Effects of spectral variety and molecular orientation on energy trapping in the photosynthetic unit: a model calculation

It is proposed that different spectral varieties of chlorophylls exist in the photosynthetic unit of green plants in order to accelerate the transfer of excitation energy to the reaction center, and thus allow the operation of physically larger units with greater light-harvesting power. This proposal is supported by computer calculation of trapping probabilities on model arrays containing three spectral forms of antenna chlorophyll in addition to reaction center chlorophyll. The calculations assume nearest neighbor transfer steps only, and pairwise dipolar interactions of the sort that feature in the inductive resonance mechanism of Förster. Mutual orientation of transition moment vectors also affects the number of jumps and the time needed for energy trapping. Spectral variety increases the trapping rate by a factor of 4 to 5 over the uniform chlorophyll rate on the arrays examined, and the proper use of orientation may introduce a similar factor. The arguments here provide a plausible explanation not only for the existence of the more abundant forms of chlorophyll a, but also for the specialized form “C705” and for chlorophyll b.