1878 



SPECTROSCOPY AND FLUORESCENCE OF PIGMENTS CHAP. 37C 



the low fluorescence yield at 430 mju as compared to that at 550 Tan.) 

 The strength of sensitized fluorescence indicates effective resonance transfer 

 of excitation energy from the phycobilins to chlorophyll a; the transfer 

 efficiency is about the same for phycocyanin (630 m/x) and phycoerythrin 

 (560 m^). 



A similarly constructed action spectrum for the excitation of phycocy- 

 anin fluorescence (based on combined data of French and Young, and 



750 



Xj = 578 mjx 



V /'- i sum of ph. 



--^■^■.M 



pnycocyanm if 



.chlorophyll-.. / 



100 



650 



750 



700 



€50 

 wave length in rrifx 



Fig. 37C.52. Fluorescence spectra of Oscillatoria excited by equal quantum fluxes 

 at 420 and 578 m/^, in a layer with negligible absorption (after Duysens 1952); ordi- 

 nates in (quanta)/( quantum X cm.'' X sec), multiplied by an arbitrary factor. Spectra 

 analyzed in terms of fluorescence spectra of chlorophyll and phycocyanin. The differ- 

 ence found at 730 ran (for Xexc = 420 ran) between the sum of the two fluorescence 

 spectra and the fluorescence spectrum of the algae indicates the presence of an unidenti- 

 fied fluorescent pigment. Excitation by 570 ran, absorbed mainly by phycocyanin, 

 produces stronger chlorophyll fluorescence than excitation by 420 m^u, strongly absorbed 

 by chlorophyll. 



Duysens) shows the yield of fluorescence of phycocyanin to be proportional 

 to the total absorption by the two phycobilins; this indicates effective 

 energy transfer also from phycoerythrin to phycocyanin. No transfer 

 appears to occur from chlorophyll (in the blue-violet band) to either of the 

 two phycobilins. 



An explanation of the paradoxical behavior of chlorophyll a fluores- 

 cence was mentioned in chapter 24, and also discussed in chapter 32. It 

 postulated two types of pigment complexes, one "active," containing phyco- 

 bilin together with (fluorescent) chlorophyll a (about 40% of total), and 

 the other "inactive," containing the greater part (about 60%) of chloro- 



