Pigment interactions in chlorosomes of various green bacteria

Abstract

Resonance Raman experiments were performed on different green bacteria. With blue excitation, i.e. under Soret resonance or preresonance conditions, resonance Raman contributions were essentially arising from the chlorosome pigments. By comparing these spectra and those of isolated chlorosomes, it is possible to evaluate how the latter retain their native structure during the isolation procedures. The structure of bacteriochlorophyll oligomers in chlorosomes was interspecifically compared, in bacteriochlorophyllc- and bacteriochlorophylle- synthesising bacteria. It appears that interactions assumed by the 9-keto carbonyl group are identical inChlorobium limicola, Chlorobium tepidum, andChlorobium phaeobacteroides. In the latter strain, the 3-formyl carbonyl group of bacteriochlorophylle is kept free from intermolecular interactions. By contrast, resonance Raman spectra unambiguously indicate that the structure of bacteriochlorophyll oligomers is slightly different in chlorosomes fromChloroflexus auranticus, either isolated or in the whole bacteria.

Pigment Binding Site Properties of Two Photosystem II Antenna Proteins

“…This is notably the case for the methine bridge stretching modes; this band, observed at about 1600 cm Ϫ1 when the central magnesium is six-coordinated, is up-shifted to 1610 -1615 cm Ϫ1 for five-coordinated magnesium. In resonance Raman spectra of chl b molecules, this band is extremely weak at best (23,24), and thus, determining the coordination state of the central magnesium proves much more difficult. There is an intense band in resonance Raman spectra of chl b arising from C a -C b stretching modes (23) that is sensitive to the molecular core size.…”

“…In spectra obtained with 457.9 nm excitation, it may be noted that the bands arising from the stretching modes of the keto carbonyl groups of chl b exhibit only low intensity. As discussed above, this excitation line induces a resonance mainly with the B x component of the Soret electronic transition, thus favoring the contribution of stretching modes of the formyl carbonyl of these molecules, and not of the keto carbonyl group (24). The increase in intensity of these bands for 441.6 nm excitation may reflect an increase in chl a resonance and/or the location of this wavelength within the B y absorption band of chl b.…”

Comparison between chlorosomes containing bacteriochlorophyll-c and chlorosomes containing bacteriochlorophyll-d isolated from two substrains of green sulfur photosynthetic bacterium Chlorobium vibrioforme NCIB 8327

“…The shifts of 13-C@O stretching mode in the chlorosomes were almost the same, ca. 60 cm À1 , indicating that the strength of the hydrogen bond between the two groups of BChls was quite similar in C-and Dchlorosomes [27]. RR spectroscopy showed that the basic structural motif of BChl self-aggregates in natural chlorosomes is essentially the same between them.…”

Resonance Raman spectroscopy

“…Around 1,660 cm -1 , the attribution of the contributing bands to acetyl (or formyl) or to keto stretching modes is not straightforward. Formyl group contributions may however be assessed using the fact that they lie on the X axis of the B(Chl) molecules, and thus are preferentially enhanced when excited in the Bx transition (Feiler et al 1994a). Other factors, such as the dielectric properties of the environment may also influence these frequencies, though to a lesser extent ).…”