Comparison of the Energy-Transfer Rates in Structural and Spectral Variants of the B800–850 Complex from Purple Bacteria

Abstract

Photosynthetic light harvesting can occur with a remarkable near-unity quantum efficiency. The B800–850 complex, also known as light-harvesting complex 2 (LH2), is the primary light-harvesting complex in purple bacteria and has been extensively studied as a model system. The bacteriochlorophylls of the B800–850 complex are organized into two concentric rings, known as the B800 and B850 rings. However, depending on the species and growth conditions, the number of constituent subunits, the pigment geometry, and the absorption energies vary. While the dynamics of some B800–850 variants have been exhaustively characterized, others have not been measured. Furthermore, a direct and simultaneous comparison of how both structural and spectral differences between variants affect these dynamics has not been performed. In this work, we utilize ultrafast transient absorption measurements to compare the B800 to B850 energy-transfer rates in the B800–850 complex as a function of the number of subunits, geometry, and absorption energies. The nonameric B800–850 complex from Rhodobacter (Rb.) sphaeroides is 40% faster than the octameric B800–850 complex from Rhodospirillum (Rs.) molischianum, consistent with structure-based predictions. In contrast, the blue-shifted B800–820 complex from Rs. molischianum is only 20% faster than the B800–850 complex from Rs. molischianum despite an increase in the spectral overlap between the rings that would be expected to produce a larger increase in the energy-transfer rate. These measurements support current models that contain dark, higher-lying excitonic states to bridge the energy gap between rings, thereby maintaining similar energy-transfer dynamics. Overall, these results demonstrate that energy-transfer dynamics in the B800–850 complex are robust to the spectral and structural variations between species used to optimize energy capture and flow in purple bacteria.

Intraband dynamics and exciton trapping in the LH2 complex of Rhodopseudomonas acidophila

“…what was observed in recent work where the excitation frequency dependence was not considered in detail 43,44 Dynamics in the Strongly Coupled B850 Ring As a consequence of the much higher density of states and the stronger electronic coupling, the dynamics in the B850 ring deviates strongly from that of B800. The inter-pigment coupling is one order of magnitude stronger than for the B800 ring (~200-400 cm -1 for the αβ pair in B850) 34 .…”

“…Although studied also with time-resolved methods, B800 has been under less intense scrutiny than the more strongly coupled B850 ring -with much of the work being in the context of B800→B850 energy transfer rather than intraband dynamics. While several studies have targeted the intraband dynamics [39][40][41][42][43][44] , the inherent difficulty in generating laser pulses short enough to resolve the dynamics This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.…”

Excitation Energy Transfer from Bacteriochlorophyll b in the B800 Site to B850 Bacteriochlorophyll a in Light-Harvesting Complex 2

“…Excitation energy is transferred from B800 to B850 BChl a in LH2 proteins with time constants of 0.7–0.9 ps at room temperature. − The spectral overlap integral between B800 and B850 BChl a is an important parameter that governs rapid and efficient EET in LH2. Systematic alteration of this parameter while maintaining the other ones constant provides useful information on EET mechanisms in LH2 proteins.…”

Energy Transfer Dynamics in Light-Harvesting Complex 2 Variants Containing Oxidized B800 Bacteriochlorophyll a

“…Despite the separation between the B800 and B850 bands, the observed EET rate within 1 ps has been rationalized by the contribution of the higher-lying excitonic eigenstates of B850 BChl a . − Elucidation of the EET dynamics is essential to evaluate the photophysical mechanisms of LH2 and its related proteins. Time-resolved spectroscopy is a powerful technique to investigate the electronic dynamics and EET processes of LH2 proteins in detail; however, many reports have focused on native LH2 isolated from purple photosynthetic bacteria. − Therefore, variations in the energy donor and acceptor in LH2 and its related proteins are required to better understand the EET dynamics. For alteration of the acceptor, time-resolved measurements have been reported for light-harvesting complex 3 (LH3), ,, whose energy acceptor is B820 BChl a and the B820 Q y band is shifted to a shorter wavelength than B850 in LH2. , …”