Low-Temperature Protein Dynamics of the B800 Molecules in the LH2 Light-Harvesting Complex: Spectral Hole Burning Study and Comparison with Single Photosynthetic Complex Spectroscopy

Abstract

Previously published and new spectral hole burning (SHB) data on the B800 band of LH2 light-harvesting antenna complex of Rps. acidophila are analyzed in light of recent single photosynthetic complex spectroscopy (SPCS) results (for a review, see Berlin et al. Phys. Life Rev. 2007, 4, 64.). It is demonstrated that, in general, SHB-related phenomena observed for the B800 band are in qualitative agreement with the SPCS data and the protein models involving multiwell multitier protein energy landscapes. Regarding the quantitative agreement, we argue that the single-molecule behavior associated with the fastest spectral diffusion (smallest barrier) tier of the protein energy landscape is inconsistent with the SHB data. The latter discrepancy can be attributed to SPCS probing not only the dynamics of of the protein complex per se, but also that of the surrounding amorphous host and/or of the host-protein interface. It is argued that SHB (once improved models are developed) should also be able to provide the average magnitudes and probability distributions of light-induced spectral shifts and could be used to determine whether SPCS probes a set of protein complexes that are both intact and statistically relevant. SHB results are consistent with the B800 --> B850 energy-transfer models including consideration of the whole B850 density of states.

Parameters of the Protein Energy Landscapes of Several Light-Harvesting Complexes Probed via Spectral Hole Growth Kinetics Measurements

“…and SPCS experiments [22][23][24][25][26] , which, we believe, at least at liquid helium temperatures, probe the barrier distributions in the excited state of the pigment-protein system (the latter conclusion is still a matter of debate, though, 24,38 ), can be interpreted with system switching between significantly more than two conformational states. Such situation is best described employing the concept of the multi-well energy landscape with several hierarchal tiers 22,44,45 .…”

“…38 the spectral diffusion observed in SPCS experiments is most likely light-induced and therefore essentially the same phenomenon as NPHB. With that in mind, we are going to apply the NPHB model of Ref.…”

“…According to 13,34,35,38 , in the absence of the energy transfer (this is the case for this manuscript focusing on the lowest-energy states of the complexes) the time dependence of the absorption spectrum affected by SHB can be described with…”

“…2). (The SHB yield can be independently estimated from photon budgets and shift rates of the SPCS experiments 38 ).…”

“…Important implication of the results briefly described above and presented in detail in Ref. 38 is that SHB, probing ensemble averages, could be used to verify if light-induced spectral diffusion behavior observed in SPCS experiments represents the behavior which is typical for the protein under study, and independent of sample preparation details (e.g. nature of the amorphous host surrounding the protein), and/or other experimental procedures.…”