Quantum chemical studies on the role of residues in calcium ion binding to Calmodulin

We perform density functional theory (DFT) based quantum chemical calculations on calcium ion coordination to an isolated loop of Calmodulin. We show that the coordination molecular orbitals in the ground state, having contributions from the valence orbitals of the metal ion and the loop atoms, bring out the roles of the coordinating and the non-coordinating residues to stabilize the coordination geometry in agreement to the mutational studies. The coordinating molecular orbitals are observed to be robust under various truncations of the binding loop and capping at the terminal residues.