270 



ELECTRON TRANSPORT 



300 



400 500 600 700 

 WAVELENGTH , m/i 



800 900 



Fig. 1. Absorption spectrum of Rhodospirillum nibriim chro- 

 matophores. The chromatophores were prepared by treatment 

 for 2 minutes in a lOKC Raytheon sonic oscillator. After re- 

 moval of the cell debris by centrifugation at 20,000 x g, the 

 chromatophores were sedimented by centrifugation for one hour 

 at 100,000 X g and then washed twice by centrifugation in Tris 

 buffer. Complete details concerning the growth of the bacter- 

 ium and chromatophore preparation are given in reference 10. 

 The chromatophores were suspended in 33 mM Tris buffer, 

 pH 8.0 and .1 M sucrose, and contained 10 /igrams of BChl per 

 ml of solution. Anaerobic conditions were obtained by three 

 evacuations interspaced with flushing by argon gas (10). The 

 absorption spectra were obtained with a Gary Model 14 record- 

 ing spectrophotometer. For this experiment the absorption was 

 measured with water in the reference cell. 



that in the present case a slow scan was made under continuous illum- 

 ination. Thus, the spectrum so obtained would include any "slow" and/or 

 irreversible changes that occurred in addition to the rapid changes 

 which usually serve as the basis for other reported difference spectra. 

 The prominent features of the difference spectrum shown in Fig. 2 are 

 the minimum at 600 m//, the minimum around 550 rafi (most likely due 

 to carotenoids), the broad maximum around 430 m^ (due to an unknown 

 component in the chromatophores), a minimum around 387 m/i with a 

 corresponding maximum around 360 m// (due to a blue shift of a BChl 

 absorption band) and a minimum around 260 m/^, which is most likely 

 due to ubiquinone reduction. 



When the absorbancy changes obtained in the presence of DPIPH2 

 are compared with those due to chromatophores alone, two main dif- 

 ferences are apparent. The obvious difference is the absorption band 



