TRIPLET EXCITATION OF PRECURSORS OF SPIRILLOXANTHIN BOUND TO THE CHROMATOPHORES OF RHODOSPIRILLUM-RUBRUM AS DETECTED BY TRANSIENT RAMAN-SPECTROSCOPY

Analysis by HPLC, and by subsequent mass spectrometry, of the carotenoid extract of the chromatophores prepared from the cells of a 3-day culture of Rhodospirillum rubrum S1 shows the presence of precursors of spirilloxanthin (rhodovibrin, anhydrorhodovibrin, and 3,4-dihydrospirilloxanthin) as minor components in addition to the major component, spirilloxanthin. Only spirilloxanthin is found in the extract of the chromatophores from the cells of a 6-day culture. Transient Raman spectra of the carotenoids bound to the above two different kinds of chromatophore have been recorded by using 532 nm, approximately 100 ps mode-locked (76 MHz) and Q-switched (800 Hz) pulses; a T1 Raman spectrum has been obtained by a one-color, pump-and-probe technique. The T1 Raman spectra of rhodovibrin and spirilloxanthin free in tetrahydrofuran solution have also been recorded by using 337 nm pump and 532 nm probe approximately 10 ns pulses (10 Hz); a T1 Raman spectrum has been obtained by a two-color, pump and probe technique (delay 1.8-mu-s) using anthracene as a sensitizer. Spectral comparison indicates that the precursors as well as spirilloxanthin bound to the chromatophores can be excited to the T1 state. The result strongly suggests that the precursors are tightly bound to the light-harvesting complex and are involved in nergy transfer.