Chlorophyll infrared absorption bands

Table 14.4. Infrared Absorption Bands of Chlorophylls and Derivatives Frequencies Corrected (cm ) (after Holt and Jacobs, 1955). (Marks, 1969.)... 

The reduction of ring IV in chlorophylls a or b changes the optical absorption spectrum of the molecule dramatically. Whereas the long-wavelength absorption band of a cytochrome is relatively weak (see fig. 14.4), chlorophyll a has an intense absorption band at 676 nm (fig. 14.5). Chlorophyll b has a similar band at 642 nm. Bacteriochlorophylls a and b have strong absorption bands in the region of 770 nm (see fig. 15.5). The chlorophylls thus absorb red or near-infrared light very well. 

Important in the assignment of these transfer steps is obviously the identification of the absorption bands in the spectrum of the reaction center. In the near-infrared absorption region there are three bands visible for the R. sphaeroides reaction center. With the use of spectral differences induced by reduction or oxidation it was possible to assign the longest wavelength band in the Qy region (870 nm) to the special pair, the middle band (800 nm) to the accessory chlorophylls, and the third (770 nm) to the pheophytins. 

Chlorophyll has an important part to play from 0.4 to 2.6 //m. In the visible, it dominates the spectral region due to two very strong absorption bands at 0.45 and 0.65 fim. In the near infrared, with the exception of the 1.45 and 1.95 ixm bands which are characterized by H2O absorption, chlorophyll becomes a good reflector. 

Elucidation of the state of the chlorophyll molecules in vivo is essential for understanding the primary processes of photosynthesis. Vibrational spectroscopy is a potentially powerful tool for investigating the structure of chlorophyll and interactions between chlorophyll and its environment. In fact, infrared spectroscopy has been shown to be useful for studying chlorophyll-chlorophyll and chlorophyll-solvent interactions iyi vitvo (J.J. Katz et al., 1966). Resonance Raman spectroscopy, on the other hand, provides a unique technique for probing the state of chlorophyll in vivo, because the Raman bands arising from the chlorophyll moiety can be selectively observed using a laser line in resonance with the electronic absorption of chlorophyll (M. Lutz et al., 1976). 

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