622 



ABSORPTION SPECTRA OF PIGMENTS IN VITRO 



CHAP. 21 



upon the transition from the prophin system to the dihydroporphin {chlorin 

 or rhodin) system. ("Rhoclins" are chlorins derived from chlorophyll 6; 

 c/. page 447, Vol. I.) Chlorins are green (as their name implies) and this 



Table 21.V 

 PoRPHiN Bands 



indicates strong absorption in the red, as well as in the blue and in the violet, 

 and transparency in the middle of the visible spectrum. Figure 21.12 

 shows the spectrum of a compound of the etio type, chloroporphyrin- 

 e4 dimethyl ester, together with that of its hydrogenation product, chlorin- 

 e4 dimethyl ester. Wliile the bands in green and yellow appear weakened 

 and displaced toward shorter waves in consequence of hydrogenation, a 

 new and very intense band arises in the red (at about 660 m/x) , which com- 

 pletely overshadows all the other bands of the orange-yellow system (its 

 intensity is about equal to that of the blue-violet band, not shown in fig. 

 21.12). The hydrogenation of a second pyrrole nucleus, which is charac- 

 teristic of bacteriochlorophyll and other derivatives of tctrahydroporphin 

 (cf. Vol. I, page 447), causes a renewed transformation of the spectrum 

 (fig. 21.10, 6). As stated on page 617 {cf. fig. 21.6), the strongest band of 

 bacteriochlorophyll is situated in the near infrared. According to Stern 

 and Pruckner (1939) the "bacterio" type bands in the visible region (red, 

 yellow and green) are generally weaker (with the notable exception of one 

 comparatively strong green band), and situated at shorter waves than 

 the corresponding bands of the chlorin type. It thus seems that the ef- 

 fect of the second hydrogenation is similar to that of the first one, i. e., 

 the majority of the previously existing bands (at least, of those in the region 

 above 450 m/x) are weakened and shifted toward the violet, while a new band 

 of dominant intensity arises at the long-wave end of the spectrum We 

 made use of this interpretation on page 618, when we suggested that the 

 orange (rather than the infrared) Ijand of bacteriochlorophyll be considered 

 as analogous to the red band of ordinary chlorophyll (see also page 631). 



The rule that the dominant red or infrared band in the absorption 

 spectrum of porphin derivatives is associated with the presence of one or 

 two hydrogenated nuclei may be very significant from the point of view of 

 the mechanism of the photosensitizing action of chlorophyll and bacterio- 

 chlorophyll, and it is therefore important to mention some apparent excep- 

 tions to this rule. One such exception was mentioned on page 619 — 



