very labile, and hydrolyses readily under mild alkaline conditions. When a known amount of alkali is used, the excess can be titrated and the ester function measured. This is not specific, however, and a method based on an enzymatic measurement of the acetate has been developed in an ISO work group. The modified starch is hydrolyzed under acidic conditions, which releases acetic acid and permits filtration of the resulting solution. Acetic acid is then measured by a commercially available enzyme test kit. Both bound and free acetyl groups can be measured, and the method is applicable... 

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