1796 



SPECTROSCOPY AND FLUORESCENCE OF PIGMENTS 



CHAP. 37c 



jugat.ed bond system and normal to this direction, respectively. One 

 can expect, however, to find this split less wide than in tetrahydroporphin. 

 The spectrum of chlorophyll (and other dihydroporphin derivatives) 

 shows, instead of the two widely separated long-wave bands of bacterio- 

 chlorophyll (located, in ether, at 772 and 575 m/x, respectively), only one 

 main band (at 660 mn in chlorophyll a in ether, cf. fig. 21.1). However, 

 the relatively weak "orange" band (located at 613 m/x in ethereal solution), 



20000r 



16000 - 



12000- 



8000 - 



4000 - 



613 m^ 



16,300 cm"' 

 a„= 1.7x10* 



660 m^ 



15,200 cm"' 

 am = 9x10* 



575 m^ 



17,400 cm "' 

 «„ = 2.0x10* 



772 m^ 



13,000 cm."' 

 om-- 9.6x10* 



PROTOCHLOROPHYLL 



CHLOROPHYLL 



BACTERIOCHLOROPHYLL 



Fig. 37C\2. Low frequenry hand.s of protochlorophyll, chlorophyll a, and bacterio- 

 chlorophyll (in ether). The symbols || and _L refer to the long axis in baoteriochloro- 

 phyll and the corresponding direction in chlorophyll (which is not a sj-nimetr}' axis). 



which was interpreted in table 21. VI as the first vibrational sub-band of the 

 red band, could be due partially, or even mainly, to an independent elec- 

 tronic transition. (This was suggested, as an alternative, on p. 631 in 

 Part 1 of Vol. II.) The bands at 660 and 613 m^ would then correspond 

 to the long-wave band pair of bacteriochlorophyll, but with narrower sepa- 

 ration. This hypothesis is illustrated in fig. 37C.2. An alternative is to 

 assume that one component of the doublet is "prohibited" in dihydro- 

 porphin. 



Stupp and Kuhn (1952) concluded, from fluorescence polarization measurements 

 {cf. below section 4b) that the weak "green" band near 530 m/i (527.5 m^u in table 21. VI), 

 rather than the "orange" band at 613 m/z, must be attributed to a separate electronic 

 transition with an electrical momentum normal to that of the main red band. If this 

 conclusion is accepted, values of the constants of the " || band" of chlorophyll a, given 

 in figure 37C.2, must be changed to 530 mn, 18,900 cm.~S and am = 0.4 X lO"", re- 

 spectively. The separation between the || and the ± component would thus be in- 

 creased to 3700 cm."' — approaching that assumed for bacteriochlorophyll (4400 cm.~')- 

 However, the experimental results of Stupp and Kuhn are in need of confirmation 

 {cf. section 4b). 



