Experimental and Theoretical Characterization of the High-Affinity Cation-Binding Site of the Purple Membrane

Abstract

Binding of Mn 2+ or Mg 2+ to the high-affinity site of the purple membrane from Halobacterium salinarium has been studied by superconducting quantum interference device magnetometry or by ab initio quantum mechanical calculations, respectively. The binding of Mn 2+ cation, in a low-spin state, to the high-affinity site occurs through a major octahedral local symmetry character with a minor rhombic distortion and a coordination number of six. A molecular model of this binding site in the Schiff base vicinity is proposed. In this model, a Mg 2+ cation interacts with one oxygen atom of the side chain of Asp 85, with both oxygen atoms of Asp 212 and with three water molecules. One of these water molecules is hydrogen bonded to both the nitrogen of the protonated Schiff base and the Asp 85 oxygen. It could serve as a shuttle for the Schiff base proton to move to Asp 85 in the L-M transition.