Complexation of glycine by manganese (II) in the gas phase: A theoretical study

The interaction of Mn²⁺, being in three possible spin states, with glycine has been studied using quantum chemical calculations in the gas phase. Seven modes of interaction have been considered in all cases. Investigation of manganese complexes having various possible spin state including high, intermediate and low spins shows that the most stable complexes are high spin (total electron spin S = 5/2). Calculations show that the most stable mode of binding involves the simultaneous interaction of Mn²⁺ with two oxygen atoms of the zwitterionic conformers of glycine (η²-O,O (CO₂^-)), while the second preferred binding site consists of chelation between the carbonyl oxygen and the amino nitrogen (η²-O,N). These results indicate that the relative affinity of Mn²⁺ and the glycine zwitterion makes this isomer of glycine more stable than others, even though it is not stable in its isolated form. The results are in accord with those of previous reports for some doubly charged cations and in some cases with monocations. The nature of the interaction between manganese metal cation and glycine is also discussed employing natural population and molecular orbital analyses. It has been found that electrostatics contribution plays a crucial role in this interaction. Complexation energy (D₀) of the low lying energy isomer, for the most stable sextet spin state, has been obtained as about 156 kcal/mol. It has also been shown that the computed vibrational frequencies could aid in identification of the most stable Mn²⁺-glycine complex.