Excitation Energy Transfer between Higher Excited States of Photosynthetic Pigments: 2. Chlorophyll b is a B Band Excitation Trap

Abstract

Chlorophylls (Chls) are known for fast, subpicosecond 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 Förster resonance energy transport (FRET) scheme, unmodulated B–B EET has an unexpectedly high range. Unsuppressed, it could pose an existential threat-the damage potential of blue light for photochemical reaction centers (RCs) is well-known. This insight reveals so-far undescribed roles for carotenoids (Crts, cf. previous article in this series) and Chl b (this article) of possibly vital importance. Our model system is the photosynthetic antenna pigment–protein complex (CP29). The focus of the study 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 of plants and algae, and RCs contain only Chl a, this would automatically trap potentially dangerous B state population in the antennae, preventing forwarding to the RCs.

Excitation Energy Transfer between Higher Excited States of Photosynthetic Pigments: 1. Carotenoids Intercept and Remove B Band Excitations

“…This enhances the effective B band depletion by Crts, as it does not matter if a photon is directly absorbed by Crts or if the S 2 excitation is resulting from B → S 2 EET; the result is the same. This aspect will also be explored in the next article in this series …”

“…It is also shown that a simple and efficient solution to the Chl B–B EET problem is the presence of Crts. Crts efficiently compete for Chl B state excitations, both via initial spectral filtering (discussed in detail in the next article of this series 71 ) and then through a high likelihood of acting as FRET acceptors (in CP29, at least 39% B–B excitation reduction). Due to rapid internal conversion and their strongly shifted emission spectrum, they cannot donate the energy back into the B band.…”

“…Finally, there is the explicit possibility that Chl b plays a role in the B–B EET context. The next article in this series 71 aims to shed light on a potential new function of Chl b as a photoprotective pigment.…”

Correction to “Excitation Energy Transfer between Higher Excited States of Photosynthetic Pigments: 1. Carotenoids Intercept and Remove B Band Excitations”

“…In our article titled “Excitation Energy Transfer between Higher Excited States of Photosynthetic Pigments: 1. Carotenoids Intercept and Remove B Band Excitations” (DOI: ), , we computationally approximated the fluorescence yields of chlorophylls (Chls) and carotenoids (Crts). During the calculations of the theoretical values, an input/output error in the code resulted in reading the absorption spectrum twice instead of reading the emission and absorption spectrum.…”