AREAS OF HYDRATION IN THE PURPLE MEMBRANE OF HALOBACTERIUM-HALOBIUM - NEUTRON-DIFFRACTION STUDY

A map of the exchangeable H atoms which contribute to the in-plane diffraction of purple membrane has been obtained by neutron diffraction using H2O/D2O exchange. Considering each of the protein and lipid areas of the membrane as homogeneous, the map shows that there is more exchange in the lipid areas than in the protein areas. The results are consistent with the expected hydration around lipid headgroups and with 60% of the protein total labile H atoms exchanging if about 24 molecules of water are associated with the protein in a general way. In this model the protein lies flush with the membrane surface. In a model where the exchange in the protein and lipid are equal, the protein would have to protrude by only 1.5 .ANG. to account for the results. There are no pockets in the protein which contain 12 or more water molecules, since they would have given rise to significant peaks in the map. The main function of bacteriorhodopsin is the light-driven vectorial translocation of protons across the membrane. If the active pump occupies only a fraction of the path across the membrane thickness, then there must be a passive pathway for proton translocation. The present results exclude the possibility that such passive transport occurs via a bulk water channel in the protein.