34 PHYSICAL FORCES AND CHEMICAL BONDS 



Consider a charged particle, an ion, in a solution which also contains 

 many small ions (e.g., Na + , CI - , S0 4 , etc.). The minus-charged ion 

 on, say, a protein molecule will attract some of the cations in solution by 

 a simple electrostatic force. As shown in Fig. 14, these cations will tend 

 to cluster around the negatively charged protein, while the negative ions 

 will be repelled. The situation would look as in the figure except that 

 the plus ions also repel each other and attract the anions. Therefore the 

 real picture will have the cations pushed out quite a bit, intermingled to 

 some extent with the negative ions, so that it is only statistically that the 

 protein has more plus charges nearby than minus charges. This picture 

 has been worked out mathematically by Debye and Hiickel, and the 

 actual distribution of cations and anions around the protein can be 

 shown to be just the kind of swirling, interpenetrating clouds that our 

 simple approach yields. At some distance from the protein, of course, 

 the electric force of the protein is vanishingly small, so that for the 

 solution as a whole there are Debye-Huckel clouds around each charged 

 molecule. 



+ 



Fig. 14. The clustering of plus and minus solution ions near the respective 

 minus and plus charged ends of a "dipolar" particle. 



These ionic clouds affect reactions appreciably, for reacting molecules 

 must drag along their sycophantic clouds, thereby reducing their mobility 

 in solution, and therefore the reactivity. Also, because of the existence 

 of these clouds of ions, molecules held together by ionic bonds or hydrogen 

 bonds will have their binding appreciably reduced. It is possible that 

 this effect is important biologically, especially since hydrogen bonds have 

 been found to be responsible for the twisting and folding of such biolog- 

 ically important molecules as proteins and nucleic acids. Indeed, since the 

 latter are held together chiefly, if not entirely, by hydrogen bonds, 

 changes in the ionic concentration near nucleic acids could well be 

 associated with their ability to separate into component strands during 

 replication. The ease of separation of the strands has been verified as 

 being greatly augmented by lowering the salt concentration while heat- 



