PHYCOBILIN BANDS IN ALGAE 



707 



strong red shift of the fiicoxanthol band as a whole, or to a broadening of this 

 band toward the longer waves. 



These observations give some information about the strength with which 

 the carotenoids are bound to the protein-pigment-lipide complex of the 

 chloroplasts. According to equation (21.4) and figure 21.23, the "red 

 shift" can be caused by association of the light-absorbing molecule with 

 other molecules (by adsorption, solution or complexing). The shift is ap- 

 proximately equal (on the energy scale) to the difference between the bind- 

 ing energies of the normal and the excited pigment molecule. In the case 

 of green leaves and algae, we found the chlorophyll bands to be shifted 

 in vivo by about 370 cm.-\ while the luteol bands were shifted by as much 

 as 1250 cm.-i (according to Meyer, and Emerson and Lewis), perhaps even 

 by 2220 and in some cases 2530 cm.-^ (according to Menke). The fuco- 

 xanthol bands in brown Laminaria are, according to Menke, shifted still 

 more widely — by 2585 and 2815 cm.-^ 



All these shifts are relative to the band position in ether solution. A better idea of 

 the binding energies could be obtained by comparison with extrapolated positions of 

 the bands of isolated pigment molecules. Such an extrapolation was made for chloro- 

 phyll and bacteriochlorophyll on page 642, but it is not yet possible for the carotenoids. 



The strong red shifts of the carotenoid bands— particularly those of 

 fucoxanthol— indicate clearly that these pigments form part, in chloro- 

 plasts, of some complex, and that the binding becomes particularly strong 

 when the carotenoid molecules are electronically excited. This fact may 

 be relevant for the transfer of electronic excitation energy from the caro- 

 tenoids (particularly fucoxanthol) to chlorophyll, a phenomenon that is 

 revealed by the occurrence of fucoxanthol — sensitized fluorescence of 

 chlorophyll in vivo (cf. chapter 24, page 814)— and that probably explains 

 also the participation of carotenoids in the sensitization of photosynthesis 

 (cf. chapter 30). 



The absorption peaks of the carotenoids appear especially clear on 

 French's (1937) spectral transmission curves of purple bacteria {cf. fig. 

 22.27), because in this case the carotenoid bands fall between the two ab- 

 sorption bands of bacteriochlorophyll, at about 600 and 400 m^- 



Differences in the carotenoid bands of brown and red varieties of Streptococcus 

 arians were described by French in a later paper (1941). 



The absorption bands of the phycobilins are clearly discernible in the 

 spectra of blue and red algae, e. g., in figure 22.20 at about 550 m/x (phyco- 

 erythrin) and in figure 22.23 in the neighborhood of 625 m/x (phycocyanin) . 

 According to Emerson and Lewis (1942), the phycocyanin maximum is 

 shifted by about 6 m/x toward shorter waves in the aqueous cell extract 

 (figs. 22.23 and 22.48), while the absorption peaks of other pigments retain 

 the positions they had in living cells. This indicates that in the extract, 



