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L. J. MULLIXS 



Fig. 5. The fit of DDT and several homologs into an interspace formed by 3 mem- 

 brane molecules of radius 17.5 A. and a separation of 2 A is shown. The grid in the 

 interspace is ruled in units of 1 A, and a circle is inscribed to denote the limit of size 

 of molecule that can penetrate in an orientation independent manner. In the left 

 column, from top to bottom, DDT, Methoxychlor (p-methoxy DDT), and 2,2 bis 

 (p-iodophenyl) 1,1,1 trichloroe thane; right column, top to bottom, 2,2 bis (p- 

 chlorphenyl) 2 chloro 1,1,1 trichloroethane, DDE (2,2 bis (p-chlorphenyl) 1,1 

 dichloroethylene), and 2,2 bis (p-chlorphenyl) ethane. Note that iodo-DDT does not 

 fit because the p-substituent is too large, while in DDE it is the change from tetrahe- 

 dral bond angle that causes non-fit. 



the number of vacant interspaces declines proportionally for all sizes as shown 

 on the left in Fig. 6. Large anesthetic molecules, because they can enter only 

 large sized interspaces, may be expected not only to decrease the number of 

 vacant interspaces but to shift the distribution maximum to lower absolute 

 values as shown on the right in Fig. 6. 



The Enzyme Dehydrohalogenating DDT 



A recent report on the presence of an enzyme in flies that are resistant to 

 DDT (Sternburg et al., 1954) together with an examination of the substrate 



