A theoretical study on the formation of hexa-coordinate phosphorus intermediate for phosphoserine's bio-mimic reaction

The mechanism of phosphoserine's bio-mimic reaction involving the formation of hexa-coordinate intermediate was studied by MNDO method. From literatur e, the intramolecular carboxylic-phosphoric mixed anhydride 2 could be form ed from starting material phosphoserine 1 by a non-synchronous concerted re action pathway and the transition state energies were 172 kJ/mol. In this p aper, it was showed that the penta-coordinate phosphorus 2 could in turn be converted into hexa-coordinate phosphorus 3 by two steps. In the first ste p, the acidic proton in the intermediate 2 dissociated from P-O-H bond to f orm 2', in which there is a weak hydrogen bond between O1 on phosphorus and the hydroxyl group on the serine side chain. Then the hydrogen of the hydr oxyl group could be transfered to O1 on phosphorus through a hydrogen-bond- bridged transition state, In the second step, the oxygen O6 anion on the se rine side chain could attack the phosphorus and the molecule geometry was c hanged into octahedral structure of hexa-coordinate phosphorus intermediate 3, The hydrogen-bond-bridge structure lowered the transition state energy to 148. 5 kJ/mol. These results could be used to explain the formation of h exa-coordinate phosphorus intermediate and the co-participation of the carb oxyl group and hydroxyl group of serine side chain during its bio-mimic rea ction.