812 



FLUORESCENCE OF PIGMENTS I IV VIVO 



CHAP. 24 



The chlorophyll a band in fig. 24.4 is in the usual position — at 685 mix. 

 However, an additional band is indicated at 730 m/u; it can be attributed 

 to chlorophyll d. The weakness of chlorophyll d absorption (mere ripple 

 in fig. 22.20!) makes one suspect that chlorophyll d fluorescence is excited 

 mostly by energy transfer from other pigments. This is strikingly con- 

 firmed by the observation, reported by Duysens (1951), that an "uniden- 

 tified pigment" — presumably chlorophyll d — whose absorption, in Por- 



-o— 



2 650 700 750 



wave length in m>i 



algae —° ° ° ^^~ 



chlorophyll —^ — 

 phycocyanin -■ — 

 sum of chlor. a 

 and phycocyanin - 

 unknown pigment - 



9> 



650 700 750 



wave length in m>i 



algae — o o o o o 



chlorophyll — ^> •<> ° ° 



phycocyanin — ■ • ■ ■• 



sum of chlor. a. 



and phycocyanin ■*■ -^ ■* 



unknown pigment — • • • 



Figure 24.5 



yhyra lacineata, is < 0.1% of that of chlorophyll a, emits, when 420 m/x 

 is used for excitation, ten times more fluorescence than chlorophyll a 

 (fig. 24.5). With excitation by 546 mfx, the chlorophyll a band at 685 m/i 

 is slightly higher than the "chlorophyll d" band at 730 m/jL. In agreement 

 with French (fig. 24.4) the phycocyanin fluorescence band at 665 m^u is 

 very weak in violet exciting light but becomes strong in green light. 



2. Fluorescence Yield and Sensitized Fluorescence in Vivo 



All observers concur that the fluorescence of chlorophyll in the living 

 cells is "weak," but absolute measurements of its yield are very few. Ver- 

 meulen, Wassink and Reman (1937) determined the quantum yields of 

 fluorescence in four Chlorella suspensions, and found values between 0.15 

 and 0.30%; Wassink and Kersten (1944) found a yield of about 0.15% 



