An extended X-ray absorption fine structure study of the high-affinity cation-binding site in the purple membrane

The structure of the high-affinity cation-binding site of bacteriorhodopsin was studied using extended x-ray absorption fine structure techniques. The results obtained for Mn²⁺ in aqueous solution and for the complex BR-Mn²⁺ (1:1 molar ratio) show great similarities, suggesting that Mn²⁺, when bound to this site, is coordinated with six atoms of oxygen, forming an octahedral disposition. The interatomic distance between the atoms of oxygen and the Mn²⁺ was found to be 2.17 Å for the complex BR-Mn²⁺, similar to Mn²⁺ in solution (2.15 Å). In addition, the absence of any other peak at greater distances in the Fourier-transformed spectrum indicates that neither phosphorus nor sulphur atoms are present in the second coordination shell. This suggests that this binding site is located in the protein, discarding the proximity of lipid polar headgroups.