ANALYSIS OF CHLOROPHYLL SPECTRUM 033 



in the maximum of the orange band, while the infrared peak is almost exactly twice as 

 high. According to figure 21. Hi, tiic maxinmni absorption coefficient of ordinary pheo- 

 phytin a in the red is about 4.2 X IQ' (in dioxane, wliere the peaks are usually sharper 

 than in methanol). It thus seems that the dominant position of the infrared band is 

 due both to its own outstanding intensity and to the comparative weakness of all other 

 bands. 



The addition of a new low olectronic level in consefiiienee of each hydro- 

 jienatioii step of Ihe poriihin system offer.s an iutei-estiMg inobleni for theo- 

 retical (liscussit)n. OlThand, one would cxpecl increased satui'ation lo de- 

 crease ratlier than to increase the niiniher of excited electronic states. 



If the red bands of dihydroporphin (and the infrared bands of tetra- 

 hydropoiphin) are brought about (or enhanced) by the presence of electi'ons 

 associated with the ad(Htional carbon-hychogen bonds, it seems pkiusible 

 that hght could specifically activate the "extra" hydrogen atoms. This 

 would make excited chlorophyll (or bacterioehlorophyll) an effective 

 hydrogen donor — a property which may be of decisive importance for the 

 photochemical function of this pigment. In Volume I (chapter 19, pages 

 552-554) the primary photochemical oxidation of chlorophyll was dis- 

 cussed as a possible mechanism of sensitization in photosynthesis. This 

 hypothesis would gain consideral)ly in plausi]:)ilit3^ if it could be j^roved that 

 absorption of light actual)}' activates chlorophyll as a hydrogen donor. 

 The effect of light on the chlorophyll-ferric iron equilibrium (c/. Vol. I, 

 page 488) is the only observation at present that lends experimental sup- 

 port to the concept of chlorophyll as a light-activated reductant. 



Stoll (1936) thought that the excitation of chlorophyll by hght activates especially 

 its "odd" hydrogen atom in position 10. 



Studies by Krasnovsky {c.j. chapter 35) indicate the capacity of chloro- 

 phyll to act also as a light-activated oxidant. 



(b) Life-Time of the Excited States of ChloropMjll 



The natural life-time of the state Y can be calculated from the integral 

 area of the red absorption band Xo -^ Fo. 



Strictly speaking, one should take into account also the probabilities of transitions 

 from Fo to the vibrating states Xi,2. . . , which could be derived from the relative intensi- 

 ties of the successive bands in the fluorescence spectrum (c/. fig. 23.2); but we are con- 

 cerned here with orders of magnitude only. 



Prins (1934), who made this integration, obtained for the number of 

 "absorption electrons" (i. e., the number of harmonic oscillators with the 

 charge e that could account for the observed intensity of absorption ac- 

 cording to classical electromagnetic theory) : 



