Chlorosomes spectra

In 1962, Olson and Romano isolated and purified a water-soluble BChl a-protein from green bacteria. Soon afterwards, the crystalline form of the protein was also obtained . It was in fact the first photosynthetic pigment-protein to be crystallized. Chemical studies showed the BChl a-protein to have a molecular weight of 150 kDa and to contain 21 BChl a molecules. It has a major absorption band in the far-red at 809 nm, which is ascribed to the Qy transition of BChl a, and a corresponding fluorescence band at 818 nm. Its circular-dichroism spectrum in the far-red absorption-band region shows multiple components which can be interpreted in terms ofexciton interactions between the BChl a molecules " . Fluorescence studies of the BChl a-protein in situ indicate that it functions as an intermediate in exd-tation-energy transfer from BChl c in chlorosomes to the BChl-dimer reaction center, P840 ... 

Absorption and fluorescence excitation spectra of Cb, vibrioforme cells under anaerobic conditions were nearly superimposable (Fig. 2), indicating that excitation transfer from BChl d in the chlorosomes to BChl a (the chromophore fluorescing at 835 nm) was essentially 100% efficient. Aerobic conditions did not affect the absorption spectrum, but caused a 90% decrease in the relative height of the peak at 730 nm (where BChl d absorbs) in the fluorescence excitation spectrum (Fig. 2). Thus aerobic conditions caused the efficiency of excitation transfer from BChl d to BChl a to decrease to about 10%. Similar effects were observed in isolated chlorosomes, in which 835 nm fluorescence comes from BChl a in the chlorosome baseplate (data not shown). 

FIGURE 1. Absorption spectrum of isolated chlorosomes of P. aestuarii at 4K. Inset the region around 800 nm on a 20-fold expanded scale (9). 

HGURE 2. Absorption spectrum of CfL aurantiacus at 4 K. The band at 873 nm is due to BChl a 866. The chlorosomal BChl a and BChl a 808 contribute about equally to the band at 801 nm. Dotted line excitation spectrum of BChl a 866 fluorescence, normalized at 872 nm (10). 

The differences between the high (5% ) and the low LDS concentrations are rather small, indicating that LDS is a good detergent for the isolation of chlorosomes. The spectra are very similar to the CD spectrum in [8]. Fig. 2 shows some of the other types of CD spectra we obtained with Bchl a containing chlorosomes. Our first idea was that the differences in the CD spectra were the effect of the different isolation procedures. However, it turned out that the CD spectra of different batches of cells/membranes were already different. Apart from the membrane CD spectrum in Fig.2 we also recorded membrane CD spectra that were completely sign-reversed (results not shown). Repeating the same isolation procedures on different batches... 

Fig. 3 shows the LD spectrum we found for the chlorosomes with the CD spectrum shown in Fig.l. It is clear that the short-wavelenght region of the Bchlc band... 

Fig. 1. Decay-associated emission spectra (DAS) of isolated BChl a containing chlorosomes as calculated by the global-analysis procedure. Excitation wavelength was 720 nm. The dashed line is the stationary fluorescence spectrum.

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