Aggregation of 8,12-diethyl farnesyl bacteriochlorophyll c at low temperature

Abstract

The effect of temperature on the aggregation of 3(l)R-8,12-diethyl farnesyl bacteriochlorophyll c in a mixture of n-pentane and methylcyclohexane (1/1, v/v) was studied by means of absorption, circular dichroism and fluorescence spectroscopy. At room temperature essentially only two aggregate species, absorbing at 702 nm (A-702) and 719 nm (A-719), were present. Upon cooling to 219 K, A-702 was quantitatively converted to A-719. Further lowering of the temperature led to the stepwise formation of larger aggregates by the conversion of A-719 to aggregate species absorbing at 743 nm (A-743) and 755 nm (A-755). All absorption changes were reversible. A-719 was highly fluorescent (maximum at 192 K: 744 nm), while A-743 and especially A-755 were weakly fluorescent. Below 130 K the mixture solidified, and no major changes in the absorption spectrum were observed upon further cooling. At 45 K, however, a relatively strong emission at 775 nm was observed. Below 200 K, the absorption, fluorescence and circular dichroism spectra resembled that of the chlorosome. These results open up the possibility to study higher aggregates of BChl c as models for the chlorosome by various methods at low temperature, thus avoiding interference by thermal processes.

Exciton Theory for Supramolecular Chlorosomal Aggregates: 1. Aggregate Size Dependence of the Linear Spectra

“…aurantiacus and Cb. tepidum chlorosomes (Olson, 1980;Brune et al, 1990;van Mourik et al, 1990;Otte et al, 1991;Griebenow et al, 1991;Lehmann et al, 1994;Niedermeier et al, 1992;Wang et al, 1995;Ma et al, 1996;Frese et al, 1997) and artificial BChl aggregates (Dudkowiak et al, 1995;Olson et al, 1985;Uehara et al, 1994;Balaban et al, 1995a;Ishii et al, 1998;Zhu et al, 1996). Surprisingly drastically different CD-spectra have been reported often for the same type of chlorosomes, but were obtained in independent preparations or by slightly different preparation procedures (see, for example, Olson, 1980;Griebenow et al, 1991; and publications discussed therein), or for different growth conditions (Ma et al, 1996).…”

Autocatalyzed Self-Aggregation of (3R)-[Et,Et]Bacteriochlorophyll cF Molecules in Nonpolar Solvents. Analysis of the Kinetics

“…Similarly, larger aggregates, the absorption of which again resembles that of the chlorosomes, form on dilution by water of (monomeric) BChl c solutions in lower alcohols; these aggregates have been claimed to form spontaneously . According to Amesz and co-workers, BChl c on direct dissolution in a 1:1 (v/v) mixture of n -pentane and methylcyclohexane at room temperature dissolves to a very low concentration only. These solutions contain two aggregates absorbing at 702 and 719 nm which upon cooling are reversibly converted to (presumably precipitated) larger aggregates with chlorosome-like vis absorption, CD, and fluorescence spectra (Figure ).…”

Dipole Strengths of the Qy(0,0) Bacteriochlorophyll c Transition¶

“…BChl c was extracted from green sulfur bacterium Prostecochloris aestuarii and purified chromatographically as described previously (9). The thin‐layer chromatography of BChl c gave a mixture of several homologs (6,8,10). The proportion of the homologs can vary with the age and growth conditions of the bacterial culture (6).…”

“…It is known that in green sulfur bacteria BChl c occurs in giant antenna complexes, and in vivo it is present in highly aggregated form (3–5). Also, in several solvents this pigment exhibits efficient aggregation (6–8). Knox and Spring (1) suspect that the aggregation properties of BChl c can be responsible for untypical changes of the transition dipole strength versus the refractive index.…”