Structure-based calculations of optical spectra of photosystem I suggest an asymmetric light-harvesting process.
Optical line shape theory is combined with a quantum-chemical/electrostatic calculation of the site energies of the 96 chlorophyll a pigments and their excitonic couplings to simulate optical spectra of photosystem I core complexes from Thermosynechococcus elongatus. The absorbance, linear dichroism and circular dichroism spectra, calculated on the basis of the 2.5 A crystal structure, match the experimental data semiquantitatively allowing for a detailed analysis of the pigment-protein interaction. The majority of site energies are determined by multiple interactions with a large number (>20) of amino acid residues, a result which demonstrates the importance of long-range electrostatic interactions. The low-energy exciton states of the antenna are found to be located at a nearest distance of about 25 A from the special pair of the reaction center. The intermediate pigments form a high-energy bridge, the site energies of which are stabilized by a particularly large number (>100) of amino acid residues. The concentration of low energy exciton states in the antenna is larger on the side of the A-branch of the reaction center, implying an asymmetric delivery of excitation energy to the latter. This asymmetry in light-harvesting may provide the key for understanding the asymmetric use of the two branches in primary electron transfer reactions. Experiments are suggested to check for this possibility.
Adolphs
Julian
J
Institut für Chemie und Biochemie, Freie Universität Berlin, Fabeckstrasse 36a, D-14195 Berlin, Germany.
Müh
Frank
F
Madjet
Mohamed El-Amine
Mel-A
am Busch
Marcel Schmidt
MS
Renger
Thomas
T
D002735:Chlorophyllides / Q000737:chemistry / Q000378:metabolism; D002942:Circular Dichroism; D006860:Hydrogen Bonding; D045342:Light-Harvesting Protein Complexes / Q000737:chemistry* / Q000378:metabolism; D008958:Models, Molecular; D045331:Photosystem I Protein Complex / Q000737:chemistry* / Q000378:metabolism; D011789:Quantum Theory; D013057:Spectrum Analysis / Q000379:methods; D055672:Static Electricity; D013329:Structure-Activity Relationship; D046940:Synechococcus / Q000737:chemistry / Q000378:metabolism; D013816:Thermodynamics
D002735:Chlorophyllides; D045342:Light-Harvesting Protein Complexes; D045331:Photosystem I Protein Complex; C007020:chlorophyllin