Application of Peptide Gemini Surfactants as Novel Solubilization Surfactants for Photosystems I and II of Cyanobacteria Shuhei Koeda, † Katsunari Umezaki, † Tomoyasu Noji, † Atsushi Ikeda, ‡ Keisuke Kawakami, § Masaharu Kondo, † Yasushi Yamamoto, † Jian-Ren Shen, ∥ Keijiro Taga, † Takehisa Dewa, † Shigeru Ito, ⊥ Mamoru Nango, § Toshiki Tanaka, † and Toshihisa Mizuno* ,† † Graduate School of Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya, Aichi 466-8555, Japan ‡ Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0192, Japan § Graduate School of Science, Osaka City University, 3-3-138 Sugimoto-cho, Sumiyoshi, Osaka 558-8585, Japan ∥ Graduate School of Natural Science and Technology, Faculty of Science, Okayama University, Okayama 700-8530, Japan ⊥ Center for Gene Research, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan ABSTRACT: We designed novel peptide gemini surfactants (PG-surfactants), DKDKC 12 K and DKDKC 12 D, which can solubilize Photosystem I (PSI) of Thermosynecoccus elongatus and Photosystem II (PSII) of Thermosynecoccus vulcanus in an aqueous buffer solution. To assess the detailed effects of PG- surfactants on the original supramolecular membrane protein complexes and functions of PSI and PSII, we applied the surfactant exchange method to the isolated PSI and PSII. Spectroscopic properties, light-induced electron transfer activity, and dynamic light scattering measurements showed that PSI and PSII could be solubilized not only with retention of the original supramolecular protein complexes and functions but also without forming aggregates. Furthermore, measurement of the lifetime of light-induced charge-separation state in PSI revealed that both surfactants, especially DKDKC 12 D, displayed slight improvement against thermal denaturation below 60 °C compared with that using β-DDM. This degree of improvement in thermal resistance still seems low, implying that the peptide moieties did not interact directly with membrane protein surfaces. By conjugating an electron mediator such as methyl viologen (MV 2+ ) to DKDKC 12 K (denoted MV-DKDKC 12 K), we obtained derivatives that can trap the generated reductive electrons from the light- irradiated PSI. After immobilization onto an indium tin oxide electrode, a cathodic photocurrent from the electrode to the PSI/ MV-DKDKC 12 K conjugate was observed in response to the interval of light irradiation. These findings indicate that the PG- surfactants DKDKC 12 K and DKDKC 12 D provide not only a new class of solubilization surfactants but also insights into designing other derivatives that confer new functions on PSI and PSII. ■ INTRODUCTION Photosystems I and II (PSI and PSII) are representative photosynthetic membrane proteins present in organisms from cyanobacteria to higher plants. They play important roles in absorbing photons from sunlight and producing electrons and holes via charge separation at the reaction center; the quantum yield is almost 1.0. 1 Recently, the construction of artificial photosynthetic systems to produce hydrogen using solar energy has been examined by combining PSI with other reductases or reducing catalysts such as hydrogenases or platinum nano- particles. 1,2 A system for evolving oxygen gas from water via water splitting using solar energy has also been conducted by immobilizing PSII on inorganic materials such as gold nanoparticles 3 or mesoporous silica. 4 Constructing more sophisticated artificial photosynthetic systems 5 consisting of multiple photosynthetic proteins with reducing catalysts or enzymes and electron mediators requires the design of an interface enabling intermediate and efficient photoinduced electron transfer and techniques for organizing such different components in desirable arrangements. Recently, various peptide-containing surfactants consisting primarily of long alkyl chain and hydrophilic, 6 β-sheet-forming, 7 or the mixed amphiphilic 8 peptide have garnered great interest owing to the unique properties arising from the peptide moiety. Subtle choices from a diverse sequence library enable us to fine- tune their assembled morphologies, 9 for example, micelles, fibers, sheets, and gels, which are applicable to various biomaterials. 10 Some peptide-based surfactants have been examined for use as solubilization surfactants for membrane proteins. 11 McGregor et al. have reported that a surfactant composed of a 24-mer amphiphilic peptide with two long alkyl chains exhibits increased thermal stability via hydrophobic interaction between the transmembrane domain of membrane protein and the hydrophobic surface of amphiphilic peptide. 12 Zhang et al. have reported a similar increase in thermal stability using an amphiphilic peptide surfactant, A6D. 13 However, the Received: November 2, 2012 Revised: August 11, 2013 Article pubs.acs.org/Langmuir © XXXX American Chemical Society A dx.doi.org/10.1021/la402167v | Langmuir XXXX, XXX, XXX−XXX