7 research outputs found
New insights into the photochemistry of carotenoid spheroidenone in light-harvesting complex 2 from the purple bacterium Rhodobacter sphaeroides
Light-harvesting complex 2 (LH2) from the
semi-aerobically grown purple phototrophic bacterium
Rhodobacter sphaeroides was studied using optical (static
and time-resolved) and resonance Raman spectroscopies.
This antenna complex comprises bacteriochlorophyll
(BChl) a and the carotenoid spheroidenone, a ketolated
derivative of spheroidene. The results indicate that the
spheroidenone-LH2 complex contains two spectral forms
of the carotenoid: (1) a minor, ‘‘blue’’ form with an S2
(11
Bu
?) spectral origin band at 522 nm, shifted from the
position in organic media simply by the high polarizability
of the binding site, and (2) the major, ‘‘red’’ form with the
origin band at 562 nm that is associated with a pool of
pigments that more strongly interact with protein residues,
most likely via hydrogen bonding. Application of targeted
modeling of excited-state decay pathways after carotenoid
excitation suggests that the high (92%) carotenoid-to-BChl
energy transfer efficiency in this LH2 system, relative to
LH2 complexes binding carotenoids with comparable
double-bond conjugation lengths, derives mainly from
resonance energy transfer from spheroidenone S2 (11
Bu
?)
state to BChl a via the Qx state of the latter, accounting for
60% of the total transfer. The elevated S2 (11
Bu
?) ? Qx
transfer efficiency is apparently associated with substantially
decreased energy gap (increased spectral overlap)
between the virtual S2 (11
Bu
?) ? S0 (11
Ag
-) carotenoid
emission and Qx absorption of BChl a. This reduced
energetic gap is the ultimate consequence of strong carotenoid–protein
interactions, including the inferred hydrogen
bondin
Carotenoid to bacteriochlorophyll energy transfer in the RC–LH1–PufX complex from Rhodobacter sphaeroides containing the extended conjugation keto-carotenoid diketospirilloxanthin
RC–LH1–PufX complexes from a genetically modified strain of Rhodobacter sphaeroides that accumulates carotenoids with very long conjugation were studied by ultrafast transient absorption spectroscopy. The complexes predominantly bind the carotenoid diketospirilloxanthin, constituting about 75% of the total carotenoids, which has 13 conjugated C=C bonds, and the conjugation is further extended to two terminal keto groups. Excitation of diketospirilloxanthin in the RC–LH1–PufX complex demonstrates fully functional energy transfer from diketospirilloxanthin to BChl a in the LH1 antenna. As for other purple bacterial LH complexes having carotenoids with long conjugation, the main energy transfer route is via the S2–Qx pathway. However, in contrast to LH2 complexes binding diketospirilloxanthin, in RC–LH1–PufX we observe an additional, minor energy transfer pathway associated with the S1 state of diketospirilloxanthin. By comparing the spectral properties of the S1 state of diketospirilloxanthin in solution, in LH2, and in RC–LH1–PufX, we propose that the carotenoid-binding site in RC–LH1–PufX activates the ICT state of diketospirilloxanthin, resulting in the opening of a minor S1/ICT-mediated energy transfer channel