Excitation energy transfer in the LHC-II trimer: a model based on the new 2.72 Å structure

Abstract

Energy transfer of the light harvesting complex LHC-II trimer, extracted from spinach, was studied in the Q(y) region at room temperature by femtosecond transient absorption spectroscopy. Configuration interaction exciton method [Linnanto et al. (1999) J Phys Chem B 103: 8739-8750] and 2.72 A structural information reported by Liu et al. was used to calculate spectroscopic properties and excitation energy transfer rates of the complex. Site energies of the pigments and coupling constants of pigment pairs in close contact were calculated by using a quantum chemical configuration interaction method. Gaussian random variation of the diagonal and off-diagonal exciton matrix elements was used to account for inhomogeneous broadening. Rate calculations included only the excitonic states initially excited and probed in the experiments. A kinetic model was used to simulate time and wavelength dependent absorption changes after excitation on the blue side of the Q(y) transition and compared to experimentally recorded rates. Analysis of excitonic wavefunctions allowed identification of pigments initially excited and probed into later. It was shown that excitation of the blue side of the Q(y) band of a single LHC-II complex results in energy transfer from chlorophyll b's of the lumenal side to chlorophyll a's located primarly on one of the monomers of the stromal side.

Two-Dimensional Spectroscopy of Chlorophyll a Excited-State Equilibration in Light-Harvesting Complex II

“…The 4.8 ps lifetime (Figure 4d) is due to the slow 2→4 transfer, i.e. equilibration between the 663/667 nm pool and the 673/680 nm pool.The simulated 2D DAS bear close similarity to the experimental 2D DAS (Figure 3a 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 10 based constraints on the rates and energy levels, 16,18,21,28 would be necessary to simulate finer details in the experimental data. We can conclude that spectral equilibration in LHCII is kinetically limited by EET occurring on a 5 ps timescale between two Chl a pools.…”

“…35,36 While the measured lifetimes for the equilibration dynamics (0.54 ps and 4.7 ps) are in good agreement with numerous previous studies, we do not observe any slower spectral equilibration processes. We contend that lifetimes in the range of 10-20 ps observed previously 9,15,16,26 are contributed by singlet-singlet annihilation, which is often evident because of the non-conservative shape of the DAS, indicating excitation decay rather than EET, and has been acknowledged in earlier works as discussed by Connelly et al …”

“…13,14 However, significantly longer lifetimes, in the range of tens of picoseconds and more, have also been reported. 9,15,16 The slowest lifetimes have been assigned to one Chl a pigment absorbing in the region of 665-670 nm. 7,17 This has been referred to as the bottleneck state and, in agreement with structure-based modelling, assigned to Chl a 604 17,18 in the crystal structure of Liu et al 19 The assignment has been challenged by more recent modelling 20 that incidentally also predicts slow Chl a -Chl a equilibration lifetimes in the tens of picoseconds region.…”

Energy transfer dynamics in trimers and aggregates of light-harvesting complex II probed by 2D electronic spectroscopy

“…2,3 In addition, modelling the excitonic couplings and site energies of the 14 chlorophylls (Chls) based on the crystal structure 2,4 has been used to predict the downhill Chl energy transfer routes and ultrafast dynamics. [5][6][7][8][9] In addition to the light-harvesting function, LHCII is involved in the non-radiative dissipation of excess excitation energy, a process collectively referred to as non-photochemical quenching (NPQ) of Chl fluorescence, [10][11][12] a key mechanism for protection against photodamage under excess radiation levels. In vitro, significant quenching manifested in shortening a) Electronic mail: lambrev@brc.hu b) Electronic mail: howesiang@ntu.edu.sg of the Chl a excitation lifetime [13][14][15][16] and development of a farred emitting state 17,18 are observed upon protein aggregation of LHCII.…”

Structure and dynamics of photosystem II light-harvesting complex revealed by high-resolution FTICR mass spectrometric proteome analysis

“…LHCn isolated from thylakoid membranes of Arabidopsis thaliana contains a mixture of 14 different light-harvesting chlorophyll alb-binding proteins (Lhcbl-6 proteins and their isomers) with similar polypeptide sequences and similar structures and functions [8,9]. The Lhcb4-6 proteins and their isomers form a monomeric, minor LHCn antenna, whereas the Lhcbl-3 proteins and their isomers form the major LHCn antenna, composed of monomers and homo-or heterotrimers [I, [10][11][12][13][14][15]. The LHCn homotrimers are formed by Lhcb1, Lhcb2, but not by Lhcb3, although heterotrimers of different combinations of Lhcb1, Lhcb2, and Lhcb3 have been reported [10,16].…”