810 FLUORESCENCE OF PIGMENTS IN VIVO CHAP. 24 



Comparison of the figures 24.3 and 23.4 shows that the displacement of 

 the fluorescence band is much stronger in the purple bacteria than in 

 green plants. The fluorescence spectrum of live Chromatium shows only 

 one band, at 926 mju, while that of the extract contains two bands, at 806 

 and 695 mju, respectively. It was mentioned on page 751 that the first 

 and more intense of these two bands can be correlated with the main ab- 

 sorption band of extracted bacteriochlorophyll, at 770 m/i, but that the 

 correlation of the 695 m^l fluorescence band with the 605 m/z absorption 

 band is doubtful. Absorption spectra of live purple bacteria show two 

 (or three) absorption bands, at 800-870 and 800 mtx, respectively (c/. p. 

 702); but here again, only the first one can be identified with the main 

 X-^ Z absorption band of dissolved bacteriochlorophyll (at 770 m^), while 

 the identification of the second one— which is comparatively weak and 

 variable in intensity— with the X -^ Y band at 605 mfx is uncertain (cf. 

 page 702). The relation between the fluorescence and the absorption 

 bands of bacteriochlorophyll is illustrated by Table 24.IIA. The table 

 shows that the fluorescence band I in vivo is shifted toward the infrared by 

 as much as 120 m^, compared to the position of the fluorescence band in 

 vitro, and by about 60 mn compared to the position of the corresponding 

 absorption band in vivo. 



Table 24.IIA 

 Spectra of Bacteriochlorophyll in Vitro and in Vivo 



I, mti II, ni/i III, mil 

 Band {X -» Z) (X -* Y) ? 



Absorption 



Cell 860-870 — 800 



Extract 770 605 — 



Fluorescence 



Cell 926 — — 



Extract 806 695 (?) — 



This tabulation indicates that, in the living cell, bacteriochlorophyll 

 fails to show one absorption band and one fluorescence band that are found 

 in extracts, but shows one (or two) extra absorption bands, without 

 corresponding fluorescence bands, which have no counterpart in the 

 solution spectrum. Duysens (1951) confirmed that Chromatium and 

 Rhodospirillum show only one fluorescence band, correlated with the 

 absorption band at 890 m^. Absorption in the 800 and 850 m^ bands 

 contributes to the excitation of this fluorescence band; so does — with a 

 50-70% lower efficiency— the absorption by cartenoids. Excitation 

 energy is thus transferred from all pigments of the bacteria to the bacterio- 

 chlorophyll form having the lowest excitation energy. 



