Abstract: We studied the chiral transition mechanism of the bis-α-alanine chelating Ca(Ⅱ) complex (α-Ala)₂→Ca(Ⅱ)  by using the M06-2X and MN15 methods of density functional theory (DFT). The results show that the chiral transition of (S-α-Ala)₂→Ca(Ⅱ) undergoes  two processes:  Firstly,  one of the α-Ala in (S-α-Ala)₂→Ca(Ⅱ) isomerizes from the S-type to the R-type to obtain R-allo-α-Ala→Ca(Ⅱ).  Secondly,  the S-α-Ala in R-allo-α-Ala→Ca(Ⅱ) isomerizes again to R-α-Ala to obtain (R-α-Ala)₂→Ca(Ⅱ).   The free energy barrier for the (S-α-Ala)₂→Ca(Ⅱ) isomerization to the R-allo-α-Ala→Ca(Ⅱ) tachycritical step under the implicit aqueous solvent  is 221.5 kJ/mol from the transition state where the H proton migrates from the chiral C atom to the  N atom， the free energy barrier drops to 93.1 kJ/mol in dominant aqueous solvent. The free energy barrier for the R-allo-α-Ala→Ca(Ⅱ) isomerization to (R-α-Ala)₂→Ca(Ⅱ) tachycritical step is 233.8 kJ/mol for the implicit aqueous solvent  from the transition state where the H proton  migrates from the chiral C atom to the  N atom, the free energy barrier  drops to 116.7 kJ/mol in the dominant aqueous solvent. Therefore,  the chiral transition rate of (S-α-Ala)₂→Ca(Ⅱ) is slow in physiological environments,  and   bis-α-Ala chelate calcium can be safely used for supplementation of living organisms with elemental calcium and α-Ala.

Key words: divalent calcium,  , α-alanine,  , chiral transition,  , density functional,  , self-consistent reaction field,  , transition state,  , free energy barrier