Analysis of structure-function correlations in light-harvesting photosynthetic antenna: Structure optimization parameters

Abstract

Our previous theoretical analysis of the consistency between the results of experimental studies of two most important stages in the primary conversion of light energy in photosynthesis—the excitation energy transfer within a light-harvesting antenna and the stabilization of this energy in reaction centres—showed that the photosynthetic unit structure is to be strongly optimized in vivo to operate with a 90% quantum yield of primary charge separation in reaction centres, which means that a macroscopic photosynthetic unit is neither uniform nor isotropic. The principles of the structural organization of a light-harvesting antenna of a model photosynthetic unit which allow optimization of excitation energy transfer from antenna molecules to reaction centres are studied here. The time of excitation energy trapping by reaction centres was computed for two-dimensional systems simulating a photosynthetic unit of purple bacteria and the longest-wavelength part of higher plant Photosystem-1 photosynthetic unit. The calculations assume a Förster inductive resonance mechanism for energy transfer within antenna and pairwise dipolar interactions. It was shown that the co-operative effect of several optimizing factors, i.e. (1) anisotropy of intermolecular distances in an antenna; (2) its spectral heterogeneity; (3) mutual orientation of transition moment vectors; (4) availability of the “focusing zone” around reaction centres; (5) definite spatial clustering of reaction centres in macroscopic antenna, can decrease 300-fold the time required for a high probability of excitation trapping to be obtained compared to a similar array of undifferentiated antenna molecules. As the analysis that we offer is of more or less general nature, it is to be hoped that our major conclusions will be valid for any photosynthetic organisms. The basic principles of the structural Organization of an optimal artificial light-converting system are formulated.