CHEMOTHERAPY 



interested in whether a group is on the ortho, meta, or para position in a 

 simple benzene derivative than whether that group is an amino, a 

 methyl, chloro, hydroxy, or nitro group. 



As a third illustration of specificity shown by simple substances 

 let us consider crystallization from solution. In general, this process 

 involves a truly remarkably specific selection of particles of a single 

 molecular species, occurring as rapidly as diffusion processes will 

 permit. The molecule of appropriate configuration finds a place 

 v/here it precisely fits, and kinetic energy of motion becomes trans- 

 formed into potential energy of position. Symmetrical molecules 

 which can "sit down" in several ways {e. g., para-disubstituted benzene 

 compounds) find this easier to do and crystallize more readily than 

 unsymmetrical molecules; and their crystals are less soluble and 

 higher in melting point. Ions which have affinity for each other are 

 able to react if by reacting they may crystallize out as an insoluble 

 compound. 



It may be worth some consideration that the demonstrations of 

 specificity in simple compounds just cited all involve as the specific act 

 the immobilization and orientation of the substance upon a surface, 

 of either a protein molecule or a crystal. We must recall that the 

 energy content of small molecules requires them to vibrate and move 

 about aimlessly in an activity far more rapid and extensive than that 

 of the protein molecules and crystal particles, which move with a 

 sluggish and restricted motion more like the Brownian movement. 

 We may picture the larger particle as furnishing a definite mold or 

 pattern into which the small particle in its bouncing, careening prog- 

 ress makes countless attempts to fit. When, by chance, there is close 

 approach at appropriate orientation to some matching area on the 

 surface, a specific adsorptive act occurs, the duration and outcome of 

 which depend upon intrinsic properties of the molecules. It may well 

 be promptly reversed and thus pass unnoticed in the confusion of 

 molecular traffic, or it may result in enzymic action, or inhibition 

 thereof, in precipitation or its inhibition, etc. We know biochemical 

 events to be replete with interactions of this general nature, and it 

 seems possible that specificity chiefly depends upon the existence of 

 anchoring areas on the surfaces of sluggishly moving large molecules 

 and cells. To learn the characteristics of their specificity we need to 

 know as much as possible about the way they are aflfected by all kinds 



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