Abstract: The mechanism of phosphoserines bio mimic reaction involving the formation of hexa coordinate intermediate was studied by MNDOmethod. From literature, the intramolecular carboxylic phosphoric mixed anhydride 2 could be formed from starting material phosphoserine 1 by a non synchronous concerted reaction pathway and the transition state energies were 172 kJ/mol. In this paper, 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 step, the acidic proton in the intermediate 2 dissociated from P_O_Hbond to form 2', in which there is a weak hydrogen bond between O 1 on phosphorus and the hydroxyl group on the serine side chain. Then the hydrogen of the hydroxyl group could be transfered to O 1 on phosphorus through a hydrogen bond bridged transition state. In the second step, the oxygen O6 anion on the serine side chain could attack the phosphorus and the molecule geometry was changed 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 hexa-coordinate phosphorus intermediate and the co-participation of the carboxyl group and hydroxyl group of serine side chain during its bio-mimic reaction.

Key words: Hexa-coordinate phosphorus, MNDO, Hydrogen-bond-bridge