Picosecond Raman spectroscopy of the B830 LH2 complex ofChromatium purpuratum BN 5500

Abstract

A setup for generating the Stokes Raman lines of benzene (556, 588 and 624 nm, ∼50 ps) by the use of the second harmonic of a Nd: YLF regenerative amplifier system (527 nm, 70 ps, 1 kHz) has been built. This was then used to detect, for the first time, the picosecond Raman spectrum of a carotenoid bound to an isolated light-harvesting complex of a photosynthetic bacterium. The 527 and 588 nm pulses have been used, respectively, for pumping and probing (delay ∼0 ps) the S₁ and T₁ states of okenone which is bound to both the isolated B830 LH2 complex and the chromatophores fromChromatium purpuratum BN 5500. Comparison of the above spectra with the S₁ and T₁ Raman spectra of all-trans-okenone, free inn-hexane solution, shows that only the T₁ state is detected with the LH2 complex, and that both the S₁ and T₁ states are detected with the chromatophores. The results indicate that in the chromatophores there are at least two types of S₁ carotenoids with different lifetimes, i.e., one in the LH2 complex which is too short-lived to be detected, most probably due to efficient energy transfer to bacteriochlorophyll, and the other in either the reaction center or the LH1 complex which is long-lived enough to be pumped and probed by 50 ∼ 70 ps pulses. The results also indicate that at least two of the actively light-harvesting carotenoid molecules are in close connection in the isolated LH2 complex since the T₁ state is generated through singlet homofission within the short S₁ lifetime.