Carotenoid-to-bacteriochlorophyll energy transfer through vibronic coupling in LH2 from Phaeosprillum molischianum

Abstract

The peripheral light-harvesting antenna complex (LH2) of purple photosynthetic bacteria is an ideal testing ground for models of structure–function relationships due to its well-determined molecular structure and ultrafast energy deactivation. It has been the target for numerous studies in both theory and ultrafast spectroscopy; nevertheless, certain aspects of the convoluted relaxation network of LH2 lack a satisfactory explanation by conventional theories. For example, the initial carotenoid-to-bacteriochlorophyll energy transfer step necessary on visible light excitation was long considered to follow the Förster mechanism, even though transfer times as short as 40 femtoseconds (fs) have been observed. Such transfer times are hard to accommodate by Förster theory, as the moderate coupling strengths found in LH2 suggest much slower transfer within this framework. In this study, we investigate LH2 from Phaeospirillum (Ph.) molischianum in two types of transient absorption experiments—with narrowband pump and white-light probe resulting in 100 fs time resolution, and with degenerate broadband 10 fs pump and probe pulses. With regard to the split Qx band in this system, we show that vibronically mediated transfer explains both the ultrafast carotenoid-to-B850 transfer, and the almost complete lack of transfer to B800. These results are beyond Förster theory, which predicts an almost equal partition between the two channels.Electronic supplementary materialThe online version of this article (doi:10.1007/s11120-017-0398-3) contains supplementary material, which is available to authorized users.

Coherence in carotenoid-to-chlorophyll energy transfer

“…However, they occur in a spectral region not predicted by the displaced harmonic oscillator model 40 for these modes, suggesting the presence of more complex mechanisms. It has been recently suggested that vibrational modes of accessory pigments could work as a sink, absorbing the excess energy released by energy transfer due to the mismatch of donor and acceptor levels 41 , 42 . In the case of PCP, the strongly beating signal in the upper-left portion of the maps would represent the deposition into the Per vibrational modes of the excess energy released after the S 2 → Q y transfer.…”

Revealing vibronic coupling in chlorophyll c1 by polarization-controlled 2D electronic spectroscopy

“…Vibronically mediated transfers are suggested to enhance the rates of electronic relaxations [4], in addition, vibronic coupling may speed up the charge separation process in reaction centers [33,34]. This phenomenon was also suggested to explain the ultrafast carotenoid to chlorophyll energy transfer in the LH2 complex [35]. Better understanding of vibronic coupling, both qualitative and quantitative is very challenging, nevertheless it might prove necessary for explaining efficient and robust primary photosynthetic phenomena.…”

Dissipation-Assisted Matrix Product Factorization

“…In Fig.2(a) and (b), we first consider the case that each site is strongly coupled to an underdamped mode with frequency 1500 cm −1 , Huang-Rhys factor s 1 = 0.5, and γ 1 = (1 ps) −1 . Such a large Huang-Rhys factor can be found in polymer-based materials in organic photovoltaics [3, 46-48, 50, 51] and photosynthetic complexes containing carotenoids [52], where vibrational progressions are well pronounced in optical responses. In addition to the underdamped modes, we couple an overdamped mode to each site, and all the modes are coupled to thermal reservoirs at room temperature.…”