A theoretical study of Na+ and Mg+2 binding to the carbonyl oxygen of N-methyl acetamide.

Molecular orbital calculations (CNDO/2) are reported for the interaction of Na+ and Mg+2 with the carbonyl of a model peptide moiety (N-methyl acetamide) as a function of the C--O ... Me distance and angle and with variation in the number of ligands for the purpose of determining the steepness of the distance dependence of the binding energy and for the purpose of determining the reduction of charge on the ion with increasing numbers of ligands. The greater energy derived on divalent ion binding and the steeper distance dependence indicate that selective, divalent over monovalent, ion binding will occur whenever the liganding system can provide a coordination shell of appropriate dimension. The calculations indicate that the preferred C--O ... Me angle is not 180 degrees. Of particular note is the decrease of charge on the cation on binding to N-methyl acetamide. One ligand bound to Na+ reduces the charge from 1.0 to 0.7 electron units and four ligands bound to Mg+2 reduces the charge from 2.0 to 0.7 electron units. This is of primary significance in carrier and channel mechanisms for cation permeation of lipid membranes; and although the numerical values are qualitative, the implication is for allowance of multiple occupancy of channels by monovalent cations.