THE PHYC0BILIN3 477 



The protein-pigment bond is particularly strong in the phycobilins. 

 It is not disrupted by organic solvents, which cause the denaturation of 

 the proteins, quite unlike the chlorophyll-protein bond. Even after 

 digestion with pepsin, parts of the broken-down protein molecules still 

 cling to the pigment. Lemberg suggested therefore that the pigment 

 is bound to the protein by a true chemical bond, for example, a peptide 

 link, R'CO— NHR", where R'COOH is the pigment and R"NH2 is the 

 protein. 



Lemberg (1930) split this link by hot hydrochloric acid. The 

 absorption bands were shifted by this treatment towards the violet end 

 of the spectrum, by as much as 60 m/x; but the chromophore seemed 

 otherwise intact. It can be dissolved in chloroform or alcohol, but is 

 insoluble in water and benzene. The elementary analysis of the protein- 

 free "phycocyanobilin" by Lemberg (1929) gave values in agreement 

 with the formula, C34H44O8N4 (molecular weight, 636). The "phyco- 

 erythrobilin" is probably closely related in its structure to the "phyco- 

 cyanobilin." Kylin (1910) and Kitasato (1925) found that this red 

 pigment turns blue upon digestion of the protein with pepsin; Lemberg 

 (1930) proved that this change occurs only in the presence of oxygen, 

 and that it is accelerated by ferric chloride; it seems likely that the 

 blue oxidation product of erythrobilin is identical with cyanobilin. Thus, 

 once again, as in the case of the two chlorophylls, or of carotene and 

 xanthophyll, we have to deal with a pair of pigments which stand to each 

 other in the relation of an oxidation product to a reduction product; and 

 here, at least, a direct conversion appears possible. 



The phycobilin chromoproteids are amphoteric, although somewhat 

 more acidic; but they are easily esterified. They form complexes with 

 zinc and copper, but do not contain any metal in the natural state. 

 Small amounts (0.25%) of calcium, found by Lemberg (1928) in their 

 ash, were probably due to adsorbed calcium sulfate; magnesium and iron 

 are definitely absent. 



Lemberg's analysis of phycocyanobilins makes it probable that they 

 are tetrapyrrole derivatives, similar to porphyrins, chlorins, phorbins, and 

 bile pigments. The structure of the absorption spectra (e. g., the absence 

 of a strong band in the region of 400-430 mpt), and the chemical prop- 

 erties, speak against a porphin structure, and in favor of a so-called 

 hilaii structure — an open chain of four pyrrole nuclei linked by CH2 — 

 or CH— bridges. Formula 17.III shows the "bilan," the hypothetical 

 mother substance of the bile pigments. Genetic relationships between 



N C N C N C N 



H2 H2 H2 



Formula 17.111. Bilan C19H16N4. 



