Transient Absorption Study of Two-Photon Excitation Mechanism in the LH2 Complex from Purple Bacterium Rhodobacter sphaeroides Ilya Stepanenko, † Viktor Kompanetz, ‡ Zoya Makhneva, § Sergey Chekalin,* ,‡ Andrei Moskalenko,* ,§ and Andrei Razjivin* ,† † A. N. Belozersky Institute of Physico-Chemical Biology, Moscow State University, Leninskie Gory, 119992 Moscow, Russia ‡ Institute of Spectroscopy RAS, 142190 Troitsk, Moscow Region, Russia § Institute of Fundamental Problems of Biology RAS, 142290 Pushchino, Moscow Region, Russia ABSTRACT: The mechanism of two-photon excitation of a peripheral light-harvesting complex LH2 (B800-850) from purple bacterium Rhodobacter sphaeroides was explained on the basis of femtosecond transient absorption data. Fast bleaching of the B850 absorption band was measured under two-photon excitation by 1350 nm femtosecond pulses, showing fast subpicosecond arrival of excitation energy to B850 circular aggregates. Any spectral changes connected with the B800 absorption band of B800-BChl molecules were absent. A similar picture was observed under one- photon excitation of the LH2 complex by 675 nm femtosecond pulses. We believe these effects may be attributed to direct excitation of high-energy excitonic states of a B850 circular aggregate or its vibrational manifold in accordance with the model of Abe [Chem. Phys. 2001, 264, 355-363]. ■ INTRODUCTION In the photosynthetic apparatus of purple bacteria, each photoactive reaction center (RC) is surrounded by a “core” light-harvesting complex (LH1). In addition to the complex LH1, many bacteria have two to six copies of a “peripheral” light-harvesting complex (LH2) per one RC. The light- harvesting complexes of purple bacteria contain two types of pigment molecules: the main pigment (bacteriochlorophyll - BChl) and auxiliary pigments (carotenoids - Cars). Photo- physical properties of these pigments by themselves and in the light-harvesting complexes are rather unusual and remain the subject of intense research. In 1982 we revealed a number of unusual properties of the BChl long-wavelength absorption band (Q y band) of light harvesting antenna of purple bacteria, 1,2 which later led us to the assumption that BChl molecules in the antenna form a symmetrical circular aggregate showing a strong exciton interaction. 3 This model is in accordance with the optical properties of light-harvesting antenna of purple bacteria, in particular the data of pico- and femtosecond laser absorption spectroscopy 4 and hole-burning experiments. 5 Subsequent X- ray studies of light-harvesting complexes LH2 6,7 and LH1 8 confirmed the arrangement of the BChl molecules in symmetrical circular excitonically coupled aggregates in these complexes. Notably, the refinement of BChl positions according to X-ray data is consistent with the results of previous calculations of the optical properties of complex LH2. 9 While describing the optical properties of light-harvesting complexes, one must take into account the excitonic nature of the BChl aggregates, which determines its energy state structure. In the case of a B850 ring with 18 BChl molecules, there are two exciton level sets (two Davydov components each of nine states). The linear absorption spectrum of the lower (red-shifted) set consists of a strong band corresponding to the degenerated state k = ±1 (-) and a weak long-wavelength line corresponding to the k =0 (-) state. The exciton levels of the upper (blue-shifted) set give no visible contribution to linear absorption (a line corresponding to the k =0 (+) state is too weak to be seen in the linear absorption spectrum). The position of the lowest k =0 (-) exciton state has been found using hole-burning spectroscopy, being at 865 nm or ∼11550 cm -1 . 10,11 However, the location of the high-energy state k =0 (+) has not been reliably determined. A weak band at 780 nm (∼12800 cm -1 ) in the circular dichroism spectrum was ascribed to it in ref 12. Later, however, this feature was assigned to vibronic manifold of free BChl molecules. 13 Using polarized fluorescence excitation spectroscopy and model simulations, the value of 760 nm (∼13200 cm -1 ) was determined in 14. It is suggested that the upper edge of exciton bandwidth can be detected by reflection spectroscopy 15 or two-photon spectros- copy. 16 According to ref 16, in a symmetrical circular aggregate (B850), the upper dipole-forbidden state k =0 (+) turns out to be two-photon-allowed and may be investigated by two-photon absorption or excitation spectroscopy. The value of the k =0 (+) state energy is important for determining the exciton coupling energy in B850 aggregate of Received: April 9, 2011 Revised: January 8, 2012 Published: January 23, 2012 Article pubs.acs.org/JPCB © 2012 American Chemical Society 2886 dx.doi.org/10.1021/jp2033214 | J. Phys. Chem. B 2012, 116, 2886-2890