Effects of denaturants on the absorption spectrum of the bacteriochlorophyll-protein from the photosynthetic bacterium Chloropseudomonas ethylicum

Abstract

1. The absorption spectrum of the water-soluble bacteriochlorophyll-protein complex in the 230- to 900-nm range is chiefly due to 20 bacteriochlorophylls per macromolecule and remains essentially unchanged in 5% Triton X-100, 6 M urea, 0.1 M AlCl 3, 1 M NaCl or 1 M MgCl 2. However, the bacteriochlorophyll is rapidly bleached by 0.1 M FeCl 3. 2. The spectrum of the complex is stable in the pH range 3–12. Below pH 3, at room temperature, the bacteriochlorophyll is converted into bacteriopheophytin at a rate proportional to the fifth power of [H +]. The rate increases with increasing ionic strength. Below pH 1.5 a blue intermediate accumulates prior to the formation of pheophytin. Similar blue intermediates are formed above pH 12 and upon treatment of the complex with sodium lauryl sulfate at pH 8. This suggests a conformational change in the native complex as a common initial step in these transformations. 3. The absorption spectrum of the complex changes relatively little upon addition of methanol, until in 90% methanol bacteriochlorophyll is extracted from the complex. 4. Spectrophotometric study of heat denaturation of the complex is complicated by thermal degradation of the chlorophyll. Thirty sec at 100° causes a solution of the complex to coagulate. Concomitantly, the bacteriochlorophyll absorption band at 809 nm is split into 2 bands at 785 nm and 830 nm.