Transient Absorption Study of Two-Photon Excitation Mechanism in the LH2 Complex from Purple Bacterium Rhodobacter sphaeroides

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].

Decoherence- and Dimerization-assisted Energy Transport in Protomer of Light-Harvesting Complexes

“…We wish to show the influence of decoherence and dimerization in the process of energy transporting between Bchl molecules. To our best knowledge, although the consensus is still lacking about the value of the coupling energy for biological application, both experimental and theoretical arguments [33] have been raised supporting coupling energies between 10 ps −1 and 24 ps −1 . In this paper, we choose the value of coupling energies as J = 10 ps −1 , g ∈ (12, 24) ps −1 , and the transfer time less than 1 ps.…”

“…[25] In the photosynthetic apparatus of purple bacteria, each "core" light-harvesting complex (LH1) surround each photoactive reaction center (RC). [26] 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)).…”

Red chlorophyll excitation dynamics in Arthrospira platensis photosystem I trimeric complexes as studied by femtosecond transient absorption spectroscopy

“…The experiments were performed with a femtosecond-pulsed laser setup consisting of a chirped-pulse amplified Ti:sapphire laser with a repetition rate of 1 kHz and an optical parametric amplifier (TOPAS), by employing a typical pump-probe layout as described in [19]. To probe light-induced absorption changes, part of the laser pulses was used to generate a femtosecond white-light continuum in a water cell.…”

Transient Absorption of Chlorophylls and Carotenoids after Two-Photon Excitation of LHCII

“…A first indication that direct 2PE of (bacterio)­chlorophylls may occur in the presumed S 1 region of the carotenoid(s) in purple bacterial LH2 was published by Krikunova et al Furthermore, this approach is impossible to apply to isolated carotenoids in solution because of the lack of detectable carotenoid fluorescence. A few reports applying transient absorption spectroscopy, however, showed that the carotenoid S 1 state is indeed excited by 2PE in both isolated carotenoids − and light-harvesting systems. ,, …”

“…A few reports applying transient absorption spectroscopy, however, showed that the carotenoid S 1 state is indeed excited by 2PE in both isolated carotenoids 25−28 and light-harvesting systems. 23,29,30 2PE-induced transient absorption spectroscopy has not been applied to LHCII yet, and the assumption of selective 2PE of S 1 of the carotenoids in LHCII has been recently challenged by measurements of 2PE profiles of Chl a and Chl b, which clearly can be excited by 2PE with excitation wavelengths in the 1100−1300 nm spectral region. 31 2PE spectra of Chl a and Chl b, including 2PE spectra of carotenoid−tetrapyrrole dyads, have also been reported by Gacek et al, 32 The (1PE) absorption and 2PE spectrum of LHCII are shown in Figure 1.…”