PHOTOSYNTHETIC PIGMENTS 33 



to separate them. Once separated they may be esterified and 

 purified by extraction with chloroform. The prosthetic 

 groups were first isolated by Lemberg, who was able to 

 prove their relationship to the bile pigments. With phy- 

 coer^thrin he identified the bile pigment meso-bilierythrin 

 and with phycocyanin meso-biliviolin. Both pigments are 

 tetrapyrrolic derivatives with the pyrrol rings in open 

 chain. The erythrins and the violins appear to be isomeric 

 structures, but there is still some doubt concerning the 

 structure of the bilierythrin. The two compounds are read- 

 ily interconvertible and both give on oxidation meso-bili- 

 verdin, the structure of which is well established. The prefix 

 meso-indicates that the two side chains corresponding to 

 the vinyl group in positions 2 and 4 in haem are converted 

 into ethyl groups. 



CH3 C,H5 CH3 CaH.COOH HOOCC2H4 CH3 CH3 C2H5 



N fi ^ 



meso-biliviolin (Lemberg) 



The pigments are presumed to be attached through the 

 carboxyl group to a peptide link of the protein, probably by 

 a twofold attachment. Each molecule of chromoprotein con- 

 tains at least 8 molecules of prosthetic group (the value 

 obtained by chemical analysis of the yield of isolated pros- 

 thetic groups) or perhaps as many as i6 (from comparison 

 of spectral absorption coefficients). If the latter estimate is 

 correct the ratio of prosthetic groups to protein weight is 

 similar to that in haemoglobin (4 haems per 68,000) or in 

 cytochrome c (i haem per 14,000). 



Optical properties 



Phycoerythrin absorbs in the visible mainly in the yellow 

 region of the spectrum and shows three maxima. Phyco- 

 cyanin absorbs somewhat further into the red (Fig. 3.5). In 

 dilute salt solutions phycoerythrin shows an orange fluores- 

 cence whilst phycocyanin shows a red fluorescence. In both 



