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.

Novel nonphotochemical hole burning action spectra are persented that yield the low-temperature absorption profiles of B896 and B870 and their underlying structures (linear electron-phonon coupling and site inhomogeneous broadening). The results establish that B896 and B870 are associated with the far more intense B875 and B850 bacteriochlorophyll a absorption bands, respectively, of the light harvesting I and I1 complexes. The homogeneous widths of the B896 and B870 zero-phonon holes are the same within experimental uncertainty, 3.2 cm-' at 4.2 K, which corresponds to a total optical dephasing time of 6.6 ps. A number of interpretations for B870 and B896 are considered. Favored is one in which they are due to the lowest energy levels of the B850* and B875* exciton bands (asterisk denoting the S,(Q,) state). Based on studies of the dephasing of excitons in organic crystals, the 6.6-p dephasing of B896* is attributed to exciton scattering with energetic inequivalent neighboring unit cells. Such scattering and B870 to B875 energy transfer are suggested to be contributors to the dephasing of B870*. The effect of glasslike structural heterogeneity on the optical selection rules for unit cells of cyclic symmetry is also considered.

We report low temperature (T) optical spectra of the isolated CP47 antenna complex from Photosystem II (PSII) with a low-T fluorescence emission maximum near 695 nm and not, as previously reported, at 690-693 nm. The latter emission is suggested to result from three distinct bands: a lowest-state emission band near 695 nm (labeled F1) originating from the lowest-energy excitonic state A1 of intact complexes (located near 693 nm and characterized by very weak oscillator strength) as well as emission peaks near 691 nm (FT1) and 685 nm (FT2) originating from subpopulations of partly destabilized complexes. The observation of the F1 emission is in excellent agreement with the 695 nm emission observed in intact PSII cores and thylakoid membranes. We argue that the band near 684 nm previously observed in singlet-minus-triplet spectra originates from a subpopulation of partially destabilized complexes with lowest-energy traps located near 684 nm in absorption (referred to as AT2) giving rise to FT2 emission. It is demonstrated that varying contributions from the F1, FT1, and FT2 emission bands led to different maxima of fluorescence spectra reported in the literature. The fluorescence spectra are consistent with the zero-phonon hole action spectra obtained in absorption mode, the profiles of the nonresonantly burned holes as a function of fluence, as well as the fluorescence line-narrowed spectra obtained for the Q(y) band. The lowest Q(y) state in absorption band (A1) is characterized by an electron-phonon coupling with the Huang-Rhys factor S of approximately 1 and an inhomogeneous width of approximately 180 cm(-1). The mean phonon frequency of the A1 band is 20 cm(-1). In contrast to previous observations, intact isolated CP47 reveals negligible contribution from the triplet-bottleneck hole, i.e., the AT2 trap. It has been shown that Chls in intact CP47 are connected via efficient excitation energy transfer to the A1 trap near 693 nm and that the position of the fluorescence maximum depends on the burn fluence. That is, the 695 nm fluorescence maximum shifts blue with increasing fluence, in agreement with nonresonant hole burned spectra. The above findings provide important constraints and parameters for future excitonic calculations, which in turn should offer new insight into the excitonic structure and composition of low-energy absorption traps.