Effect of alkaline treatment on bacteriochlorophyll a, quinones and energytransfer in chlorosomes from Chlorobium tepidum and Chlorobium phaeobacteroides

Chlorosomes isolated from two types of green sulfur bacteria, Chlorobium te pidum which contains bacteriochlorophyll c (BChl c) and the BChl e-containi ng Chlorobium phaeobacteroides, were subjected to alkaline treatment (pH 12 .7 at 40 degrees C for 20 min). This caused selective degradation of BChl a , whereas BChl c or e were not affected. Chlorobiumquinone in the chlorosom es was partially degraded by the alkaline treatment but menaquinone was unc hanged. Fluorescence decay kinetics showed that alkaline treatment disrupte d energy transfer from BChl c or e to BChL a under reducing conditions. How ever, this did not give rise to any substantial increase in the excited sta te lifetime of BChl e in C. phaeobacteroides chlorosomes, while for C. tepi dum a decrease in the BChl c Lifetime was found. The steady-state fluoresce nce of chlorosomes is highly dependent on the redox potential such that emi ssion is quenched in oxidizing environments. Alkaline treatment diminished this quenching effect and caused a doubling in the BChI c or e emission int ensity under aerobic conditions. Single-photon timing experiments conformed that alkaline treatment inhibits the energy trapping process operative und er aerobic conditions. These effects of alkaline treatment on the fluoresce nce intensity and decay kinetics are likely to be related to the depletion in BChl a or in chlorobiumquinone or a combination of these.