title: Insight into the structural role of carotenoids in the photosystem I: a quantum chemical analysis.
abstract: The structural stabilization role of carotenoids in the formation of photosynthetic pigment-protein complexes is investigated theoretically. The pi-pi stacking and CH-pi interactions between beta-carotenes and their surrounding chlorophylls (and/or aromatic residues) in Photosystem I (PS1) from the cyanobacterium Synechococcus elongatus were studied by means of the supermolecular approach at the level of the second-order Møller-Plesset perturbation method. PS1 features a core integral antenna system consisting of 22 beta-carotenes intertwined with 90 chlorophyll molecules. The binding environments of all 22 beta-carotenes were systematically analyzed. For 21 out of the 22 cases, one or more chlorophyll molecules exist within van der Waals' contacts of the beta-carotene molecule. The calculated strengths of pi-pi stacking interactions between the conjugated core of beta-carotene and the aromatic tetrapyrrole rings of chlorophyll are substantial, ranging from -3.54 kcal/mol for the perpendicular-positioned BCR4004...CHL1217 pair to -16.01 kcal/mol for the parallel-oriented BCR4007...CHL1122 pair. A strong dependence of the pi-pi stacking interaction energies on the intermolecular configurations of the two interacting pi-planes is observed. The parallel-oriented beta-carotene and chlorophyll pair is energetically much more stable than the perpendicular-positioned pair. The larger the extent of pi-pi overlapping, the stronger the interaction strength. In many cases, the beta-ring ends of beta-carotene molecules are found to interact with the tetrapyrrole rings of chlorophyll via CH-pi interactions. For the latter interactions, the calculated interaction strengths vary from -7.03 to -11.03 kcal/mol, depending on the intermolecular configuration. This work leads to the conclusion that pi-pi stacking and CH-pi interactions between beta-carotene and their surrounding chlorophylls and aromatic residues play an essential role in binding beta-carotenes in PS1 from S. elongatus. Consequently, the molecular basis of the structural stabilization function of carotenoids in formation of the photosynthetic pigment-protein complexes is established.
authors.affiliation: Department of Chemistry, University of Toledo, Toledo, Ohio 43606, USA.
authors.forename: Yanli
authors.initials: Y
authors.lastname: Wang
authors.forename: Lisong
authors.initials: L
authors.lastname: Mao
authors.forename: Xiche
authors.initials: X
authors.lastname: Hu
mesh_terms: D001703:Biophysics / Q000379:methods*; D002244:Carbon / Q000737:chemistry; D002338:Carotenoids / Q000737:chemistry* / Q000378:metabolism; D002734:Chlorophyll / Q000737:chemistry; D016000:Cluster Analysis; D000458:Cyanobacteria / Q000378:metabolism; D019281:Dimerization; D008956:Models, Chemical; D008958:Models, Molecular; D008968:Molecular Conformation; D010788:Photosynthesis; D045331:Photosystem I Protein Complex / Q000737:chemistry*; D011487:Protein Conformation; D013223:Statistics as Topic; D013816:Thermodynamics; D019207:beta Carotene / Q000378:metabolism
chemical_list: D045331:Photosystem I Protein Complex; D019207:beta Carotene; D002734:Chlorophyll; D002338:Carotenoids; D002244:Carbon