SPECTRA OF PHOTOSYNTHETIC PIGMEXTS 3G5 



mercial equipment for precise measurement of the energy distribution of 

 the very weak light produced by fluorescent pigments. In general terms 

 it may be said that fluorescence spectroscopy is just as useful for the qual- 

 itative identification of pigments as absorption spectroscopy is, assuming, 

 of course, that the pigments of interest do fluoresce. It is probably not 

 so useful as absorption spectroscopy for quantitative measurements of 

 pigment concentration. There is, however, an important type of infor- 

 mation that can be obtained only from fluorescence spectra, and that is 

 the distribution of energy between various pigments in a complex mixture 

 of fluorescent substances which is being irradiated. Such applications 

 will be taken up in the following section after the fluorescence spectra 

 of the photosynthetic pigments have been discussed. 



Chlorophyll in solutions of organic solvents gives a brilliant red fluo- 

 rescence whose spectral energy distribution has been measured by Zscheile 

 and Harris (1943). Two of their curves for chlorophyll a fluorescence 

 spectra in ether are given in Fig. 6-9a, which also shows the absorption 

 spectrum of chlorophyll a in the same solvent. The tall fluorescence 

 curve obtained in a dilute solution shows the true spectrum in ether. 

 The great distortion of the fluorescence curve which can be introduced 

 by reabsorption of the fluorescent light by the pigment itself is observed 

 in the small curve obtained through a thick layer of solution. The 

 reabsorption changes the apparent position of the peak that lies close to 

 the absorption band and also greatly reduces its height in comparison 

 with that of the 725-m^i band, which is hardly affected. A similar effect 

 of reabsorption is found in a comparison of the spectra of chlorophyll in a 

 pale and in a dark green leaf in Fig. 6-lOc. Figure 6-9b gives the absorp- 

 tion and fluorescence spectra of chlorophyll b in ether. Watson and 

 Livingston (1948) discuss the self-quenching of chlorophyll fluorescence. 



Although different absorption spectra are found for phycoerythrin from 

 various species, the fluorescence spectra are identical for at least two 

 samples having very different absorption spectra. Phycoerythrin from 

 Porphyridium cruentum has the spectra given in Fig. 6-9c, and that from 

 Porphyra kindly given us by Prof. L. R. Blinks shows a different absorp- 

 tion spectrum but a nearly identical fluorescence spectrum. In Fig. 6-9c 

 the crosses represent the effectiveness of different wave lengths in exciting 

 the fluorescence of this solution of phycoerythrin (V. K. Young, 1950, 

 unpublished data). The absorption and fluorescence spectra of phyco- 

 cyanin from Oscillatoria (Duysens, 1951a) are shown in Fig. 6-9^. The 

 fluorescence spectra of the photosynthetic bacteria will be given in the 

 last section of this chapter. There have been some reports of carote- 

 noid fluorescence, but if it does exist, it is not readily observable in the 

 common representatives of that class of compounds either in live cells 

 or in extracts. 



