THEORY OF CHLOROPHYLL SPECTRUM 1797 



The attribution of the long-wave (red) band in chlorophyll, as well as in 

 bacteriochlorophyll, to a vibration parallel to the symmetry axis, and of 

 the short-wave (orange) component to a vibration perpendicular to this 

 axis, is plausible because the molecular structure changes, in the series 

 porphin-dihydroporphin-tetrahydroporphin, much less strongly in the 

 direction normal to this axis than parallel to it {cf. fig. 37C.1). Of the 

 two bands interpreted above as components of the red doublet, the short- 

 wave (orange) band changes comparatively little, in position or intensity, 

 in the transition from porphin to tetrahydroporphin {i. e., from proto- 

 chlorophjdl to bacteriochlorophyll) ; while the long-wave (red) band is 

 strongly shifted and enhanced by this transition (fig. 37C.2). 



This difference becomes less pronounced if the constants of the "|| band" in chloro- 

 phyll are changed, as suggested above, to account for the polarization experiments of 

 Stupp and Kuhn. 



The blue-violet band is a doublet, in chlorophyll as well as in most of its 

 derivatives. (AUomerized chlorophyll appears to be an exception, cf. fig. 

 21 .4A.) The relative intensities of the main "blue" peak and of its "violet" 

 satellite change strongly from preparation to preparation {cf. p. 607). 

 Whenever the violet satellite appears particularly prominent, suspicion 

 arises that pheophytin is present as an impurity; however, careful chro- 

 matographic purification never eliminates the satellite band entirely, but 

 only reduces its prominence. The separation and relative intensity of the 

 violet satellite band depends considerably on the solvent {cf. fig. 21.26). 

 Its interpretation is at present uncertain. A vibrational sub-band could 

 occur on the short-wave side of the blue peak. Isomerism (or tautomerism) 

 could account for a doublet structure. A final possibility is that the two 

 bands correspond to the two theoretically expected electronic transitions. 

 In the last case the blue band of chlorophyll a (at 429 m^ in ether) and its 

 violet satellite (at 410 m^u in ether) are to be considered as components of an 

 electronic doublet, the first one polarized |!, and the second one J_ to the 

 short axis of the molecule. They are then analogous to the two ultra- 

 violet bacteriochlorophyll bands (395 and 360 mn, respectively) ; but the 

 "satellite" is located at the short-wave side of the main component in 

 chlorophyll, and on the long-wave side of it in bacteriochlorophyll. 



According to Stupp and Kuhn (1952), the polarization of chlorophyll fluorescence, 

 excited by absorption in the blue-violet region, can be interpreted by a superposition 

 of two bands, with electric momenta in two mutually perpendicular directions {cf. 

 fig. 37C.21). 



The wide error margin of the calculations, which is obvious from table 

 37C.I, the arbitrariness in the selection of weak bands interpreted as inde- 

 pendent electronic transitions, and the uncertain isomeric homogeneity 



