MOLECULES AND STRUCTURE FORMATION 13 



the interaction energy varies inversely with the square of the dis- 

 tance (Overbeek, 1952; Prosser and Kitchener, 1956). 



The last item of Table 1 refers to large molecules carrying many 

 groups capable of being positively or negatively charged, or un- 

 charged. At the isoionic point and at low ionic strength, mutual 

 electrical polarizations will tend to establish charge patterns on the 

 two molecules which lead to an electrostatic attractive energy 

 which decreases in proportion to the distance. At short distances, 

 interaction through charge fluctuation might provide an interesting 

 explanation for the participation of the protein moiety of an enzyme 

 molecule in the mechanism of hydrolytic enzyme reactions (Kirk- 

 wood, 1957). 



Of great interest to the biologist are those systems containing 

 large, usually asymmetric, particles at the correct concentration, 

 pH, and ionic strength which separate into two phases: one phase 

 in which the solute concentration is reduced below average and in 

 which orientation of the particles is absent, and a second phase 

 in which the solute concentration is increased above average and the 

 particles are well oriented and exist in a three-dimensional pattern. 

 Examples are to be found in bentonite (platelets), tobacco mosaic 

 protein (rodlets), and many systems in which "colloids" carrying 

 net charges of different sign are mixed. The last-named examples, 

 which must be of importance in providing an understanding of the 

 way in which large molecules interact over intervening aqueous 

 gaps, have recently been analyzed by Overbeek ( 1957 ) and by 

 Michaeli et al (1957). They treat the system theoretically as a 

 competition between charge interactions, which tend to accumulate 

 the charged particles, and entropy, which tends to disperse them. 

 Of considerable interest are the results obtained when the charged 

 particles, the polyanion and polycation, are present in different con- 

 centrations; for now the concentrated phase, the coacervate, will 

 be more symmetric in concentration than the starting mixture, while 

 the dilute phase will be more asymmetric. Separation into phases 

 takes place at solute concentrations which make these systems of 

 immediate biological interest. 



The structure of water in biological systems is also a matter of 

 importance. Water of hydration, at least that associated with polar 

 group^, will be firmly bound and will be involved in defining the 

 contour and properties of the surface of the condensed structure to 

 which it is attached. In the lamellar systems mentioned previously 



