title: Quenching of chlorophyll triplet states by carotenoids in reconstituted Lhca4 subunit of peripheral light-harvesting complex of photosystem I.
abstract: In this study, triplet quenching, the major photoprotection mechanism in antenna proteins, has been studied in the light-harvesting complex of photosystem I (LHC-I). The ability of carotenoids bound to LHC-I subunit Lhca4, which is characterized by the presence of the red-most absorption components at wavelength >700 nm, to protect the system through quenching of the chlorophyll triplet states, has been probed, by analyzing the induction of carotenoid triplet formation. We have investigated this process at low temperature, when the funneling of the excitation toward the low-lying excited states of the Chls is stronger, by means of optically detected magnetic resonance (ODMR), which is well-suited for investigation of triplet states in photosynthetic systems. The high selectivity and sensitivity of the technique has made it possible to point out the presence of specific interactions between carotenoids forming the triplet states and specific chlorophylls characterized by red-shifted absorption, by detection of the microwave-induced Triplet minus Singlet (T-S) spectra. The effect of the red forms on the efficiency of triplet quenching was specifically probed by using the Asn47His mutant, in which the red forms have been selectively abolished (Morosinotto, T., Breton, J., Bassi, R., and Croce, R. (2003) J. Biol. Chem. 278, 49223-49229). Lack of the red forms yields into a reduced efficiency of the triplet quenching in LHC-I thus suggesting that the "red Chls" play a role in enhancing triplet quenching in LHC-I and, possibly, in the whole photosystem I.
authors.affiliation: Dipartimento di Scienze Chimiche, Università di Padova, via Marzolo 1, 35131 Padova, Italy. donatella.carbonera@unipd.it
authors.forename: Donatella
authors.initials: D
authors.lastname: Carbonera
authors.forename: Giancarlo
authors.initials: G
authors.lastname: Agostini
authors.forename: Tomas
authors.initials: T
authors.lastname: Morosinotto
authors.forename: Roberto
authors.initials: R
authors.lastname: Bassi
mesh_terms: D019943:Amino Acid Substitution / Q000235:genetics; D017360:Arabidopsis / Q000737:chemistry / Q000235:genetics; D001216:Asparagine / Q000235:genetics; D002338:Carotenoids / Q000737:chemistry* / Q000502:physiology; D002734:Chlorophyll / Q000737:chemistry* / Q000502:physiology; D061146:Chlorophyll Binding Proteins; D002851:Chromatography, High Pressure Liquid; D004735:Energy Transfer / Q000235:genetics; D006639:Histidine / Q000235:genetics; D045342:Light-Harvesting Protein Complexes / Q000737:chemistry* / Q000235:genetics / Q000502:physiology; D009682:Magnetic Resonance Spectroscopy; D045331:Photosystem I Protein Complex / Q000737:chemistry* / Q000235:genetics / Q000502:physiology; D010940:Plant Proteins / Q000737:chemistry* / Q000235:genetics / Q000502:physiology; D021122:Protein Subunits / Q000737:chemistry* / Q000235:genetics / Q000502:physiology; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D013696:Temperature
chemical_list: D061146:Chlorophyll Binding Proteins; D045342:Light-Harvesting Protein Complexes; D045331:Photosystem I Protein Complex; D010940:Plant Proteins; D021122:Protein Subunits; D011994:Recombinant Proteins; D002734:Chlorophyll; D002338:Carotenoids; D006639:Histidine; D001216:Asparagine