COMPARISON OF IN-LINE AND NON-IN-LINE ASSOCIATIVE AND DISSOCIATIVE REACTION PATHWAYS FOR MODEL REACTIONS OF PHOSPHATE MONOESTER HYDROLYSIS

The energies, geometries and transition state orders for several diffe rent potential phosphate ester group hydrolysis mechanisms are determi ned at different levels of phosphate group and nucleophile protonation to define factors which might favour one phosphoryl group transfer me chanism over another. With respect to the phosphate ester group, proto nation is considered as a generic model for phosphate anion charge neu tralisation by, for example, alkylation or metal ion chelation. The re sults indicate that the protonation state of either the phosphate grou p and/or the nucleophile can produce dramatic changes in the relative energies of intermediates and transition states for any particular hyd rolytic mechanism and that the magnitude of the changes can be suffici ently large to cause a change in favoured mechanism. For example, the results of the calculations predict that the favoured mechanism should change from a dissociative one at low levels of phosphate group proto nation, to an associative one at higher levels of phosphate group prot onation. Furthermore, the results indicate that under conditions in wh ich a highly stable pentacoordinate intermediate is formed, the favour ed reaction pathway involves a non-inline displacement of the leaving group by the nucleophile rather than an inline displacement. The relev ance of these findings to understanding enzyme-catalysed phosphoryl gr oup transfer is discussed.