Cation affinities of cyclohexadepsipeptide: Ab initio study

Abstract

The interactions of cations (Li+, Na+, Be2+, Mg2+) with a cyclohexadepsipeptide composed of glycines and glycolic acids have been investigated using ab initio calculations. The crucial role played by the orientation of the ion-dipolar moiety could possibly explain the binding preference upon complexation with alkali cations since the dipole moment of the amide carbonyl moiety is greater than that of the ester carbonyl moiety. We find that cations prefer to bind amide carbonyl oxygen atoms rather than ester oxygen atoms. This should also explain why the binding affinities of the cyclohexadepsipeptide for cations are larger than those of 18-crown-6 and [1(6)]starand in the gas phase. However, in divalent cationic cases which have twice the charge of the monovalent cationic species, the coordination numbers related to charge-charge interactions tend to be somewhat more important than the ion-dipolar moiety interactions. The self-consistent reaction field (SCRF) results for hexahydrated complexes of Na+ with the cyclohexadepsipeptide indicate that cations prefer to bind amide carbonyl oxygen atoms rather than ester oxygen atoms in solution as well as in the gas phase. The complexation of two cyclohexadepsipeptide molecules with one cation (i.e., 2:1 sandwich-type complexes) in the gas phase has also been discussed to affirm the possible existence of such complexes suggested by Ovchinnikov. In these complexes, a cation binds mainly amide carbonyl oxygen atoms. If glycines and glycolic acids are replaced by other residues, these modified cyclodepsipeptide ionophores could show different selectivities for cations with varying flexibilities.