FLUORESCENCE OF CHLOROPHYLL in vilro 



1831 



in the — highly viscous — castor oil. The excitation dipole appears to be 

 parallel to the emission dipole in the orange and red (580 him), and per- 

 pendicular to it at 540 mju ; below 520 niM the two types of excitation seem 

 to be combined, with the "parallel type" becoming more prevalent below 

 430 m/i. Calculation shows that a polarization degree of +0.2 results 

 if the contribution of the "parallel type" excitation is 1.8 times that of the 

 "perpendicular type." 



400 



500 



600 



Xm m^ 



Fig. .37C.21. Polarization (P) of chlorophyll a fluores- 

 cence as function of wavelength of (polarized) exciting 

 light (after Kuhn and Stupp 1952). Lower curve : absorp- 

 tion spectrum (A) of chlorophyll a in castor oil. 



The significance of these findings for the theoretical interpretation of 

 the chlorophyll bands, and the estimation of the life time of the excited 

 state, was mentioned in section 1 above. It seems, however, that the 

 matter is in need of further experimental study. Jacobs and Coleman 

 (unpublished) could find no reversal of polarization in the 540 m/x band 

 of either chlorophyll or chlorophyllide, but did note such an effect in 

 pheophorbide. Since the pheophytin absorption in this region is much 

 stronger than that of chlorophyll, even a small content of Mg-free product 

 in the chlorophyll preparation used could perhaps account for Kuhn and 

 Stupp's results. 



(c) Quenching 



Watson and Livingston (1950) continued the study of self-quenching 

 ("concentration quenching") of chlorophyll fluorescence, described in 

 chapter 23 (c/. fig. 23.7). Measurements with chlorophyll a in ether, 

 methanol and acetone confirmed the absence of concentration quenching 

 below 2 X 10-3 M, and supported the interpretation (offered on p. 774) of 



