Giant circular dichroism of chlorosomes fromChloroflexus aurantiacus treated with 1-hexanol and proteolytic enzymes

Abstract

The circular dichroism (CD) spectrum of isolated chlorosomes fromChloroflexus aurantiacus showed a conservative, S-shaped signal with a negative maximum at 723 nm, a positive maximum at 750 nm and a zero-crossing at 740 nm. Proteolytic treatment of chlorosomes with trypsin at 37°C did not change the CD signal or the absorption spectrum in contrast to treatment with proteinase K, where a twofold increase in rotational strength and a slight decrease of the absorption band at 740 nm were observed. Treatment with saturating 1-hexanol concentrations resulted in a blue shift of the absorption band at 740 nm as well as in changes of the CD spectrum. These changes reversed when the sample was diluted to half the saturating 1-hexanol concentration. In contrast to that, we observed an irreversible formation of a giant CD signal using the combination of 1-hexanol and proteinase K treatment. Electron micrographs of chlorosomes treated with both 1-hexanol and proteinase K showed large aggregates of multiple chlorosome size. By comparison of proteinase K induced effects with trypsin effects it appeared that the 5.7 kDa polypeptide has a structural role in the organisation of BChlc in the chlorosome.

Structural differences in chlorosomes from Chloroflexus aurantiacus grown under different conditions support the BChl c‐binding function of the 5.7 kDa polypeptide

“…The relative CD intensity (~~/~~~ of the low-light chlorosomes doubled upon digestion with proteinase K but it did not change after treatment with trypsin [18]. In the case of the 48 h high light chlorosomes the relative intensity increased fivefold after the degradation of the 5.7 kDa polypeptide with proteinase K and remained constant with trypsin (Fig.…”

“…There is some evidence that BChl c in the chlorosome is organized as oligomers [8,9], which may be bound to protein in order to maintain parallel transition moments between clusters of BChl c [2.5,26]. There are several reports that BChl c aggregates in hexane [27,28], in aqueous suspension, or after LDS treatment [4], and after proteolytic treatment [4,18] exhibited higher rotational strength than BChl c in chlorosomes of ChlorojZexus auruntiucus. Similarly, GEF-chlorosomes [29] showed stronger CD bands than chlorosomes isolated with detergents and density gradient centrifugation, according to a comparative study [28].…”

“…We conclude that, if the intensity of the CD signal around 740 nm in the chlorosomes of ChIoroJEexus aurantiacus is depending on the aggregate size of BChl c Table 2 Remaining absorption intensity and blue shift of the 740 nm absorption band after proteolytic treatment with trypsin or proteinase K at 30°C for 2 h [ 4,18], the BChl c oligomers in high-light chlorosomes are smaller than in low-light chlorosomes. In that sense, the degradation of the 5.7 kDa polypeptide in high-light chlorosomes would produce a stronger increase in rotational strength, because only then the liberated pigments form larger oligomers.…”

Excitons in tubular molecular aggregates

“…These results lead us to conjecture that the differences in the reported experimental results originate from the fact that different sample preparation and handling routines lead to chlorosomes of different length [6,14]. Electromicrographes lend further support to this conclusion [20]. We now turn to the tubular C8S3 aggregates.…”

“…The strong variation of the CD with the cylinder length offers an explanation to a well-known problem in the literature on chlorosomes, namely the strong variation of this spectrum as observed in different experiments [15][16][17][18][19][20]. Two types of spectra are typically reported in the experimental literature.…”