1836 



SPECTROSCOPY AND FLUORESCENCE OF PIGMENTS 



CHAP. 37c 



re-emission by each pigment (by 10-30%, dependent on concentration, 

 which was varied from 0.35 to 19.4 X 10~^ mole/1.). The interpretation of 

 the results is complicated by the fact that sensitization, self-quenching 

 and a '——>■ b "cross-quenching" all grow with increasing concentration. 

 An approximate kinetic analysis led to the following tentative conclusions. 

 (1) Energy transfer from Chlb to Chla becomes significant above 2 X 10^"* 

 mole/1. — an order of magnitude earlier than self-quenching of either a or b 



z 

 < 



o 



z 

 < 



o 



z 

 o 

 o 



UJ 



o 

 </) 



LlI 



a: 

 o 



=5 



429 mi 



', o,^ Excited by 453 m^ 



Chio '^•>!^i^ct"°"''^o 





:.> 



Chl * 



_L 



u. 650 700 750 



WAVE LENGTH, m/i 



Fig. 37C.23. Fluorescence spectra of chlorophyll a + b in ether (1.2 X 10 ~' M) 

 excited by X = 429 (70% absorbed by chlorophyll a) and by X = 453 ni/i (5% absorbed 

 by chlorophyll a). Corrected for self-absorption. Fluorescence in (quanta emitted)/- 

 (quanta absorbed), multiplied by arbitrary factor. The peak at 670 ni/i is due to chloro- 

 phyll a, the hump at 650 m/x to chlorophyll b. Vertical arrow shows large contribution 

 of chlorophyll a to fluorescence at 670 n\n excited by X 453 m^, where the absorption of 

 chlorophyll a is small; it indicates energy transfer from chlorophyll b to chlorophyll a 

 (after Duysens 1952). 



(this may be attributed to stronger overlapping of the absorption band of a 

 with the fluorescence band of 6, as compared to the overlapping of the ab- 

 sorption band of each pigment and its own fluorescence band). (2) The 

 quenching of chlorophyll b fluorescence by chlorophyll a is highly efficient; 

 that of a fluorescence by b is negligible. (3) The quenching of 6 by a 

 increases with concentration faster than the sensitization of a. (It appears 

 as if the first effect may be proportional to the square, the second one to the 

 first power, of concentration.) Above 10~^ mole/1., quenching begins to 

 reduce markedly the yield of sensitization, and the latter drops to zero 

 above 10 "2 mole/1. 



Duysens (1952) determined the ratio of the intensities of fluorescence 

 excited in an ethereal solution, 1.2 X 10^^ il/ in both chlorophyll a and 



