ENERGY TRANSFER BETWEEN DIFFERENT PIGMENT MOLECULES 1303 



fluorescence yield at 480 mn (using the assumption that the energy taken 

 up by Chi h at this wave length is transferred to a with 100% efficiency). 



The spectrum of fluorescence was found to be the same with excitation 

 at 480 m/i (where h absorbs much light) and at 420 mn (where a is the main 

 absorber). No fluorescence band was noticeable around 650 m/z, where the 

 peak of the fluorescence of chlorophyll b in vivo should be located. Both 

 observations confirm the quantitative transfer of energy from 6 to a. 

 (We note, however, that table 24.1 lists several determinations of the chloro- 

 phyll b fluorescence peak in Ulva and Elodea, at 655-657 van; but perhaps 

 these photographic data are unreliable.) 



Energy transfer was also observed between the chlorophylls a and b in solution 

 (about 1.2 X 10 ~' M in each pigment), by comparison of the intensities of chlorophyll a 

 fluorescence excited at 429 mn (70% absorption by a, 30% by b) and 453 m^u (5% absorp- 

 tion by a, 95% by b). The ratio of the two intensities was only 1.7 (instead of 14, as ex- 

 pected in the absence of a transfer). This indicates a 40-50% efficiency of transfer from 

 b to a. As shown in fig. 37C.23, a marked fluorescence of b itself is emitted in this 

 case (because the transfer to a is not 100% effective). At 5 X 10~^ moIe/1. the transfer 

 efficiency was down to about 35%, and at 1.2 X 10 ~^ mole/1, to about 5%. According 

 to Forster's calculations (equation 32.6), the probabiUty of the transfer a — *- a is 50% 

 at 7.7 X 10 "^ mole/1., if a life-time of 3 X 10 ~^ sec. is assumed; with a life-time of 1 X 

 10~' sec. (natural life-time 4 X 10~* sec; 25% fluorescence yield), this "critical" con- 

 centration, [k]o, becomes 7.7 X 10 ~* X v^ ^^ 1 X 10 ~' mole/1. As mentioned above 

 (p. 1300), the efficiency oi b ^-a transfer should be about one half of that between two 

 molecules of chlorophyll a, or 0.25 at 1 X 10""^ mole/1. To bring it up to 50% the concen- 

 tration should be increased by \/2 — 1.4, i. e. to about 1.5 X 10~^ mole/1., while Duy- 

 sens' experimental value is about 1.2 X 10 ~* X 11 mole/1. (The probability of transfer 

 increases, according to Forster, with the square of concentration; the above quoted 

 estimates of Duysens— 50% at 12 X 10"* mole/1., 35% at 5 X 10"^ mole/1., and 5% 

 at 1.2 X 10 ~^ mole/1. — do not agree well with this law, but claim no precision.) 



In brown algae, the transfer of excitation energy from fucoxanthal to 

 chlorophyll a, first demonstrated by Button, Manning and Duggar (1943) 

 and confirmed by Wassink and Kersten (1946) (cf. chapter 24, p. 814), was 

 again observed by Duysens (1952). According to the latter, the efficiency 

 of energy transfer (as measured at 500 m/z) is: 



Carotenoids (mainly fucoxanthol) — >■ chlorophyll: '^ 70% 



This value can be compared with the ratio of quantum yields of photo- 

 synthesis at 500 and 660 m/i, which, according to Tanada (cf. fig. 30.9A), 

 is approximately 0.8. 



Measurements of the action spectra of photosynthesis and of chloro- 

 phyll a fluorescence in the blue-green alga Oscillatoria, also made by Duy- 

 sens (1952), showed (similarly to the experiments on red algae, see below) 

 that quanta absorbed by phycocyanin are transferred with considerable 

 (perhaps > 90%) efficiency to fluorescent and photosynthetically active 

 chlorophyll a; but that the greater part (55-60%) of chlorophyll a is pres- 



