The CP43 Core Antenna Complex of Photosystem II Possesses Two Quasi-Degenerate and Weakly Coupled Q_y-Trap States

Abstract

The CP43 chlorophyll a-core protein complex plays an important role in funneling excitation energy absorbed by more peripheral antenna complexes of photosystem II (PSII) to the reaction center (RC). Identification and characterization of the lowest energy Q y -states of CP43 is important for understanding the kinetics of excitation energy transfer (EET) from CP43 to the RC. We report the results of several types of spectroscopic experiments performed at liquid He temperatures on the isolated CP43 complex from spinach. Nonphotochemical hole burning (NPHB) and triplet bottleneck hole burning spectroscopies as well as zero-phonon hole (ZPH) action and Stark hole burning spectroscopies were employed. Two quasi-degenerate trap states at 682.9 nm (B state) and 683.3 nm (A state) are identified. The widths of their mainly inhomogeneously broadened Q y -absorption bands are 45 and 120 cm -1 , respectively. The uncorrelated site excitation distribution functions (SDF) of the two states are nearly the same as their absorption bands since the electron-phonon coupling is weak (optical reorganization energies of ∼6 cm -1 ). The NPHB spectra establish that the B state is the primary trap for EET from higher energy Q y -states. The permanent dipole moment change (∆µ) of the S 0 f Q y transition for both the B and A states is small, f‚∆µ ) 0.25 ( 0.05 and 0.47 ( 0.05, respectively, where f is the local field correction factor. These values, together with the weak electron-phonon coupling and other results, indicate that both states are highly localized on a single Chl a molecule. Holewidth measurements led to the remarkable finding that the rates of A f B and B f A EET processes are extremely slow, ∼(6 ns) -1 . This suggests that the Chl a molecules of the two states belong to different layers of Chl a molecules located at opposite sides of the membrane. The intriguing question of why CP43 possesses two quasi-degenerate trap states that are so weakly coupled is addressed. The possibility that they play a role in the photoinhibitory and photoregulatory processes is raised.

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

“…It very closely resembles those reported earlier 12,14 and is nearly identical to recent data 18 . The hole-burning action spectrum of CP43 (also presented in Fig.…”

“…The A-state 12 , likely localized on Chl 44 54 (notation by Loll et al 55 ), is the major contributor to absorption at 686.8 nm where the HGK measurement described below was performed (arrow). Note that the action spectrum normalized to the absorption spectrum in the region dominated by the A state overshoots the absorption in the region of the narrow B state at 683 nm.…”

“…It appears that the shape of the pseudo-PSB in case of 684< B <686 nm is determined by the actual shape of the phonon sideband, not by the shape of the SDF, and, therefore, we used the phonon sideband shape following from SHB data (See Table 1). As a result somewhat broader Lorentzian contribution to PSB width was employed here compared to 12,18 . There is general agreement that the Astate of CP43 is localized on a single Chl a molecule, and, therefore, the integrated 24 absorption cross-section was taken to be equal to that of one Chl a molecule.…”

“…2C) at 683.1 nm (also probed at 682.3 nm) and LHCII monomer (Fig. 2D) 12,18 . Note that within the framework of the model for EET between two quasi-degenerate states described in 52 , the small contribution of the B state which might still be present at 686.8 nm belongs to the B-type pigments being the lowestenergy pigments in the complex, and therefore, further downhill EET from these pigments is impossible.…”

“…SHB has been applied to explore electronic level structure, electron-phonon coupling and energy and charge transfer processes in a broad variety of complexes [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] , as summarized in a recent review 21 . In terms of low-temperature protein dynamics, the presence of low-energy excitations responsible for the ~T 1.3 dependence of the homogeneous line width, which is characteristic for amorphous solids, was demonstrated for many PS complexes [3][4][5][6][7][8][9][10]12 .…”