Gas phase interaction of (L)-proline with Be2+, Mg2+ and Ca2+ ions: A computational study

Geometry optimisation and metal ion affinities (MIAs) of the binding configurations of Be2+, Mg2+ and Ca2+ to (L)-proline were calculated using the hybrid Density Functional Theory (DFT-B3LYP) and second order Mollet-Plesset perturbation theory (MP2) methods. Be2+ was found to bind preferentially in a charge transfer type arrangement through the carbonyl oxygen (-C=O) and the lone pair of the imino-group nitrogen atom (-NH-). On the contrary Mg2+ and Ca2+ were found to prefer binding in a bi-dentate manner through the carboxylate group of L-proline (OCO) in a zwitterion form. The main types of interactions found to influence the binding preference Of M2+ ions to L-proline were (i) charge transfer in the case of Be2+ ions and (ii) electrostatic interactions in the case of Mg2+ and Ca2+ ions. Inspection of the IR stretching of the N-H and the O-H groups of (L)-proline with M2+ ions in a chelating configuration (to both O and N atoms) indicated a considerable shift to higher frequency with decreasing MIA. On the other hand, the MIA for the zwitterion L-proline with M2+ tracks the reciprocal distance of the M2+-OCO bond further confirming that the nature of the bond is mainly electrostatic. Comparison with other molecules containing the carboxylic function is also included in order to gain more insight on the types of interaction of this amino acid with metal ions in general.