MOLECULES AND STRUCTURE FORMATION 5 



The fact that covalent bonds have associated with them preferred 

 or hmited distances, directions, and rotations in conjunction with 

 the sizes of the participating atoms (Pauhng, 1940) means that the 

 possible conformations of the molecule may be specified. As the 

 number of atoms increases, possibilities arise for various types of 

 interaction between groups of atoms within the same covalently 

 linked assembly. In molecules of large size, such as the proteins, 

 carbohydrates, and nucleic acids, these interactions are numerous 

 and involve all known types of forces. For example, these inter- 

 actions are involved in determining the structures of the poly- 

 peptide chain helices of proteins (Pauling ct fl/., 1951), the inter- 

 actions of proteins (Waugh, 1954), and the double helix of nucleic 

 acids (Watson and Crick, 1953). Recognition of the importance of 

 the group of short-range forces in determining specificity of inter- 

 action, as well as the intramolecular stabilizations just mentioned, is 

 due largely to the work of Pauling and co-workers. 



Table 1 gives a general idea of the various types of short-range 

 and long-range interactions. The characteristics of the first of these, 

 the covalent bond, have already been mentioned. The hydrogen 

 bond is a permanent dipole interaction, involving a hydrogen atom 

 covalently bonded to one electronegative atom and the unshared 

 electron pair of another electronegative atom ( Pauling, 1940; Orgel, 

 1959). This bond has a high interaction energy, a result of the small 

 size of the proton which permits the close approach of dipoles. To 

 break a hydrogen bond requires an investment of about 5 kcal per 

 mole. However, if H-bond exchange with water is possible, the 

 investment is much reduced and may be reversed if the groups 

 involved make unusually strong hydrogen bonds with water. Thus 

 the strength of a particular H-bond is dependent on the circum- 

 stances permitting a simultaneous interaction with water. 



The simple ionic bond, a strong interaction in the absence of 

 water, leads to attraction or repulsion according to Coulomb's law. 

 The interaction energy varies with the dielectric constant of the 

 intervening medium and inversely with the distance between cen- 

 ters. The former varies strongly with distance in an aqueous 

 medium and decreases as the distance between centers decreases 

 (Pressman et al., 1946). Assuming that water hydrates an interact- 

 ing pair such as an ammonium ion and carboxyl ion, close approach 

 will produce an interaction energy of about 5 kcal per mole ( Pauling 

 et at., 1946). We should remember also that charged groups lead 



