Active intramolecular charge-transfer state in LH1 with diketospirilloxanthin from Rhodobacter sphaeroides

Václav Šloufa, Gürkan Keşana, Elizabeth C. Martinb, C. Neil Hunterb, Tomáš Polívkaa,c
a
Faculty of Science, University of South Bohemia, České Budějovice, Czech Republic; bDepartment of Molecular Biology and Biotechnology, University of Sheffield, United Kingdom; cInstitute of Plant Molecular Biology, Czech Academy of Sciences, České Budějovice, Czech Republic

Spectroscopic properties of the carotenoid spheroidenone in LH1 and LH2 complexes from Rhodobacter (Rba.) sphaeroides substantially differ. While the transient absorption spectrum of spheroidenone bound to LH2 exhibits behaviour similar to that in solution, an extra far-red band is observed in LH1. This band is associated with the intramolecular charge-transfer (ICT) state of spheroidenone [1].

Here we present a femtosecond time-resolved study of LH1 from Rba. sphaeroides, which was genetically modified to produce carotenoids from the spirilloxanthin pathway. The dominant carotenoid in this LH1 is 2,2’-diketospirilloxanthin (dspx). The main aim is to study the carotenoid ICT activity in this complex.

Spectroscopic data confirm the fast energy transfer from the S2 state and weak/no energy transfer from the S1 state of the carotenoid to bacteriochlorophyll. More importantly, the ICT-related signal is recorded. On the contrary, no such signal was observed in LH2 containing dspx as follows from an earlier published study [2]. The LH1-LH2 pair with dspx thus exhibits similar spectroscopic behaviour as that with spheroidenone. Moreover, spectroscopy helps estimate the details about carotenoid configuration in LH1. One of the carbonyls must be close to an s-trans configuration, while the other’s configuration should be different to fulfill the condition of asymmetry leading to the ICT state activity.

Because dspx is not a naturally occurring carotenoid in LH1 of Rba. sphaeroides, the question of the significance of the ICT activity for the organism is inadequate. However, this study brings evidence that not only spheroidenone, but also a very distinct carotenoid dspx are capable of exhibiting an ICT state activity in LH1.

References

[1] V. Šlouf et al., Proc Natl Acad Sci 2012, 109, 8570–8575.
[2] D. M. Niedzwiedzki et al., BBA - Bioenergetics 2015, 1847, 640–655.