198 MACROMOLECULAR COMPLEXES 



actually distributed in a laver about 60 A thick at the surface of the 

 chromatophore. 



It is generally assumed that the photosynthetic pigments exist in 

 a condensed state, such as a monolayer, in vAvo. This concept has 

 been inferred repeatedly (see Rabinowitch, 1945, 1951, 1956) from 

 considerations of the absorption spectrum, fluorescence, energy 

 transfer, etc. In Chromatin m, energy is transferred from the carot- 

 enoids to bacteriochlorophyll with an efficiency of 30 to 40 per cent 

 (Duysens, 1952); neyertheless, these accessory pigments are not 

 necessary as functional or structural components (Fig. 10) of the 

 chromatophore. In addition, there is supposed to be a fundamental 

 difference between the relationships of the two types of pigment 

 with the protein. The bacteriochlorophyll is a clayate molecule; 

 that is, the phytol "tail" is hydrophobic, but the porphyrin "head" 

 is hydrophilic and can enter into intimate relation with protein. 

 The carotenoids are pure lipids and cannot form strong bonds with 

 protein. 



The pigments of the chromatophore will fit in a monolayer about 

 25 A thick at the internal face of the proposed protein layer. The 

 area available at this interface [4n ( 100 A)-] is (1.26 X 10') A". The 

 carotenoids of purple bacteria are aliphatic C4,, compounds about 

 25 A in length. Since the yan der Waals radius of the methyl group 

 is 2 A, each of the 300 carotenoid molecules will require about 13 A- 

 at the interface, for a total of 4000 A". The remaining area, ( 1.22 

 X 10'' )A-, allows 200 A- for each of the 600 molecules of bacterio- 

 chlorophyll. This yalue falls between the area of the porphyrin 

 itself ( 242 A- ) and the area occupied by the "head" of the chloro- 

 phyll molecule in the tilted position assumed in monolayers (Trur- 

 nit and Colmano, 1959 ) . Similar calculations indicate that the 3000 

 cephalin molecules can be ordered with the polar groups directed 

 into the central aqueous phase and with the non-polar fatty acid 

 moieties forming a lipid phase in conjunction with the carotenoids 

 and the phytol "tails." This asymmetric organization, though specu- 

 latiye, is consistent with the known physicochemical properties, the 

 composition data, and the electron microscope observations. 



The de novo origin of bacterial chromatophores seems to have 

 been established by observations on RhocIospiriUum ruhnim (Pardee 

 et al, 1952; Vatter and Wolfe, 1958). Such observations suggest 

 that the chromatophores are assembled from smaller particles. In 

 terms of the "fixed" components, the simplest representative build- 



