Chapter 79 - The mechanism of energy transfer and trapping in Photosystem I

Abstract

The collection and transfer of excitation energy to the reaction centers of the photosynthetic unit of bacteria, cyanobacteria, and higher plants, known as light harvesting, is one of the most exquisitely effective processes in nature, with efficiencies approaching 100%. Discovering the design principles at work in these photosynthetic antenna and reaction center complexes has been a preoccupation of ultrafast spectroscopy for many years. The structure of the core antenna of Photosystem I (PSI) stands in striking contrast to the bacterial system. The 96 non-equivalent chlorophyll (Chi) molecules are very densely packed in a three dimensional arrangement with little apparent symmetry or regularity. This immediately raises the question of whether there are new design principles at work in PSI as compared to the bacterial system. In particular, no obvious energy funnel is apparent; indeed, there are antenna Chls that absorb to the red of the primary electron donor, P700. This chapter summarizes the experimental timescales for energy transfer in PSI and compares them with results from a detailed quantum dynamical calculation. The remarkable success of the calculations in reproducing the experimental timescales encourages exploration of the design principles at work in PSI and enables one to present snapshots of the way in which energy flows through this spatially and energetically disordered system.