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We designed novel bilayer-forming amphiphiles based on the cyclic oligo-Asp-based peptide gemini (PG) surfactants cr-D2C12 and cr-D3C12, which consist of -Cys(Asp)nCys- (n = 2 or 3) as a core peptide and two Cys residues containing a dodecylamidomethyl group. Dynamic light scattering and transmission electron microscopy measurements revealed the formation of spherical bilayer membranes that could incorporate the light-harvesting antenna complex 2 (LH2) from Rhodopseudomonas acidophila . Furthermore, this proteoliposome-like conjugate could be assembled onto cationized glass and mica to form planar bilayer membranes incorporating LH2. Using atomic force microscopy, we observed LH2 protruding (ca. 1.2-1.5 nm) from flat terraces of the planar bilayer membranes formed from cr-D2C12 or cr-D3C12. Thus, our designed PG surfactants are a new class of bilayer-forming amphiphiles that may be applied to the study of various membrane proteins.

LH1-α and -β polypeptides, which make up the light-harvesting 1 (LH1) complex of purple photosynthetic bacteria, along with bacteriochlorophylls, have unique binding properties even for various porphyrin analogs. Herein, we used the porphyrin analogs, Zn-Chlorin and the Zn-Chlorin dimer, and examined their binding behaviors to the LH1-α variant, which has a His-tag at the C-terminus (MBP-rubα-YH). Zn-Chlorin and the Zn-Chlorin dimer could bind to MBP-rubα-YH and form a subunit-type assembly, similar to that from the native LH1 complex. These complexes could be immobilized onto Ni-nitrilotriacetic acid-modified Au electrodes, and the cathodic photocurrent was successfully observed by photoirradiation. Since Zn-Chlorins in this complex are too far for direct electron transfer from the electrode, a contribution of polypeptide backbone for efficient electron transfer was implied. These findings not only show interesting properties of LH1-α polypeptides but also suggest a clue to construct artificial photosynthesis systems using these peptide materials.

The PufX protein, encoded by the pufX gene of Rhodobacter sphaeroides, plays a key role in the organization and function of the core antenna (LH1)-reaction centre (RC) complex, which collects photons and triggers primary photochemical reactions. We synthesized a PufX/maltose-binding protein (MBP) fusion protein to study the effect of the PufX protein on the reconstitution of B820 subunit-type and LH1-type complexes. The fusion protein was synthesized using an Escherichia coli expression system and purified by affinity chromatography. Reconstitution experiments demonstrated that the MBP-PufX protein destabilizes the subunit-type complex (20°C), consistent with previous reports. Interestingly, however, the preformed LH1-type complex was stable in the presence of MBP-PufX. The MBP-PufX protein did not influence the preformed LH1-type complexes (4°C). The LH1-type complex containing MBP-PufX showed a unique temperature-dependent structural transformation that was irreversible. The predominant form of the complex at 4°C was the LH1-type. When shifted to 20°C, subunit-type complexes became predominant. Upon subsequent cooling back to 4°C, instead of re-forming the LH1-type complexes, the predominant form remained the subunit-type complexes. In contrast, reversible transformation of LH1 (4°C) and subunit-type complexes (20°C) occurs in the absence of PufX. These results are consistent with the suggestion that MBP-PufX interacts with the LH1α- polypeptide in the subunit (α/β)-type complex (at 20°C), preventing oligomerization of the subunit to form LH1-type complexes.