MOLECULES IN SOLUTION 33 



mally gives up one electron. Thus the OH - ion should really be thought 

 of as = coupled to H+. This is an ionic bond. When we examine the 

 NH bond, a similar situation is found. For example, in ammonia, NH 3 , 

 the nitrogen would be triply charged negatively to reach the stable con- 

 figuration. Now if the geometrical structures of molecules were such that 

 an NH 3 group should be near an oxygen atom (such as one forming part 

 of a C = group) one of the plus-charged hydrogens from the NH 3 

 group could oscillate back and forth between its group and the oxygen 

 atom, making essentially electrical bonds and thereby binding the two 

 groups together. Since the hydrogen nucleus is nearly 2000 times as 

 massive as the electron, the oscillations are much slower than for electron 

 bonds, and the resultant bond is correspondingly weak. But since large 

 molecules of biological importance contain many such groups, there can 

 be many hydrogen bonds which add up to a respectably large total 

 binding energy. For example, the nucleic acids' characteristic double- 

 stranded structure appears to be held together entirely by hydrogen 

 bonds. 



To give an idea of the magnitudes of these bonds, the following table 

 shows typical values in each of the five categories of bonds: 



ionic 100 kcal/mol 



covalent 100 kcal/mol 



partial ionic 50 kcal/mol 



resonance 40 kcal/mol 



hydrogen 2-10 kcal/mol 



One final feature to be discussed is called saturation. In the ionic bond, 

 the ions interact electrically with all other electrically charged groupings 

 in their neighborhood; the ionic bond is therefore said to be unsaturated, 

 since other ions can join in. The covalent bond we have pictured is due to 

 a sharing of electrons, and the result would be that the shared electrons 

 would lie primarily between the two atoms. Therefore no other atoms 

 could normally become involved — without breaking this covalent bond. 

 Thus the covalent bond is said to be saturated, since it normally involves 

 just one pair of atoms. Such considerations become important in esti- 

 mating the effects of the entry of other substances in the neighborhood of 

 the existing bonded substances. 



MOLECULES IN SOLUTION 



Practically all the chemistry occurring in biological systems takes 

 place in solutions, almost all of which are water solutions. Thus it is im- 

 portant to look at the effects of water on the forces and bonds we have 

 outlined. 



