The CP43 core antenna complex of photosystem II possesses two quasi-degenerate and weakly coupled Q(y)-trap states

The CP43 chlorophyll a-core protein complex plays an important role in funn eling excitation energy absorbed by more peripheral antenna complexes of ph otosystem II (PSII) to the reaction center (RC). Identification and charact erization of the lowest energy Q(y)-states of CP33 is important for underst anding the kinetics of excitation energy transfer (EET) from CP43 to the RC . We report the results of several types of spectroscopic experiments perfo rmed at liquid He temperatures on the isolated CP43 complex from spinach. N onphotochemical hole burning (NPHB) and triplet bottleneck hole burning spe ctroscopies 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 mainl y inhomogeneously broadened Q(y)-absorption bands are 45 and 120 cm(-1), re spectively. The uncorrelated site excitation distribution functions (SDF) o f the two states are nearly the same as their absorption bands since the el ectron-phonon coupling is weak (optical reorganization energies of similar to6 cm(-1)). The NPHB spectra establish that the B state is the primary tra p for EET from higher energy Q(y)-states. The permanent dipole moment chang e (Delta mu) of the S-0 --> Q(y) transition for both the B and A states is small, f.Delta mu = 0.25 +/- 0.05 and 0.47 +/- 0.05, respectively, where fi s the local field correction factor. These values, together with the weak e lectron-phonon coupling and other results, indicate that both states are hi ghly localized on a single Chi a molecule. Holewidth measurements led to th e remarkable finding that the rates of A --> B and B --> A EET processes ar e extremely slow, similar to (6 ns)(-1). This suggests that the Chi a molec ules of the two states belong to different layers of Chi a molecules locate d at opposite sides of the membrane. The intriguing question of why CP43 po ssesses two quasi-degenerate trap states that are so weakly coupled is addr essed. The possibility that they play a role in the photoinhibitory and pho toregulatory processes is raised.