1858 



SPECTROSCOPY AND FLUORESCENCE OF PIGMENTS 



CHAP. 37C 



(which contains "B 890" only) ilhimination causes a decrease in absorp- 

 tion above 805 m/x, and an increase below 805 mju. In the first region, the 

 changes are strongest at 872 and 812 uifx, in the second one, at 790 m^u. 

 A similar picture was obtained with the colloidal extract from Chromatium 

 (which has three absorption peaks). The maximum changes in fig. 37C.35 

 are of the order of 3 to 5%. 



These effects were strongest in the absence of hydrogen donors, i. e., 

 when photosynthesis was prevented; as suggested by Bell (cf. above), 

 spectral changes may be more pronounced under these conditions, because 



f 0.001 



001 _ 



002 - 



CYTOCHROME C 

 OXIDIZED- REDUCED. 



i-Plt-i 



O 

 Q 



^^^65^ 



RHODOSPIRILLUM RUBRUM 

 IRRADIATED- DARK 



WAVE LENGTH (m^) 



600 



550 



500 



4 50 



•400 



Fig. 37C.37. Changes in absorption spectrum in the green-blue-violet region 

 in illuminated bacteria (after Duysens 1954^. 



the usual path by which the photochemically changed pigment returns 

 into its normal state, is closed. 



It will be noted that the first maximum of the bleaching effect coincides 

 with the absorption peak of "B 890." If the whole change is attributed to 

 conversion of "B 890" into a different pigment, at least 8% of "B 890" 

 must be changed. (Fig. 37C.36a and h show the spectrum of the changed 

 pigment, calculated with two arbitrarily chosen percentage changes — 

 8 and 20% ; one sees that, if one would assume < 8% conversion, the cal- 

 culated absorption coefficient at 820 mju would become negative.) Prob- 

 ably, much more than 8% of "B 890" is changed, in light, into a pigment 

 (or pigment complex) with a spectrum only shghtly different from that of 

 "B 890" itself (as assumed in fig. 37C.246). 



Later, Duysens (1952, 1954^) noted that important reversible changes 

 occur, in irradiated purple bacteria, also in the region 400-570 mju. These 

 are represented in fig. 37C.37, for Rhodo spirillum ruhrum suspended in 

 peptone solution (or in 0.03 M sodium acetate + phosphate buffer, pH 

 6.8), under anaerobic conditions and in relatively low light. The change 

 is complete within a time of the order of one second. 



