ENERGY TRANSFER BETWEEN DIFFERENT PIGMENT MOLECULES 1307 



In purple bacteria the overlap between the fluorescence spectrum of 

 Imcteriochlorophyll "800" and the absorption spectrum of bacteriochloro- 

 phyll "850," as well as that between the fluorescence spectrum of bacterio- 

 chlorophyll "850" and the absorption spectrum of bacteriochlorophyll 

 "890," is considerable, so that, with <p = 0.01, a [A-]o value of about 0.025 

 mole/1, can be estimated for both transfers. This is presumably smaller 

 than the actual concentration of the two bacteriochlorophylls serving as 

 energy acceptors, and this makes a high efficiency of the transfers BChl 

 "800" -^ BChl "850" -»► BChl "890" plausible; the direct transfer from 

 BChl "800" to BChl "890" should play only a subordinate role. 



To estimate theoretically the probability of resonance transfer from 

 carotenoids to chlorophyll, Duysens assumed <p = 10""* (0.01% fluores- 

 cence yield of carotenoids in the absence of transfer). Assuming, for 

 purple bacteria, a carotenoid fluorescence band at 590 m^t, a value of [k]o = 

 0.13 mole/1, can be calculated for the energy transfer to the three bacterio- 

 chlorophylls (which all have an absorption band near 590 m^t). This 

 makes the observed high efficiency of this type of transfer in purple bacteria 

 plausible. In green algae, on the other hand, the observed, relatively low 

 efficiency of the carotenoids (<50%) can be plausibly explained by a smal- 

 ler band overlap. Whatever transfer does occur in this case, can be due 

 largely to a primary energy transfer from the carotenoids to chlorophyll b, 

 whose "blue" absorption band must be located, in vivo at about 465 m^ — 

 close to the probable position of the fluorescence bands of the carotenoids. 

 In red algae, a 50% eflficiency of transfer from carotenoids to chlorophyll 

 can be estimated theoretically to require 0.05 mole/1, phycoerythrin. The 

 actual efficiency is lower (about 20% in Porphyridinm cruentum), confirm- 

 ing, according to Duysens, the spatial separation of the chromoproteids 

 from the lipochromes. 



The exceptionally high efficiency of energy transfer from fucoxanthol 

 to chlorophyll, indicated by Montfort's, Manning's and Tanada's experi- 

 ments (chapter 30), cannot be explained entirely by a larger overlap in- 

 tegral; it may indicate either a relatively high intrinsic fluorescence yield 

 of fucoxanthol or a close association of this pigment with chlorophyll. 



The excitation quantum of chlorophyll, acquired in the blue-violet absorption band, 

 is "acceptable" to carotenoids; but the high yield of red chlorophyll fluorescence in vivo, 

 excited at 420 m/x, indicates that this transfer is improbable compared to internal con- 

 version of the "blue" excited state into the "red" one, within the chlorophyll molecule. 



Sensitized fluorescence of dyestuffs in solutions, indicating resonance 

 transfer of energy from one dyestuff to another, was first observed by Per- 

 rin and Choucroun (1927, 1929), (phenosafranin -^ tetrabromoresorufin) , 

 then by Forster (1947, 1948) (fluorescein -* erythrosin, in water) and 

 (1949^) (trypaflavin -^ rhodamine B, in methanol). (The transfer chloro- 



