FLUORESCENCE OF PHYCOBILINS 799 



2. Fluorescence of Phycobilins 



The phycobilins are usually described as brilliantly fluorescent. How- 

 ever, since no exact determinations of the yield of fluorescence exist, it 

 is impossible to judge whether the fluorescence is so much stronger than 

 that of chlorophyll, or whether its greater brilliancy is due to the fact that 

 the fluorescence bands of the phycobilins lie near the region of the greatest 

 sensitivity of the human eye, whereas those of chlorophyll are in the far red, 

 and partly in the infrared. 



Association with proteins does not impair the fluorescence of phycobil- 

 ins. On the contrary, according to Lemberg (1930), the isolated pigments 

 fluoresce less strongly than the chromoproteids. 



The fluorescence of red algae was first described by Stokes, in the same 

 paper in which he also reported the discovery of the fluorescence of green 

 leaves (c/. page 805). Since then,* the fluorescence of both the living 

 algae, and of their aqueous extracts, has repeatedly been observed, e. g., 

 by Schiitt (1888), Hanson (1909), Turner (1916), Lemberg (1928), Dhere, 

 and Fontaine (1931), Roche (1933), Dhere and Raffy (1935), Van Norman. 

 French and Macdowafl (1948), Arnold and Oppenheimer (1949) and 

 French (1951). 



Van Norman, French and Macdowall (1948) gave a photometric curve 

 for the fluorescence of an extract obtained by grinding a species of the red 

 alga Iridaea under water and centrifuging at high speed. It shows a sharp 

 peak at about 580 m/i, clearly related to the first long-wave absorption 

 band of phycoerythrin at 566 m/x (cf. fig. 23. 9A, from French 1951). 



A shoulder appears on the long-wave side of the 580 m/x fluorescence 

 band, indicating the presence of a second maximum at about 630 m/x. 

 This probably is the second (0-^1) fluorescence band of phycoerythrin 

 (leading to a vibrational ground state). The first fluorescence band of 

 phycocyanin (correlated with the first absorption band of this chromopro- 

 teid, the peak of which appears, in the same extract, at about 615 m^) 

 lies at 660 m^, according to the curve obtained by French (1951) by 

 subtraction of the phycoerythrin fluorescence from the fluorescence 

 spectrum of the crude aqueous extract from a red alga (fig. 23. 9B) . Earlier, 

 Dhere and Fontaine (1931) gave 578 and 648 m/x, respectively, as the axes 

 of the fluorescence bands of the two phycobilins in aqueous extract. Dh^re 

 and Raffy (1935) noted a second phycocyanin band at about 728.5 m/x. 



According to French's figures, the fluorescence bands of the phycobilins 

 are shifted to the red of the absorption bands, by 14 m/x in the case of 

 phycoerythrin, and by 45 m/x in tliat of phycocyanin. 



