AGGREGATION OF 8,12-DIETHYL FARNESYL BACTERIOCHLOROPHYLL-C AT LOW-TEMPERATURE

The effect of temperature on the aggregation of 3(1) R-8,12-diethyl fa rnesyl bacteriochlorophyll c in a mixture of n-pentane and methylcyclo hexane (1/1, v/v) was studied by means of absorption, circular dichroi sm 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 conver ted 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 absorptio n changes were reversible. A-719 Was highly fluorescent (maximum at 19 2 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 abso rption 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 hig her aggregates of BChl c as models for the chlorosome by various metho ds at low temperature, thus avoiding interference by thermal processes .