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.

Introduction 4546 2. General Information on the Structure of Photosynthetic Complexes and StructureÀFunction Relationships 4548 2.1. Photosystem I (PSI) and Photosystem II (PSII) 4548 2.2. Basic Aspects of Bacterial Photosynthesis 4549 3. Interpigment Interactions, Excitation Energy Transfer (EET), and Charge Separation (CS) Rates-General Considerations 4550 4. Fundamentals of Spectral Hole-Burning (SHB) and Fluorescence Line-Narrowing Spectroscopy (FLNS) and Single Photosynthetic Complex Spectroscopy (SPCS) 4552 4.1. Zero-Phonon Lines, Homogeneous and Inhomogeneous Broadening 4553 4.1.1. ZPLs and Phonon Sidebands (PSBs) 4554 4.1.2. ElectronÀPhonon Coupling and Homogeneous Line Shapes 4555 4.2. Nonphotochemical, Photochemical, and Transient SHB Spectroscopy 4557 4.3. Mechanism of Nonphotochemical Hole-Burning (NPHB) 4558 4.4. Kinetics of NPHB 4559 4.5. Zero-Phonon Action (ZPA) Spectroscopy: Site Distribution Function (SDF) 4560 4.6. Hole Shapes and FLN Line Shapes-Electron Phonon Coupling and ΔFLNS 4561 4.7. Ground and Excited State Vibrational Frequencies 4567 4.8. SHB in Excitonically Coupled Systems 4567 4.9. Basic Principles of SPCS 4568 4.10. Basic Principles of Two-Dimensional Electronic Spectroscopy (2D ES) 4569 5. Examples of Applications of NPHB, FLNS, SPCS, and 2D ES to Photosynthesis 4570 5.1. Light-Harvesting and EET in Antenna Complexes 4570 5.1.1. Peripheral Antenna Systems of Photosystem II (

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