Origin of the 701-nm fluorescence emission of the Lhca2 subunit of higher plant photosystem I.
Photosystem I of higher plants is characterized by red-shifted spectral forms deriving from chlorophyll chromophores. Each of the four Lhca1 to -4 subunits exhibits a specific fluorescence emission spectrum, peaking at 688, 701, 725, and 733 nm, respectively. Recent analysis revealed the role of chlorophyll-chlorophyll interactions of the red forms in Lhca3 and Lhca4, whereas the basis for the fluorescence emission at 701 nm in Lhca2 is not yet clear. We report a detailed characterization of the Lhca2 subunit using molecular biology, biochemistry, and spectroscopy and show that the 701-nm emission form originates from a broad absorption band at 690 nm. Spectroscopy on recombinant mutant proteins assesses that this band represents the low energy form of an excitonic interaction involving two chlorophyll a molecules bound to sites A5 and B5, the same protein domains previously identified for Lhca3 and Lhca4. The resulting emission is, however, substantially shifted to higher energies. These results are discussed on the basis of the structural information that recently became available from x-ray crystallography (Ben Shem, A., Frolow, F., and Nelson, N. (2003) Nature 426, 630-635). We suggest that, within the Lhca subfamily, spectroscopic properties of chromophores are modulated by the strength of the excitonic coupling between the chromophores A5 and B5, thus yielding fluorescence emission spanning a large wavelength interval. It is concluded that the interchromophore distance rather than the transition energy of the individual chromophores or the orientation of transition vectors represents the critical factor in determining the excitonic coupling in Lhca pigment-protein complexes.
Croce
Roberta
R
Istituto di Biofisica, CNR, Trento, c/o ITC via Sommarive 18, Povo, Trento 38100, Italy. croce@itc.it
Morosinotto
Tomas
T
Ihalainen
Janne A
JA
Chojnicka
Agnieszka
A
Breton
Jacques
J
Dekker
Jan P
JP
van Grondelle
Rienk
R
Bassi
Roberto
R
D016880:Anisotropy; D017360:Arabidopsis / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000737:chemistry* / Q000378:metabolism; D002734:Chlorophyll / Q000737:chemistry; D061146:Chlorophyll Binding Proteins; D002851:Chromatography, High Pressure Liquid; D002942:Circular Dichroism; D018076:DNA, Complementary / Q000737:chemistry; D006860:Hydrogen Bonding; D045342:Light-Harvesting Protein Complexes / Q000737:chemistry* / Q000378:metabolism; D009154:Mutation; D016011:Normal Distribution; D045331:Photosystem I Protein Complex / Q000737:chemistry* / Q000378:metabolism; D010940:Plant Proteins / Q000737:chemistry*; D011994:Recombinant Proteins / Q000737:chemistry; D013050:Spectrometry, Fluorescence; D013053:Spectrophotometry; D013696:Temperature
D029681:Arabidopsis Proteins; D061146:Chlorophyll Binding Proteins; D018076:DNA, Complementary; C490277:Lhca2 protein, Arabidopsis; D045342:Light-Harvesting Protein Complexes; D045331:Photosystem I Protein Complex; D010940:Plant Proteins; D011994:Recombinant Proteins; D002734:Chlorophyll