Light harvesting, electron transfer and electron cycling of a native photosynthetic membrane adsorbed onto a gold surface

Photosynthetic membranes comprise a network of light harvesting and reaction center pigment–protein
complexes responsible for the primary photoconversion reactions: light absorption, energy transfer and
electron cycling. The structural organization of membranes of the purple bacterial species Rb. sphaeroides has
been elucidated in most detail by means of polarized light spectroscopy and atomic force microscopy. Here
we report a functional characterization of native and untreated membranes of the same species adsorbed
onto a gold surface. Employing fluorescence confocal spectroscopy and light-induced electrochemistry we
show that adsorbed membranes maintain their energy and electron transferring functionality. Goldadsorbed
membranes are shown to generate a steady high photocurrent of 10 μA/cm2 for several minutes
and to maintain activity for up to three days while continuously illuminated. The surface-adsorbed
membranes exhibit a remarkable functionality under aerobic conditions, even when exposed to light
intensities well above that of direct solar irradiation. The component at the interface of light harvesting and
electron cycling, the LH1 complex, displays exceptional stability, likely contributing to the robustness of the
membranes. Peripheral light harvesting LH2 complexes show a light intensity dependent decoupling from
photoconversion. LH2 can act as a reversible switch at low-light, an increased emitter at medium light and
photobleaches at high light.