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.