Photoprotection in a purple phototrophic bacterium mediated by oxygen-dependent alteration of carotenoid excited-state properties

Abstract

Carotenoids are known to offer protection against the potentially damaging combination of light and oxygen encountered by purple phototrophic bacteria, but the efficiency of such protection depends on the type of carotenoid. Rhodobacter sphaeroides synthesizes spheroidene as the main carotenoid under anaerobic conditions whereas, in the presence of oxygen, the enzyme spheroidene monooxygenase catalyses the incorporation of a keto group forming spheroidenone. We performed ultrafast transient absorption spectroscopy on membranes containing reaction center-light-harvesting 1-PufX (RC-LH1-PufX) complexes and showed that when oxygen is present the incorporation of the keto group into spheroidene, forming spheroidenone, reconfigures the energy transfer pathway in the LH1, but not the LH2, antenna. The spheroidene/spheroidenone transition acts as a molecular switch that is suggested to twist spheroidenone into an s-trans configuration increasing its conjugation length and lowering the energy of the lowest triplet state so it can act as an effective quencher of singlet oxygen. The other consequence of converting carotenoids in RC-LH1-PufX complexes is that S(2)/S(1)/triplet pathways for spheroidene is replaced with a new pathway for spheroidenone involving an activated intramolecular charge-transfer (ICT) state. This strategy for RC-LH1-PufX-spheroidenone complexes maintains the light-harvesting cross-section of the antenna by opening an active, ultrafast S(1)/ICT channel for energy transfer to LH1 Bchls while optimizing the triplet energy for singlet oxygen quenching. We propose that spheroidene/spheroidenone switching represents a simple and effective photoprotective mechanism of likely importance for phototrophic bacteria that encounter light and oxygen.

Fig. 1.

Absorption spectra of RC-LH1-PufX complexes containing spheroidene (sph) and spheroidenone (spn). The excitation wavelengths are indicated by vertical arrows.

Fig. 2.

Transient absorption spectra recorded after excitation of the S₂ state of carotenoid in (A) RC-LH1-PufX complexes containing spheroidene (sph) and spheroidenone (spn). The transient absorption spectra were recorded 0.5 ps after excitation at 475 nm (sph) and 514 nm (spn). (B) Comparison of transient absorption spectra of the RC-LH1-PufX(spn) complex, LH2 complex containing spn, and spn in acetonitrile. Excitation wavelengths were 514 nm (RC-LH1-PufX), 515 nm (LH2), and 500 nm (acetonitrile). Data on LH2 and spheroidenone in solution are taken from refs.  and . All spectra are normalized to S₁–S_n maximum.

Fig. 3.

EADS resulting from global fitting the data in the visible spectral region measured for (A) RC-LH1-PufX(spn) and (B) RC-LH1-PufX(sph) complexes. The data at wavelengths shorter than 550 nm are omitted for the RC-LH1-PufX(spn) complex due to strong scattering from excitation. n.d., nondecaying component.

Fig. 4.

EADS resulting from global fitting the data in the near-IR spectral region measured for (A) RC-LH1-PufX(spn), (B) RC-LH1-PufX(sph) complexes. n.d., nondecaying component.