STATE OF PIGxMENTS IN CHLOROPLASTS 1753 



simultaneous decrease in the sensitivity of chlorophyll to bleaching, and 

 concluded that chlorophyll exists in the cells in two forms. They assumed 

 one to be a monomeric, "photoactive," fluorescent form, with an ab- 

 sorption peak at 670 m/i; the other, a polymeric, "photo-inactive," non- 

 fluorescent form, with a band at 678 m/i. After 16 hours of greening, the 

 absorption peak moved from 670 to 678 mn, indicating that the proportion 

 of "photoactive" or "bleachable" chlorophyll dechned sharply. (The 

 total amount of chlorophyll grew, while that of the "photoactive" com- 

 ponent remained almost constant.) 



This interpretation of the chlorophyll spectrum in vivo was analogous 

 to that suggested by Krasnovsky et al. for the two main absorption peaks 

 of bacteriochlorophyll in the living cell (c/. Chapter 37C, section 6c). 



Whether the postulate of the two forms of chlorophyll is correct or not, 

 it is certainly implausible (and in the case of bacteriochlorophyll, incorrect) 

 to attribute (as Krasnovsky suggests) the fluorescence (and photochemical 

 activity) to the form with the absorption band at the shorter waves. 

 Experiments on energy transfer, described in Chapter 32, clearly proved 

 that excitation energy moves toward the pigment with the lowest excitation 

 level, and that this pigment (or this pigment form) is responsible for both 

 the fluorescence and the photochemical activity of the whole system. 



It remains to be proved that the band shift from 670 to 678 ran with 

 increasing pigment density is not due to changes in scattering. It may 

 also be caused by "organization" of molecularly dispersed chlorophyll 

 into coherent monolayers (cf. chapter 37C, section 3). 



The position of the red absorption band of chlorophyll in the living cell 

 (between 670 and 680 mn, as compared to 650-660 mn in organic solvents) 

 has been for a long time used as the basis for speculations (and experi- 

 ments) concerning the nature of the "chlorophyll complex" in vivo. This 

 matter was discussed in Chapter 21, without arriving at a definite conclu- 

 sion; and the problem must still be considered as open at this writing ten 

 years later. The alternative — but not mutually exclusive — interpretations 

 of the "red band shift" in vivo are pigment association with a protein, and 

 resonance interaction between closely packed chlorophyll molecules. 



Rodrigo (1953) prepared a number of artificial protein-chlorophyll com- 

 plexes, and found that one of them, a protein extracted from white leaves 

 of Pelargonium zonale, had an absorption peak at 680 m/x. 



In Chapter 37C we will describe experiments on chlorophyll crystals 

 and monolayers, showing a red resonance shift depending on the density 

 of the packing (and, to a certain extent, on the resonance volume). The 

 concentration of the chlorophyll molecules in vivo (assuming they form 

 monomolecular layers on grana discs or chloroplast laminae) is such that a 



