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, this article) and Chl b (next article in this series) of possibly vital importance. Our model system is the photosynthetic antenna pigment-protein complex (CP29). Here, we show that the B → Q IC is assisted by the optically allowed Crt state (S2): The sequence is B → S2 (Crt, unrelaxed) → S2 (Crt, relaxed) → Q. This sequence has the advantage of preventing ∼39% of Chl-Chl B-B EET since the Crt S2 state is a highly efficient FRET acceptor. The B-B EET range and thus the likelihood of CP29 to forward potentially harmful B excitations toward the RC are thus reduced. In contrast to the B band of Chls, most Crt energy donation is energetically located near the Q band, which allows for 74/80% backdonation (from lutein/violaxanthin) to Chls. Neoxanthin, on the other hand, likely donates in the B band region of Chl b, with 76% efficiency. Crts thus act not only in their currently proposed photoprotective roles but also as a crucial building block for any system that could otherwise deliver harmful "blue" excitations to the RCs.

• Publikační typ
• časopisecké články MeSH

Chlorophylls and bacteriochlorophylls, together with carotenoids, serve, noncovalently bound to specific apoproteins, as principal light-harvesting and energy-transforming pigments in photosynthetic organisms. In recent years, enormous progress has been achieved in the elucidation of structures and functions of light-harvesting (antenna) complexes, photosynthetic reaction centers and even entire photosystems. It is becoming increasingly clear that light-harvesting complexes not only serve to enlarge the absorption cross sections of the respective reaction centers but are vitally important in short- and long-term adaptation of the photosynthetic apparatus and regulation of the energy-transforming processes in response to external and internal conditions. Thus, the wide variety of structural diversity in photosynthetic antenna "designs" becomes conceivable. It is, however, common for LHCs to form trimeric (or multiples thereof) structures. We propose a simple, tentative explanation of the trimer issue, based on the 2D world created by photosynthetic membrane systems.

• Klíčová slova
• bacteriochlorophylls, carotenoids, chlorophylls, excitation energy transfer, light-harvesting complexes, photoprotection, photosynthesis, photosystems, pigment-protein complexes,
• MeSH
• bakteriální proteiny chemie   metabolismus   MeSH
• fotosyntéza MeSH
• konformace proteinů MeSH
• molekulární modely MeSH
• multimerizace proteinu MeSH
• přenos energie MeSH
• rostlinné proteiny chemie   metabolismus   MeSH
• rostliny metabolismus   MeSH
• sinice metabolismus   MeSH
• světlosběrné proteinové komplexy chemie   metabolismus   MeSH
• Publikační typ
• časopisecké články MeSH
• přehledy MeSH
• Názvy látek
• bakteriální proteiny MeSH
• rostlinné proteiny MeSH
• světlosběrné proteinové komplexy MeSH