Two-Dimensional Self-Organization of the Light-Harvesting Polypeptides/BChl a Complex into a Thermostable Liposomal Membrane

The detergent-isolated light-harvesting polypeptide (LH)/bacteriochlorophyll a (BChl a) complex from the photosynthetic bacterium Rhodospirillum rubrum was organized in thermostable liposomal membranes comprising membrane-spanning tetraether lipids from Sulfolobus acidocaldarius to develop a two-dimensional self-organization of the LH/BChl a complex in the lipid membrane. UV−vis absorption spectral data showed that the LH/BChl a complex was stable in the tetraether lipid membrane. As indicated by the Qy band of BChl a in the complex, the thermostability of the complex in liposomal membranes made from different lipids increased in the following order: dioleoylphospholipids < distearoylphospholipids ≪ tetraether lipids. The increased thermostability of the complex in the tetraether lipid membrane was probably due to hydrophobicity and microviscosity of the isoprenoid chains of the tetraether lipid. Linear dichroism spectra indicated that the BChl a in the complex incorporated into the tetraether lipid membrane was perpendicularly oriented and was homogeneous to the transmembrane axis. This implies that the orientation of the Qy band of BChl a in the complex was parallel to the membrane plane, consistent with the orientation in the photosynthetic membrane.