Environmental effects on phosphoryl group bonding probed by vibrational spectroscopy: implications for understanding...

We have used vibrational spectroscopy to study bonding in monosubstituted dianionic phosphates, both to learn more about basic properties intrinsic to this important class of biological substrates and to assess the ability of vibrational spectroscopy to provide a "sensor" or probe of the local environment experienced by the phosphoryl group. We examined the bonding properties of the phosphoryl group via vibrational spectroscopy for a series of compounds in which the phosphoryl substituent was varied systematically and extensively. A broad linear correlation of the bridging P-O(R) bond length and the pK(a) of the substituent alcohol was observed. The results indicate that the P-O(R) bond changes by only approximately 0.04 A with alcohol substituents that vary in pK(a) by approximately 12 units, suggesting that phosphoryl group bonding responds in a subtle but regular manner to changes in the local environment. We also determined the effect on the phosphoryl bonding from changes in the solvent environment. Addition of dimethyl sulfoxide (DMSO) elongates the bridging bond, presumably as a result of lessened solvation to the nonbridging oxygens and conservation of bond order to phosphorus. Finally, we have addressed the relationship between ground-state bonding properties and reactivity, as changing the leaving group substituent and adding DMSO have large rate effects, and it was previously proposed that lengthening of the bond to be broken is the cause of the increased reactivity. The results herein suggest, however, that the change in the bridging bond energy is small compared to the changes in energy that accompany the observed reactivity differences. Further analysis indicates that electrostatic interactions can provide a common driving force underlying both bond lengthening and the observed rate increases. We suggest that ground-state distortions of substrates bound to enzymes can provide a readout of the electrostatic active site environment, an environment that is otherwise difficult to assess.