Abstract
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.
Highlights
In higher plants and green algae, Chls a and b serve as the major antenna pigments in the light-harvesting (antenna) complexes (LHCs)
It is clear that light-harvesting antenna complexes serve to enlarge the absorption cross sections of the respective photochemical reaction centers (RCs) but are vitally important in short- and long-term adaptation and regulation of the energy-transforming processes in the photosynthetic apparatus, in response to external and internal conditions
An enigma remains: why are so many light-harvesting complexes trimeric? Given the structural, genetic, pigment-binding and functional diversity of LHCs, as presented above, it appears that the trimeric construction principle may be a result of the environment in which these complexes are located in or are attached to, namely, two-dimensional (2D) membrane systems
Summary
The PSII of all oxygenic photosynthetic organisms contains two core antenna complexes (Figure 4), designated as CP43 and CP47, due to their apparent molecular weights. Within each monomeric PCP subunit, pigments are organized in two clusters related by a pseudo-twofold symmetry (Figure 6) Each of these clusters contains one Chl a surrounded by four peridinins within Van der Waals distance. Depending on the computational methods used, this mixed state may even be initially populated via direct absorption close to the typical xanthophyll allowed state From this state, which apparently cannot be labeled as pure S2 or S1, Förster-type EET to the closest Chls a may occur. These results, are very sensitive to the employed computational approach, since the lowest excited state of carotenoids is very difficult to assess [78]. Experimental evidence for the theoretically modelled type of mixed EET (with S2 preference) exists [79]
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