Inter-pigment interactions in the peridinin chlorophyll protein studied by global and target analysis of time resolved absorption spectra

Inter-pigment interactions define the functioning of light-harvesting protein complexes. To describe the particularly complex molecular dynamics and interactions of peridinin and chlorophyll in the peridinin chlorophyll protein of Amphidinium carterae, we applied global and target analysis to a series of ultrafast transient absorption experiments. We have created and validated a model that consistently describes and characterizes the interactions and evolution of excited and ground-state populations after excitation in all different experiments. The series of energy transfer steps that follow excitation are described by our model of cascading populations and numerous rate constants that correspond to intra-molecular thermal relaxation, fast and slow peridinin-to-chlorophyll energy transfer steps, and chlorophyll excited-state annihilation. By analyzing the spectral response of ground-state peridinins to excited chlorophylls we have identified which specific peridinin molecule is most closely coupled to the chlorophylls. No evidence was found that the intra-molecular charge transfer (ICT) state of peridinin, identified in studies of peridinin in solution, is a separate entity in the protein. The peridinin that exhibited slow peridinin-to-chlorophyll energy transfer was characterized by a difference spectrum free from ICT features, consistent with the importance of coupled ICT and S₁ states for energy transfer.