32S 



Govind jee 



their effect on the fluorescence yield of "P 700" and Chi a 670, 

 we first measured the action spectra of fluorescence at room, and 

 at liquid nitrogen temperatures. 



I 



i 



FLUORESCENCE EXCITATION SPECTRUM 



LIQUID NITROGEN 

 TEMPERATURS 



ROOM TEMPERATURE 



SPINACH 

 CHLOROPLAST 



550 600 650 '00 750 600 660 700 



WAVELENGTH, m>i 



Figure 11, Fluorescence exciLatlon spectra of isolated sp'nach 

 chloroplasts, measured at 22° C (right) and at -196" C (left); 

 note a twentyfold increase in the fluorescence yield. The appear- 

 ance of a new band at 700 nvJ in Che cooled sample is clearly 

 demonstrated; tentatively, it can be assigned to P 700 — a pig- 

 ment (or pigments) absorbing at 700 n^. Ferricyanide (10~-*,M) 

 quenches fluorescence yield (see open circles) at all the wave- 

 lengths— due to the formation of non-fluorescent oxidation 

 products of the pigments (after Louisa Yang and Covindiee). 



First, we show (see figure 11) the action spectrum of fluores- 

 cence of a chloroplast suspension, measured at 758 m\x and at 

 -196° C. A peak at 705 m\j. appears (which is not present at room 

 temperature). This provides independent confirmation of Butler's-*" 

 prior findings. The peak is believed to be due to P 700. 



In Anacystis , fluorescence spectra (see figure 12) (excited by 

 436 and 605 m|i light, absorbed primarily by chlorophyll a and 

 phycocyanin respectively) show (1) that chlorophyll a fluores- 

 cence, when excited via energy transfer from phycocyanin, is much 

 stronger than when it is excited directly; (2) that excitation in 

 the Soret band of chlorophyll a is not transferred to phycocyanin; 

 (3) that energy transfer yield from phycocyanin to chlorophyll a 

 is much less than 1007o, as some energy is lost as phycocyanin 

 fluorescence (peak at 650 m|j) ; (4) upon cooling to -196° C, new 

 peaks appear at 696 m|j., 718 m\i, and a shoulder at 760 m|j,. 



