ENERGY MIGRATION AND THE PHOTOSYNTHETIC UNIT 1297 



One could suggest that bands leading to the "crystal levels" in naphthalene crj^stals 

 are analogous to the sharp absorption bands of Scheibe's polymers, and that energy 

 quanta absorbed in these bands are "communal property" of the lattice. In contrast 

 to this, quanta taken up in bands leading to "molecular levels" can l)e considered as be- 

 longing to individual lattice points, and capable of migration only by the "slow" reson- 

 ance mechanism. 



From the point of view of the state of chlorophyll m vivo, the spectra of 

 chlorophyll monolayers are of special interest. Of the two types of such 

 layers (Jacobs ct al. 1954), the "optically dense" (probably, bimolecular) 

 layers show a band position entirely different from that of chlorophyll in the 

 living cell; but the "optically thin" (undoubtedly, monomolecular) layers 

 show the band in about the same position as the living cells (675 m^; cj. 

 table 37C.II). Two questions must be answered, however, before this coin- 

 cidence is accepted as significant. Is the band position in the monolayer 

 due to chromophore interaction, or to the binding of chlorophyll molecules 

 to water? And, similarly, is the band position in the living cell due to 

 chromophore interaction, or to the attachment of chlorophyll to other cell 

 constituents (e. g., water, or proteins)? If the red shift were as wide as in 

 chlorophyllide monolayers (or in "dense" chlorophyll layers), its attribu- 

 tion to interaction between chlorophyll molecules would be much safer, 

 since such shifts are unlikely to arise from solvation. Despite this uncer- 

 tainty, it is at least a legitimate working hypothesis to assume that the 

 band position in vivo is due mainly to the interaction between chlorophyll 

 molecules within monolayers (which they probably form on protein discs). 

 The width of the absorption bands in chlorophyll crystals and mono- 

 layers make it likely (although, according to p. 1294, not quite certain) 

 that no "fast" migration of excitation energy occurs in them; while the 

 absence of fluorescence (or its shift into the infrared?) sets a rather low 

 limit for the possible extent of "slow" resonance migration. It will be 

 noted in this connection that the extensive band shift in crystals and 

 monolayers, although it indicates a "stabilization" of the "red" excitation 

 level by about 2000 cm.-^ as compared to isolated chlorophyll molecules, 

 can be adequately explained, in the first approximation, by virtual dipole 

 interaction between the chromophores in the lattice, without recourse to 

 quantum mechanical resonance phenomena, and therefore offers no evi- 

 dence concerning possible excitation energy migration in the lattice. 



In vivo, on the other hand, the small but measurable fluorescence yield 

 leaves, as estimated on p. 1289, just enough leeway for resonance energy 

 migration to satisfy the needs of a modest "photosynthetic unit." The 

 red band shift in vivo may be the composite result of virtual dipole interac- 

 tion between pigment molecules in a monolayer, the attachment of pig- 

 ment molecules to proteins and other cellular constituents, and a small 



