Binding of Fe3+ ions to halobacterial purple membranes as studied by Mössbauer spectroscopy

Purple membranes (PM) from Halobacterium were reconstituted with 57Fe ions and investigated by Mössbauer spectroscopy within the temperature range from 5 to 300 K at the Fe/bacteriorhodopsin (BR) ratio 0.6-300. When the Fe/Br ratio was below 2, Fe3+ bonded to PM mostly as hydroxymonomeric particle [FeOH]2+.5H2O, the apparent charge of the iron ion being two. When the Fe/BR ratio exceeded two, the dimeric form [FeOH](2+)4.8H2O along with a cluster form dominated. The temperature dependences of the mean square displacement show that the mobility of Fe ions changes from the solid-state type to the quasi-diffusional one at temperatures approximately 200 and approximately 230 K for the dimeric or monomeric and cluster iron forms, respectively. The nature of the cation binding sites and their location on the PM surface are discussed. A possible role of the divalent cation binding to PM in the mechanism of BR proton pumping is suggested.