COUPLED REDUCTION OF UBIQUINONE 



271 



CHROMATOPHORES + DPIPH2 



CHROMATOPHORES 



300 



400 500 



WAVELENGTH, m/i 



600 



700 



Fig. 2. The effect of DPIPH2 upon the light-induced absorbancy changes in 

 R. riibrum chromatophores. The suspension contained 10 //grams BChl per nil 

 and Tris buffer and sucrose as shown for Fig. 1. Where indicated, 66 /iM DPIPH2 

 (reduced with ascorbic acid) was also present. The to .1 absorbancy scale was 

 used, and illumination was obtained by means of two 6-volt, 18-ampere tungsten 

 lamps situated above the cell compartment. Light was transmitted through a 

 No. 2403 Corning filter to the circular absorption cells by means of a lucite 

 plastic tube which served to guide the light rays to the absorption cell. The 

 intensity of illumination was saturating for the reactions studied. 



of DPIP, which has become oxidized by the photochemical system con- 

 tained in the chromatophores. The changes due to chromatophores 

 themselves are superimposed upon this broad absorption band of 

 DPIP. The second important change is in the ultraviolet region, where 

 a pronounced decrease in absorption around 280 rrxji occurred. The 

 most logical compound to be related to the band at 280 m/i is ubiquinone 

 (9). Clayton has recently shown that illumination of Chromatium 

 chromatophore suspensions results in changes corresponding to a 

 photo reduction of the ubiquinone contained within these chromatophores. 

 The discrepancy between the observed minima in the two cases (270 

 m/i reported for Chromatium and around 280 m/^for R. nihnim in this 

 investigation) is due to the fact that there are some absorption changes 

 in this region accompanying the oxidation of DPIPH2. The conversion 

 of DPIPH2 to DPIP results in slight increases in absorbancy in the 

 range from 260 to 290 m//. The difference spectrum obtained with the 

 dye itself (oxidized minus reduced) shows a minimum at 285 and a 

 maximum at 270 m//. It is the superimposition of this DPIP difference 

 spectrum upon that due to chromatophores that shifts the observed 



