Thiophosphate monoesters

B. Synthesis of Thiophosphate Monoesters Chiral by Virtue of Oxygen Isotopes. 102... 

F. Configurational Analysis of Oxygen Chiral Thiophosphate Monoesters. 112... 

As noted previously, studies of the mechanisms of phosphate monoester solvolysis have been extended to the mechanisms of the analogous phosphorothioate ester solvolysis because the thiometaphosphate anion is believed to be more stable than the metaphosphate anion. Thus, a general method based upon P NMR spectroscopy for the configurational analysis of chiral thiophosphate monoesters (see Fig. 10) was described recently by Cullis and co-workers (38). 

These comparative studies constituted the first example of an enzyme-catalyzed hydrolysis reaction whose stereochemical course was unaffected by sulfur substitution. At the time these experiments were performed, the stereochemical courses of the reactions catalyzed by glycerol kinase (83, 84) and by the bacterial adenylate cyclase (85, 86) had already been compared in the laboratories of Knowles and Gerlt, respectively, and these were also found to be unaffected by the sulfur substitution. A number of other comparisons of this type have been made, and in no case were the stereochemical consequences of the reactions studied with chiral phosphate esters and the chiral thiophosphate analogs found to differ. This agreement suggests that the necessary use of oxygen chiral thiophosphate monoesters to study the stereochemical course of phospho-monoesterases will provide pertinent results for ascertaining whether phosphory-lated intermediates are involved in the reaction mechanism. 

Although it is convincing that metaphosphate ion is not a free intermediate in substitution reactions in aqueous solution, this does not exclude the possibility of reactions of related species proceeding via analogous intermediates. Reactions of thiophosphate monoesters are more rapid than those of phosphate monoesters. If three sulphur atoms replace three of the oxygens of metaphosphate, a stable salt can be isolated (Roesky et al, 1986). Harnett and Lowe (1988) examined the stereochemical course of the non-enzymic hydrolysis of adenosine (P- O, thio) diphosphate in In... 

The proposed differential energetic effects on transferring the ground State and the transition state for hydrolyzing phosphate and thiophosphates monoester dianions from water to DMSO was tested by Hengge s group... 

The mechanism of an actual hydrolysis reaction catalyzed by this prototype phosphomonoesterase has never been studied stereochemically. This apparent omission is presumably explained by the very low catalytic efficiency of the enzyme toward phosphorothioate monoesters as compared to phosphate monoesters (75) certainly, chiral [ O, 0]phosphorothioate 0-ester substrates already exist, and methodology is available for the configurational analysis of the chiral [ 0, 0, 0]thiophosphate that would be produced if the chiral substrate were hydrolyzed in H2. In fact, the low catalytic reactivity of phosphorothioate O-esters and the high reactivity of phosphorothioate S-esters has been explained by the enzyme utilizing nucleophilic catalysis (an associate mechanism) to achieve hydrolysis of the phosphate ester bond 40). 

Such a line-broadening effect of 0 in P NMR has been used to locate the position of a O label (Tsai, 1979) and to calculate the percent enrichment of (Huang and Tsai, 1982 Reed and Leyh, 1980). In addition, it has made possible analysis of the configuration of [ 0, 0, 0]phosphate monoesters and [ 0, 0, 0]thiophosphates by P NMR (Section VI). 

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