Excitation energy transfer between higher excited states of photosynthetic pigments: 2. Chlorophyll b is a B band energy trap

Chlorophylls (Chls) are known for fast, sub-picosecond internal conversion (IC) from ultraviolet/blue absorbing ("B" or "Soret" states) to the energetically lower, red light-absorbing Q states. Consequently, excitation energy transfer (EET) in photosynthetic pigment-protein complexes involving the B states has so far not been considered. We present, for the first time, a theoretical framework for the existence of B-B EET in tightly coupled Chl aggregates, such as photosynthetic pigment-protein complexes. We show that according to a simple Forster resonance energy transport (FRET) scheme, unmodulated B-B EET likely poses an existential threat, in particular the photochemical reaction centers (RCs). This insight leads to so-far undescribed roles for carotenoids (Crts, cf. previous article in this series) and Chl b (this article) of possibly primary importance. It is demonstrated how pigments in a photosynthetic antenna pigment-protein complex (CP29) undergo FRET. Here, the focus is on the role of Chl b for EET in the Q and B bands. Further, the initial excited pigment distribution in the B band is computed for relevant solar irradiation and wavelength-centered laser pulses. It is found that both accessory pigment classes compete efficiently with Chl a absorption in the B band, leaving only 40% of B band excitations for Chl a. B state population is preferentially relocated to Chl b after excitation of any Chls, due to a near-perfect match of Chl b B band absorption with Chl a B state emission spectra. This results in an efficient depletion of the Chl a population (0.66 per IC/EET step, as compared to 0.21 in a Chl a-only system). Since Chl b only occurs in the peripheral antenna complexes, and RCs contain only Chl a, this would automatically trap potentially dangerous B state population distantly from the RCs.