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 CP33 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 similar to6 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 (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 fis 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 Chi a molecule. Holewidth measurements led to the remarkable finding that the rates of A --> B and B --> A EET processes are extremely slow, similar to (6 ns)(-1). This suggests that the Chi a molecules of the two states belong to different layers of Chi 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.