Metaphosphates, monomeric systems

Monomeric Metaphosphates in Enzymic and in Enzyme-Model Systems... 

During the last two decades, the mechanisms of many enzymic processes have been established, and model systems have been developed that effectively mimic their action. In particular, the roles of thiamin, NAD, pyridoxal, flavins, Bl2, ferridoxin, and metals in many enzymic processes now are understood. Model systems have been developed to imitate the action of decarboxylases and esterases, to imitate the action of enzymes in binding their substrates, and to approach the stereospecificity of enzymes. Our laboratory recently has found phosphorylating agents that release monomeric methyl metaphosphate, which in turn carries out phosphorylation reactions, including some at carbonyl oxygen atoms, that suggest the actions of ATP. The ideas of biomimetic chemistry are illustrated briefly in terms of the processes mentioned above. 

These introductory remarks will have a twofold objective first, to review the history of the field, in an attempt to put the following papers into prospective second, to present some work from my own laboratory on a possible biomimetic system—the generation of monomeric metaphosphate, and its reactions that seem to parallel the action of pyruvate kinase and of amidotransferases. 

Further progress has been reported in the chemistry of CT X, -p -bonded systems. Full details of such systems stabilised by intramolecular coordination, as in, e.g., 334, have been described. The kinetically stable system 335 has been prepared and its solid state structure determined . The P-halobis(imino)-CT -phosphoranes 336 have also been prepared, and detailed NMR studies of bis(imino) phosphoranes reported . Quin s group has continued studies of the generation and characterisation of reactive a X -systems, e.g., 337 ". Methods for the generation of monomeric metaphosphate esters in solution have been investigated. A theoretical approach has been used to probe the mechanism of the reaction between phosphanylnitrenes 338 and boranes. The thiophosphonic anhydride 339 behaves as a source of the dithioxophosphorane... 

The corner diagonally opposite the pentacoordinate phosphorus is occupied by a tricoordinate phosphorus species, which is either monomeric metaphosphate (in the case of the reaction of a monoester) or a substituted derivative. Again, in the platonic sense it is easy to visualize metaphosphate as an ideal species, and it can be given reality by calculations (Sliznev et al., 1981) and by observations of species in the gas phase (Henchman et ah, 1985 Meyerson et al., 1984). However, the actual involvement of such a species in a stepwise substitution process in aqueous solution has not been demonstrated (Freeman et al, 1987). It is clear that such a species is highly electron-deficient, and it may be too reactive to exist as an intermediate (Guthrie, 1977). The quest for evidence in this system has produced elegant experiments that have provided much information about the nature of substitution at phosphorus in general. 

Phosphorus w) Compounds. Although the monomeric PO3 ion is severely electron-deficient, S.C.F. M.O. calculations have indicated a greater overlap population in both the a- and ir-systems for this species than in the isoelectronic nitrate ion. In view of this, it has been suggested that kinetic rather than thermodynamic reasons should be sought for the instability of PO3. The formation of monomeric methyl metaphosphate, Me0P02, reported in an earlier volume, has now been substantiated by identifying the methyl ester of p-diethyl-aminobenzenephosphonic acid as the product from the reaction with NN-diethyl-aniline at low temperatures. Substitution into the aromatic ring provided a measure of the electrophilicity of the monomer. 

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