The origin and spectroscopic properties of the red forms in the light harvesting complex Lhca4

Martijn Tros, Rienk van Grondelle, Roberta Croce
Department of Physics and Astronomy, Faculty of Sciences, Vrije Universiteit Amsterdam, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands

The Lhca4 antenna complex of higher plants is characterized by the presence of chlorophylls with red-shifted absorption and emission spectra. Though it is known that these so-called red forms involve the mixing of a chlorophyll exciton state with charge transfer (CT) state, their exact origin is still unknown. By studying several mutants of the Lhca4 complex differing in the properties of the red-shifted absorption chlorophylls more information about the origin of the red forms can be obtained. The excitation energy transfer within the Lhca4 mutants can be followed accurately with transient absorption spectroscopy on fs-ps timescale. These data in combination with the recently reported crystal structure of PSI can be used to build an accurate exciton model for the entire Lhca4 complex. The strong interaction between the two red Chlorophylls probably causes very strong coherences in the Lhca4 complex. By means of two dimensional electronic spectroscopy (2DES) these coherences can be measured directly. Moreover 2DES is also sensitive to dark states, and can thus provide more insight in the charge transfer (CT) state. In particular,  comparing the results for the different mutants may lead to a better understanding of the role of the CT state in the excitonic state manifold and the excited energy transfer (EET) within the complex.