Ultrafast Energy Transfer from Chlorophyll c(2) to Chlorophyll a in Fucoxanthin-Chlorophyll Protein Complex

Light-harvesting in fucoxanthin-chlorophyll protein (FCP) of diatoms is performed by a cluster of chromophores: chlorophylls a (Chl a), chlorophylls c(2) (Chl c(2)), and carotenoids fucoxanthins. It is well-known that energy captured by fucoxanthin is transferred to Chl a on a subpicosecond time scale. However, the energy flow channel connecting Chl c(2) and Chl a remained elusive. In this study, the energy transfer between Chl c(2) and Chl a molecules in the FCP complex from the diatom algae C. meneghiniana at room temperature is investigated using pump probe and coherent two-dimensional electronic spectroscopy. Measured dynamics of the absorption band associated with the Q(y) transition of the Chl c(2) reveals an ultrafast energy transfer pathway to CM a. This conclusion is supported by the theoretical simulations based on the effective oscillator model

Mapping energy transfer channels in fucoxanthin-chlorophyll protein complex.

International audienceFucoxanthin-chlorophyll protein (FCP) is the key molecular complex performing the light-harvesting function in diatoms, which, being a major group of algae, are responsible for up to one quarter of the total primary production on Earth. These photosynthetic organisms contain an unusually large amount of the carotenoid fucoxanthin, which absorbs the light in the blue-green spectral region and transfers the captured excitation energy to the FCP-bound chlorophylls. Due to the large number of fucoxanthins, the excitation energy transfer cascades in these complexes are particularly tangled. In this work we present the two-color two-dimensional electronic spectroscopy experiments on FCP. Analysis of the data using the modified decay associated spectra permits a detailed mapping of the excitation frequency dependent energy transfer flow with a femtosecond time resolution

Excitons in the LH3 Complexes from Purple Bacteria

The noncovalently bound and structurally identical bacteriochlorophyll <i>a</i> chromophores in the peripheral light-harvesting complexes LH2 (B800–850) and LH3 (B800–820) from photosynthetic purple bacteria ensure the variability of the exciton spectra in the near-infrared (820–850 nm) wavelength region. As a result, the spectroscopic properties of the antenna complexes, such as positions of the maxima in the exciton absorption spectra, give rise to very efficient excitation transfer toward the reaction center. In this work, we investigated the possible molecular origin of the excitonically coupled B820 bacteriochlorophylls in LH3 using femtosecond transient absorption spectroscopy, deconvolution of steady-state absorption spectra, and modeling of the electrostatic intermolecular interactions using a charge density coupling approach. Compared to LH2, the upper excitonic level is red-shifted from 755 to 790 nm and is associated with an approximate 2-fold decrease of B820 intrapigment coupling. The absorption properties of LH3 cannot be reproduced by only changing the B850 site energy but also require a different scaling factor to be used to calculate interpigment couplings and a change of histidine protonation state. Several protonation patterns for distinct amino acid groups are presented, giving values of 162–173 cm^–1 at 100 K for the intradimer resonance interaction in the B820 ring