The low-energy forms of Photosystem I light-harvesting complexes: spectroscopic properties and pigment-pigment interaction characteristics.

In this work the spectroscopic properties of the special low-energy absorption bands of the outer antenna complexes of higher plant Photosystem I have been investigated by means of low temperature absorption, fluorescence and fluorescence line narrowing experiments. It was found that the red-most absorption bands of Lhca3, Lhca4 and Lhca1-4 peak, respectively, at 704, 708 and 709 nm and are responsible for 725, 733 and 732 nm fluorescence emission bands. These bands are more red-shifted compared to "normal" Chl alpha bands present in light-harvesting complexes. The low-energy forms are characterised by a very large bandwidth (400-450 cm(-1)), which is the result of both large homogeneous and inhomogeneous broadening. The observed optical reorganisation energy is untypical for Chl alpha and resembles more that of BChl alpha antenna systems. The large broadening and the changes in optical reorganisation energy are explained by a mixing of an Lhca excitonic state with a charge-transfer state. Such a charge-transfer state can be stabilized by the polar residues around Chl 1025. It is shown that the optical reorganisation energy is changing through the inhomogeneous distribution of the red-most absorption band, with the pigments contributing to the red part of the distribution showing higher values. A second red emission form in Lhca4 was detected at 705 nm and originates from a broad absorption band peaking at 690 nm. This fluorescence emission is present also in the Lhca4-N47H mutant, which lacks the 733 nm emission band.