Probing the lowest energy chlorophyll a states of Photosystem II via selective spectroscopy: New insights on P680

We present the wavelength dependence of homogeneous holewidths of persistent spectral holes burnt in O₂-evolving Photosystem II core complexes isolated from spinach, in the temperature range 2.5-8 K. The data supports the assignment that those chlorophylls which undergo persistent spectral hole-burning are specific CP43 and CP47-trap states that transfer their excitation energy to the reaction center. The lifetime-limited holewidths show that when PS II is in the S₁(Q _A ^- ) (closed) state, the CP43/CP47-trap states have excited-state lifetimes in the range from 70 to 270 ps. These lifetimes correspond to excitation transfer rates to the reaction center, and are far slower than required for models in which the PS II reaction center (P680) acts as a 'shallow-trap' for excitations. For wavelengths at which both traps absorb, the hole shape is clearly a composite of two Lorentzians, corresponding to hole-burning in both states simultaneously. The temperature dependence of the homogeneous holewidth does not follow the usual T ^1.3 dependence found in many chlorophyll-protein systems. Our data indicates T ² temperature dependence, typically found in crystalline systems where the chromophore is coupled to localized phonon modes.