Contents lists available at ScienceDirect BBA - Bioenergetics journal homepage: www.elsevier.com/locate/bbabio Energy landscape of the intact and destabilized FMO antennas from C. tepidum and the L122Q mutant: Low temperature spectroscopy and modeling study Anton Khmelnitskiy a , Adam Kell a , Tonu Reinot a , Rafael G. Saer c , Robert E. Blankenship c , Ryszard Jankowiak a,b, ⁎ a Department of Chemistry, Kansas State University, Manhattan, KS 66506, USA b Department of Physics, Kansas State University, Manhattan, KS 66506, USA c Departments of Chemistry and Biology, Washington University in St. Louis, Saint Louis, MO 63130, USA ARTICLE INFO Keywords: Photosynthesis FMO C. tepidum L122Q mutant Hole-burning (HB) Excitonic structure Site energies ABSTRACT We discuss the excitonic energy landscape of the typically studied wild-type (WT) Fenna-Matthews-Olson (FMO) antenna protein from the green sulfur bacterium Chlorobaculum tepidum (referred to as WT M ), which is described as a mixture of intact (WT I ) and destabilized (WT D ) complexes. Optical spectra of WT M and the L122Q mutant (where leucine 122 near BChl 8 is replaced with glutamine) are compared to WT I FMO. We show that WT M and L122Q samples are mixtures of two subpopulations of proteins, most likely induced by protein conformational changes during the isolation/purification procedures. Absorption, emission, and HB spectra of WT M and L122Q mutant are very similar, in which the low-energy trap (revealed by the nonresonant HB spectra) shifts to higher energies as a function of fluence, supporting a mixture model. No fluence-dependent shift is observed in the WT I FMO trimers. New Hamiltonians are provided for WT I and WT D proteins. Resonant HB spectra show that the internal energy relaxation times in the WT M and L122Q mutant are similar, and depend on excitation frequency. Fast average relaxation times (excited state lifetimes) are observed for burning into the main broad absorption band near 805 nm. Burning at longer wavelengths reveals slower total dephasing times. No resonant bleach is observed at λ B ≤ 803 nm, implying much faster (femtosecond) energy relaxation in this spectral range in agreement with 2D electronic spectroscopy frequency maps. 1. Introduction The Chlorobaculum (C.) tepidum Fenna-Matthews-Olson (FMO) an- tenna complex from green-sulfur bacteria consists of three monomers arranged in C 3 symmetry, each binding eight bacteriochlorophyll a (BChl a) molecules (Fig. 1A). This antenna complex transfers excitation energy from the light-absorbing chlorosome to the reaction center, where charge-separation and photochemistry takes place. FMO is an important model system for the study of exciton dynamics and excita- tion energy transfer (EET) in photosynthetic complexes. The structure of this protein was solved and originally revealed seven BChl a per monomer [1]. More recently, however, an eighth molecule of BChl a (BChl 8) was discovered to be present at the interface between adjacent FMO monomers (Fig. 1B) [2]. However, the occupancy of the latter pigment varies with the method of protein purification used [3,4]. Because of the specific orientation of FMO in relation to the RC, it was suggested that BChl 8 might be the entry point of excitations from the chlorosome, with BChl 3 being the energy sink that transfers this energy to the P840 dimer of the RC [5,6]. BChl 3 interfaces with the cyto- plasmic membrane in which the RC is located. While there is a con- sensus that BChl 3 mostly contributes to the lowest energy sink (trap) [7–10], the initial state, i.e., which BChl molecule(s) is(are) excited first, is still a matter of debate. For example, Ritschel et al., [11] showed that a relatively faster transfer time is observed when initialization at BChl 1 or BChl 6 is considered. Our recent modeling studies of wild- type (WT) FMO also suggested that BChl 6 has the highest site energy [12]. However, it is feasible that both BChls (1 and 6), whose site en- ergies cover a very broad spectral range, could represent two primary energy transfer pathways [13]. Many Hamiltonians have been reported for FMO over the years, each describing different linear and nonlinear optical spectra [7,12,14–17]. Unfortunately, the low-temperature experimental ab- sorption spectra used in modeling studies have slightly different spec- tral positions and/or intensities of the absorption bands. One could https://doi.org/10.1016/j.bbabio.2017.11.008 Received 24 August 2017; Received in revised form 23 November 2017; Accepted 27 November 2017 ⁎ Corresponding author at: Department of Chemistry, Kansas State University, Manhattan, KS, USA. E-mail address: ryszard@ksu.edu (R. Jankowiak). BBA - Bioenergetics 1859 (2018) 165–173 Available online 01 December 2017 0005-2728/ © 2017 Elsevier B.V. All rights reserved. T