032 



ABSORPTION SPECTRA OF PIGMENTS IN VITRO 



CHAP. 21 



21.6) to a new electronic transition, X — > Z, added (in consequence of the 

 hydrogenation of a second pyrrole nucleus) to the three transitions X ^ Y, 

 X —^ A and X -^ B present in the spectrum of chlorophyll. Simultane- 

 ously with the creation of a new excited electronic level Z, the "old" levels 

 Y, A and B are shifted upward, thus accounting for the "violet shift" 

 of all bacteriochlorophyll bands "inherited" from chlorophyll. The in- 

 frared band X — > Z dominates the spectrum of bacteriochlorophyll to an 

 even greater degree than the red band, X —> Y, dominates the visible spec- 

 trum of ordinary chlorophyll. 



500 600 700 800 



WAVE LENGTH, m/i 



900 



Fig. 21.21. Absorption spectrum of bacteriopheophytin from 

 Spirillum rubrum (after French 1940). Specific absorption coef- 

 ficients, c, in mg./i., d in cm. 



Since the absolute extinction coefficients of bacteriochlorophyll are as yet unknown 

 (fig. 21.6 gives only the optical densities), it is not certain whether the predominance 

 of the 770 niM band is eau.sed by its great absohite lieight, or tjy a relatively low intensity 

 of tlie otlier bands. A specific extinction curvt; of bacteriopheophyiin was given by 

 PVench in a later paper (1940), and is reproduced in figure 21.21. It shows that the 

 molar absorption coefficient of bacteriopheophytin (in methanol) reaches 2.7 X 10^ 



