Self-Assembled Monolayer of Light-Harvesting Core Complexes from Photosynthetic Bacteria on a Gold Electrode Modified with Alkanethiols Masaharu Kondo, Yukari Nakamura, Kaoru Fujii, Morio Nagata, Yoshiharu Suemori, Takehisa Dewa, Kouji Iida, † Alastair T. Gardiner, ‡ Richard J. Cogdell, ‡ and Mamoru Nango* Department of Applied Chemistry, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan, Nagoya Municipal Industrial Research Institute, Rokuban 3-4-41, Atsuta-ku, Nagoya 456-0058, Japan, and Division of Biochemistry and Molecular Biology, Glasgow Biomedical Research Centre, Institute of Biomedical and Life Sciences, University of Glasgow, 126 University Place, G12 8TA, UK Received March 30, 2007 Light-harvesting antenna core (LH1-RC) complexes isolated from Rhodoseudomonas palustris were self-assembled on a gold electrode modified with self-assembled monolayers (SAMs) of the alkanethiols NH 2 (CH 2 ) n SH, n ) 2, 6, 8, 11; HOOC(CH 2 ) 7 SH; and CH 3 (CH 2 ) 7 SH, respectively. Adsorption of the LH1-RC complexes on the SAMs depended on the terminating group of the alkanethiols, where the adsoption increased in the following order for the terminating groups: amino groups > carboxylic acid groups > methyl groups. Further, the adsorption on a gold electrode modified with SAMs of NH 2 (CH 2 ) n SH, n ) 2, 6, 8, 11, depended on the methylene chain length, where the adsorption increased with increasing the methylene chain length. The presence of the well-known light-harvesting and reaction center peaks of the near infrared (NIR) absorption spectra of the LH1-RC complexes indicated that these complexes were only fully stable on the SAM gold electrodes modified with the amino group. In the case of modification with the carboxyl group, the complexes were partially stable, while in the presence of the terminal methyl group the complexes were extensively denatured. An efficient photocurrent response of these complexes on the SAMs of NH 2 (CH 2 ) n SH, n ) 2, 6, 8, 11, was observed upon illumination at 880 nm. The photocurrent depended on the methylene chain length (n), where the maximum photocurrent response was observed at n ) 6, which corresponds to a distance between the amino terminal group in NH 2 (CH 2 ) 6 SH and the gold surface of 1.0 nm. Introduction When light energy is absorbed in vivo by purple bacterial light-harvesting (LH) complexes, it is rapidly transferred to the reaction centers (RC) where light energy is efficiently converted into useful chemical energy. 1 In most types of purple bacteria, there are two types of antenna complexes: peripheral LH2 complexes and the LH1 complexes. 1 The structure of the LH2 complex of Rhodopseudomonas acidophila strain 10050 has been resolved to a resolution of 2.0 Å. 2 This LH2 complex consists of a ring of nine heterodimeric subunits. However, such a high-resolution structure has not yet been determined for the LH1 complex. There are low-resolution projection structures produced by transmission electron microscopy (TEM) of two- dimensional (2D) crystals of the LH1 complex and a 4.8 Å X-ray crystal structure of the LH1-RC core complex. 3-5 The recent crystal structure of the LH1-RC “core” complex from Rho- doseudomonas palustris reveals that the LH1 complex surrounds the contours of the RC so that the core complex has an overall oval rather than circular shape. 4 This structure shows that the RC is surrounded by the LH1 complex, which consists of 15 pairs of transmembrane helical R- and -polypeptides and their coordinated bacteriochlorophylls (BChls). Atomic force mi- croscopy (AFM) has also been used to observe antenna complexes in both natural and reconstituted membranes. 6-12 Organic compounds and devices whose design has been inspired by photosynthesis are being extensively studied in the field of interfacial science. Imahori et al. assembled organic compounds such as porphyrins and fullerenes onto a gold electrode or gold colloids as photoconversion devices and catalysts. 13,14 Amao et al. adsorbed chlorophyll derivatives on TiO 2 substrates to construct photoelectron conversion devices and hydrogen evolution systems. 15,16 Okura et al. demonstrated that photosystem I with cytochrome c 3 cross-linked to [NiFe]- hydrogenase is capable of hydrogen evolution. 17 Dutton et al., Miyake et al., and Cotton et al. have all reported on devices using LB films containing bacterial RCs. 18-20 Trammell et al. reported orientated binding of RC on a gold electrode by a polyhistidine tag method. 21 Our current understanding of energy transfer and charge separation reactions in the LH2 and LH1-RC complexes has enabled the first step to be taken toward generating from them artificial systems that convert light energy into usable electrical current. Previous attempts to produce an artificial, energy- converting electrode system used either the LH1 complexes 22 or RC 23 immobilized on the electrodes. Until now, however, there have only been a few attempts to immobilize intact core complexes consisting of both the LH1 complex and the RC components together onto an electrode. 24,25 We have recently developed a procedure to create a self-assembled monolayer * Corresponding author. Fax & Tel: +81-52-735-5226. E-mail: nango@nitech.ac.jp. † Nagoya Municipal Industrial Research Institute. ‡ University of Glasgow. 2457 Biomacromolecules 2007, 8, 2457-2463 10.1021/bm070352z CCC: $37.00 © 2007 American Chemical Society Published on Web 06/26/2007