CHLOROPHYLL'S LIPID ENVIRONMENT 



A. A. Benson 



Estimates ^'2 of the intermolecular distance for chlorophyll in chloro- 

 plast lamellae correspond to those of a half-molar solution. Photochem- 

 istry of dilute chlorophyll solutions hardly seems pertinent to conditions 

 existing in the chloroplast. Interactions of closely spaced chlorophyll 

 molecules oriented at the water-air interface have recently been reinves- 

 tigatedl Absorption maxima of the pure pigment were shown to be de- 

 pendent upon the film pressure, especially when it exceeded the limit of 

 reversible pressure-area relationships. Whether the presence of other 

 highly surfactant molecules might produce similar effects has not yet 

 been demonstrated but appears plausible. The lipids of chloroplasts are 

 among the strong surfactants of Nature, 



Myelin Figures 



Formation of myelin figures is a characteristic action of certain phos- 

 pholipids upon hydration^. These multilaminar structures exhibit dimen- 

 sions found in cell organelles. The early observations of Menke^ and 

 Weber^ revealed the formation of myelin figures from chloroplast com- 

 ponents in intact cells. These birefringent green films were extruded as 

 a result of disorganization of the plastids by a variety of reagents. It is 

 now of interest that the most concentrated lipid of chloroplasts exhibits 

 just such activity when the pure substance is exposed to water^ . 



Chloroplast Lipids 



The chloroplast lamellar lipids have been identified and recently have 

 been isolated^ The glycolipids of Figure 1 and the phospholipids of 

 Figure 2 are the surfactant materials which appear to stabilize the mem- 

 branes of chloroplasts. These lipids which stabilize membranes of the 

 thylakoids are predominantly amphipathic surfactant substances which, 

 like chlorophyll itself, possess hydrophyllic and hydrophobic groups. 

 The solubility properties and structure of these groups determine the 

 molecules' interfacial adsorption properties and, apparently, the struc- 

 ture and stability of oriented molecular leaflets which occur in the chlor- 

 oplast. 



The hydrophyllic moiety of chloroplast lipids is primarily the a -D- 

 galactosyl-(l-*6) -D-galactosyl group of digalactosyl diglyceride. This 

 lipid, together with about half as much of the monogalactosyl diglyceride, 



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