Quenching Capabilities of Long-Chain Carotenoids in Light-Harvesting-2 Complexes from Rhodobacter sphaeroides with an Engineered Carotenoid Synthesis Pathway.
Six light-harvesting-2 complexes (LH2) from genetically modified strains of the purple photosynthetic bacterium Rhodobacter (Rb.) sphaeroides were studied using static and ultrafast optical methods and resonance Raman spectroscopy. These strains were engineered to incorporate carotenoids for which the number of conjugated groups (N = NC═C + NC═O) varies from 9 to 15. The Rb. sphaeroides strains incorporate their native carotenoids spheroidene (N = 10) and spheroidenone (N = 11), as well as longer-chain analogues including spirilloxanthin (N = 13) and diketospirilloxantion (N = 15) normally found in Rhodospirillum rubrum. Measurements of the properties of the carotenoid first singlet excited state (S1) in antennas from the Rb. sphaeroides set show that carotenoid-bacteriochlorophyll a (BChl a) interactions are similar to those in LH2 complexes from various other bacterial species and thus are not significantly impacted by differences in polypeptide composition. Instead, variations in carotenoid-to-BChl a energy transfer are primarily regulated by the N-determined energy of the carotenoid S1 excited state, which for long-chain (N ≥ 13) carotenoids is not involved in energy transfer. Furthermore, the role of the long-chain carotenoids switches from a light-harvesting supporter (via energy transfer to BChl a) to a quencher of the BChl a S1 excited state B850*. This quenching is manifested as a substantial (∼2-fold) reduction of the B850* lifetime and the B850* fluorescence quantum yield for LH2 housing the longest carotenoids.
Preston L
PL
Dilbeck
Department of Chemistry, University of California Riverside , Riverside, California 92521, United States.
Qun
Q
Tang
Department of Molecular Biology and Biotechnology, University of Sheffield , Sheffield S10 2TN, United Kingdom.
David J
DJ
Mothersole
Department of Molecular Biology and Biotechnology, University of Sheffield , Sheffield S10 2TN, United Kingdom.
Elizabeth C
EC
Martin
Department of Molecular Biology and Biotechnology, University of Sheffield , Sheffield S10 2TN, United Kingdom.
C Neil
CN
Hunter
Department of Chemistry, University of California Riverside , Riverside, California 92521, United States.
David F
DF
Bocian
Dewey
D
Holten
Dariusz M
DM
Niedzwiedzki
D001426:Bacterial Proteins / Q000737:chemistry / Q000378:metabolism*; D001429:Bacteriochlorophylls / Q000737:chemistry / Q000378:metabolism; D002338:Carotenoids / Q000737:chemistry / Q000378:metabolism*; D004735:Energy Transfer; D045342:Light-Harvesting Protein Complexes / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D011789:Quantum Theory; D011994:Recombinant Proteins / Q000096:biosynthesis / Q000737:chemistry / Q000302:isolation & purification; D012242:Rhodobacter sphaeroides / Q000378:metabolism*; D013050:Spectrometry, Fluorescence; D013059:Spectrum Analysis, Raman
D001426:Bacterial Proteins; D001429:Bacteriochlorophylls; D045342:Light-Harvesting Protein Complexes; D011994:Recombinant Proteins; D002338:Carotenoids