EXPERIMENTAL AND THEORETICAL CHARACTERIZATION OF THE HIGH-AFFINITY CATION-BINDING SITE OF THE PURPLE MEMBRANE

Binding of Mn2+ or Mg2+ to the high-affinity site of the purple membra ne from Halobacterium salinarium has been studied by superconducting q uantum interference device magnetometry or by ab initio quantum mechan ical calculations, respectively. The binding of Mn2+ cation, in a low- spin state, to the high-affinity site occurs through a major octahedra l local symmetry character with a minor rhombic distortion and a coord ination number of six. A molecular model of this binding site in the S chiff base vicinity is proposed. In this model, a Mg2+ cation interact s 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 m olecules is hydrogen bonded to both the nitrogen of the protonated Sch iff base and the Asp(85) oxygen. It could serve as a shuttle for the S chiff base proton to move to Asp(85) in the L-M transition.