Optimal coupling of subantennas as a strategy for efficient functioning of the light-harvesting antennas in photosynthesizing organisms: Model computations

Abstract

This work continues the cycle of studies into the strategy of efficient functioning of the natural light-harvesting molecular antennas within the framework of the previously proposed concept of stringent optimization of the structure of the photosynthesizing machinery according to the functional criterion [1]. The presence of inhomogeneous antennas composed of different subantennas poses the problem of their coupling [2]. One of the structural optimizing factors is the relative orientation of Q y transition dipoles of the light-harvesting pigments in subantennas [3]. This paper briefs the mathematical modeling of the dynamics of excitation energy transfer in the superantenna of the photosynthesizing green bacteria Chloroflexus ( Cf. ) aurantiacus. This allowed us to compute the optimal relative orientation of the Q y transition dipoles in subantenna molecules using the formalism of probability matrices and to demonstrate the significance of their optimal coupling for an efficient and stable functioning of the superantenna. The superantenna of Cf. aurantiacus is localized to three subcellular structures: chlorosomes, containing the oligomeric BChl c subantenna B740; the baseplate, containing the monomeric BChl a subantenna B798; and cytoplasmic membrane, containing the BChl a subantenna B808‐866, analogous to the subantenna B800‐ 850 of purple bacteria [4]. The excitation energy transfer B740 → B798 → B808 → B866 proceeds according to the thermodynamic potential [5]. The Q y transition dipoles in BChl c clusters of subantenna B740 are mutually parallel and oriented along the long axis of the chlorosome [6], whereas the Q y transition dipoles in B808 and B866 subantenna molecules make angles of ~43° and ~8° with the plane of the membrane, respectively [7]. The orientation of Q y transition dipoles for the intermediate subantenna B798 in BChl ‡ is unknown. In this work, we modeled the energy transfer B740 → B798 → B808 to analyze the optimization of this process by optimizing the orientation of Q y transition dipoles in BChl ‡ subantenna B798. An infinite three-dimensional antenna displaying