3-Acetyl-chlorophyll formation in light-harvesting complexes of purple bacteria by chemical oxidation

Abstract

Oxidation of bacteriochlorophyll (BChl) with potassium ferricyanide in membranes and LH2 complexes (carotenoid-less and control samples) from the purple bacteria Allochromatium minutissimum and Rhodobacter sphaeroides as well as BChl photobleaching in a model system have been studied. The oxidation of BChl depended on the type of bacteria. BChl850 was rapidly oxidized in samples from Alc. minutissimum, and BChl800 and BChl850 were slowly oxidized in samples from Rb. sphaeroides. The carotenoids were not involved in protecting BChl from chemical oxidation in the lightharvesting complexes. The appearance of BChl oxidation product was registered in the absorption spectra (absorption maximum about 700 nm) and by HPLC analysis. The oxidized BChl was identified as 3-acetyl-chlorophyll. It differed from BChl by the presence of a double bond in pyrrole ring II at the 7-8 position. The extinction coefficient of 3-acetyl-chlorophyll was about 10 times less than that of BChl850 in the LH2 complex from Alc. minutissimum. In the BChl → 3-acetylchlorophyll transition, the binding constant of the latter with LH2 complex as compared with that of BChl did not change dramatically, as indicated by: (i) preserved electrophoretic mobility of the complex; (ii) the presence of 3-acetyl-chlorophyll in the complex after separation; (iii) the presence of a 3-acetyl-chlorophyll CD signal that was proportional to its absorption spectrum.