New insights into the photochemistry of carotenoid spheroidenone in light-harvesting complex 2 from the purple bacterium Rhodobacter sphaeroides

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Niedzwiedzki, Dariusz M. ; Dilbeck, Preston L. ; Tang, Qun ; Martin, Elizabeth C. ; Bocian, David F. ; Hunter, C. Neil ; Holten, Dewey (2016)

Light-harvesting complex 2 (LH2) from the\ud semi-aerobically grown purple phototrophic bacterium\ud Rhodobacter sphaeroides was studied using optical (static\ud and time-resolved) and resonance Raman spectroscopies.\ud This antenna complex comprises bacteriochlorophyll\ud (BChl) a and the carotenoid spheroidenone, a ketolated\ud derivative of spheroidene. The results indicate that the\ud spheroidenone-LH2 complex contains two spectral forms\ud of the carotenoid: (1) a minor, ‘‘blue’’ form with an S2\ud (11\ud Bu\ud ?) spectral origin band at 522 nm, shifted from the\ud position in organic media simply by the high polarizability\ud of the binding site, and (2) the major, ‘‘red’’ form with the\ud origin band at 562 nm that is associated with a pool of\ud pigments that more strongly interact with protein residues,\ud most likely via hydrogen bonding. Application of targeted\ud modeling of excited-state decay pathways after carotenoid\ud excitation suggests that the high (92%) carotenoid-to-BChl\ud energy transfer efficiency in this LH2 system, relative to\ud LH2 complexes binding carotenoids with comparable\ud double-bond conjugation lengths, derives mainly from\ud resonance energy transfer from spheroidenone S2 (11\ud Bu\ud ?)\ud state to BChl a via the Qx state of the latter, accounting for\ud 60% of the total transfer. The elevated S2 (11\ud Bu\ud ?) ? Qx\ud transfer efficiency is apparently associated with substantially\ud decreased energy gap (increased spectral overlap)\ud between the virtual S2 (11\ud Bu\ud ?) ? S0 (11\ud Ag\ud -) carotenoid\ud emission and Qx absorption of BChl a. This reduced\ud energetic gap is the ultimate consequence of strong carotenoid–protein\ud interactions, including the inferred hydrogen\ud bonding
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