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.

Persistent nonphotochemical and population bottleneck hole-burning results obtained as a function of burn wavelength are reported for the CP47 proximal antenna protein complex of photosystem 11. Attention is focused on the lower energy chlorophyll a Qv states. Results are presented for the CP47 complex from two preparations. The Chl a content per CP47 complex was determined, spectroscopically, to be 14 f 2. On the basis of the analysis of the hole spectra and the 4.2 K static fluorescence spectrum, the lowest energy state of CP47 lies at 690 nm (fluorescence origin at 691 nm). The width of the weak 690-nm absorption band from inhomogeneous broadening is 100 cm-l. The linear electron-phonon coupling of the 690-nm state is weak with a Huang-Rhys factor (S) of about 0.2 and a mean phonon frequency (0,) of 20 cm-I, which explains why the Stokes shift ( 2 S~m ) is so small. The 690-nm state is found to be excitonically correlated with a, hitherto, unobserved state at 687 nm. However, the combined absorption intensity of the 690-and 687-nm states was determined to be equivalent to only 1 Chl a molecule. Results are presented which illustrate that these two states are fragile (Le., their associated chlorophyll a molecules are readily disrupted). Thus, it is possible that the correct number of Chl a molecules is 2, not 1. Indeed, the simplest interpretation consistent with the hole-burning data has the 687-and 690-nm states being associated with a Chl a dimer with the latter close to forbidden in absorption. The results indicate that the 687-nm state relaxes to the 690-nm state in about 70 fs. The zero-phonon hole widths for the higher integrity CP47 samples are used to determine the energy-transfer times for the higher energy absorbing states at 4.2 K. The absorption intensity of a previously identified state at 684 nm is found to vary from preparation to preparation. Diminution of the intensity of the 684-nm band is accompanied by increased absorption at -670 nm. This speaks to the fragility of the 684-nm absorbing Chl a. Consideration of the nature of the 684-nm-absorbing Chl a of CP47 is mainly reserved for the accompanying paper on the D1-D2-cyt b559 reaction center and CP47-Dl-D2-cyt bss9 complexes.

The core antenna complexes of photosystem II, CP43 and CP47, were purified from two higher plants by anion-exchange chromatography, using a combination of the chaotropic agent UCIO4 and the nonionic detergent /3-dodecyl maltoside. The Qy transition was resolved at 48 K into two main bands near 682.3 and 671.5 nm for CP43, while the CP47 spectrum showed a more complex structure with main bands at 688, 681.2, 676, 670, 667, and 661 nm. Emission bands (77 K) were detected at 683 and 695 nm for CP43 and CP47, respectively. Fluorescence excitation spectra showed high efficiency of energy transfer between the different transitions of the chlorophylls and a somewhat lower efficiency from /3-carotene. The circular dichroism spectrum of CP47 indicated the presence of excitonic interactions between some chlorophylls. In contrast, CP43 showed a single negative circular dichroism band at 670 nm. The pigment content of the complexes was determined by both spectroscopic measurements and HPLC. Contents of 18 chlorophylls a and 5 /3-carotenes per CP43 polypeptide and 19 chlorophylls a and 3 /3-carotenes per CP47 polypeptide were found, using the methods of Lowry or Bradford for protein quantitation. When the protein concentration was determined from the amino acid analysis, 20 chlorophylls a and 5 /3-carotenes per CP43 and 21-22 chlorophylls a and 4 /3-carotenes per CP47 were obtained. Thus, a content of 46-48 chlorophylls a was obtained for the core complex, assuming 4-6 chlorophylls per reaction center, in agreement with the composition obtained experimentally using a highly purified oxygen-evolving core complex. This suggested that no pigments were lost during the purification procedure. Moreover, the amino acid analysis of the purified complexes revealed a high homology with the amino acid composition derived from the gene sequences reported for other higher plants.Photosystem (PS1) II is a membrane protein complex present in all oxygenic photosynthetic organisms. The smaller particle of PSII isolated so far, which is able to retain the primary processes of photosynthesis, i.e., charge separation, quinone reduction, and oxygen evolution, is a particle called the oxygenevolving core complex (OECC) (Ghanotakis & Yocum, 1986). This particle consists of eight main polypeptides, i.e., Di and D2, the a-and /3-subunits of cytochrome (Cyt) 6559, the psbl gene product of the reaction center (RC), the core antennae CP43 and CP47, and the 33-kDa extrinsic protein. Many * This work was supported by the Direccibn General de Investigacibn Cientifica y Tecnica (DGICYT, Grant PB92-0125). M.A. and G.M. are grateful to the Direccibn de Polltica Cientifica Gobierno Vasco and the CONAI-Diputacibn General Aragbn, respectively, for financial support.