MEASUREMENTS OF ABSORPTION SPECTRA OF PIGMENTS 1809 



measured the absorption spectra of miscellaneous porphin derivatives, 

 (Mg-, Zn- and Ca-complexed mesoporphyrin dimethyl ester; Mg-complexed 

 tetraphenylchlorin, and Zn- or Ca-substituted chlorophylls) in pure, 

 nonpolar solvents, and in the presence of alcohols or organic bases. The 

 association constants in table 37C.IA (to be compared with constants 

 calculated from fluorescence and listed on p. 768 in table 23.IIIA!) were 

 calculated from absorption measurements. 



Imino compounds (in which the metal is replaced by two hydrogen 

 atoms), such as pheophytin, showed no tendency at all for complexing with 

 bases. This, and the unchanged complexing tendency of allomerized 

 chlorophyll, supports the hypothesis of Evstigneev et al., attributing com- 

 plexing to the central metal atom, in preference to Livingston's initial hy- 

 pothesis of complexing through enol formation in ring V. 



In section 2a above, we have noted Freed and Sancier's evidence for 

 the occurrence of both types of association with bases, the association in 

 the cyclopentanon ring leading to "discolored" compounds (brown in 

 the case of chlorophyll a). 



(d) Spectra of Accessory Pigments 



A few new measurements of the absorption spectra of plant carotenoids, 

 particularly of those of purple bacteria, have been added to those sum- 

 marized in chapter 21 (part C). Polgar, van Niel and Zechmeister (1944) 

 gave absorption data for spirilloxanthin from Rhodospirillnm ruhrum 

 (peaks at 540.5, 503.5 and 473 m/x in dioxane; 548.5, 510 and 479 m)u in 

 benzene; 571.5, 532 and 495 van in carbon disulfide). Comparison with 

 the absorption spectrum of Karrer and Solmssen's rhodoviolascin from 

 Rhodovibrio (table 21. IX, p. 659, and fig. 37C.13) indicates the probable 

 identity of the two pigments. Two methoxyl groups have been identified 

 in both of them, and their most likely formula is therefore that suggested 

 by Karrer, C4oH5o(OCH3)2. Natural spirilloxanthin is the all-^rans isomer; 

 partial conversion to cis form produces a new peak at 495 myu. 



Some new absorption measurements on phycohilins also must be 

 mentioned. According to Lemberg and Legge (1949), the phycoerythrin 

 from Rhodophyceae ("R-phycoerythrin") has three absorption bands, while 

 that from Cyanophyceae (*'C-phycoerythrin") has only one. However, 

 fig. 21.39 shows considerable variations in the relative intensities of the 

 different bands in phycoerythrin from different red algae, and it is not 

 impossible that these variations may occasionally lead to spectnim with 

 only one prominent peak. From this point of view, the absorption spec- 

 trum of phycoerythrin from Porphyridium cruentum, measured by Koch 

 (1953), is of interest — it shows only one main peak at 535 m^t, but a satelUte 



