42 SOME PHYSICAL FORCES EXEMPLIFIED IN MAN 



another. Water molecules attract each other, dipole to dipole, and give to 

 bulk water a structure of oriented dipoles. Ions attract one end of the dipole 

 and repel the other, and the result is an array of water dipoles oriented 

 radially outwards from a central ion. The dipoles on large molecules can be 

 hydrated by attraction to water molecules. Big molecules can be attracted 

 to each other, or indeed have one part folded back and attracted to another 

 part where two dipoles fall in close proximity, or where one dipole falls close 

 to a charged group. Thus the dipolar character helps to determine not only 

 composition but also structure. 



Still weaker forces exist between induced dipoles. Even if the molecule is 

 symmetrical about an atom, a strong positive or negative charge can some- 

 times induce the molecule's electrons to move a bit, so that the charge dis- 

 tribution becomes distorted. Such induced charge separation is called an 

 induced dipole. Interactions between the mutually induced dipoles of two 

 molecules in close proximity are called the van der Waals forces. Further, it 

 is postulated that the electron cloud of a molecule is in continuous motion, 

 continually varying both the size and direction of its dipole. It induces a 

 further dipole in its neighbor, and the new "dynamic" dipole interacts with 

 the old static one in a manner which seems to confer an extra stability on 

 the intermolecular "bond." The extra force of attraction is called the "dis- 

 persion force," first postulated by London in 1930. Since one occurs when- 

 ever the other does, today the mutually induced dipole and dispersion forces 

 of attraction are referred to as the London-van der Waals forces. They are 

 very weak by comparison with Coulombic forces, principally because the 

 charges are not only small but deformable. However, in the absence of 

 charged groups and when two molecules can come into close proximity 

 (< 5 A) at a great many places over a fairly long distance (~15 carbon 

 atoms in each molecular chain), considerable binding between the two has 

 been shown to be accountable on the basis of London-van der Waals forces. 

 Such is the case in lipoproteins in which a long hydrocarbon (and therefore 

 with no polar groups and no permanent dipoles) chain becomes and remains 

 intimately bonded to a polyamino acid or protein molecule. The strength 

 and the sensitivity of this bond to interatomic spacings have been very 

 evident in recent studies of lipoproteins in nerve cell membranes of the cen- 

 tral nervous system. For example, one form of encephalitis is currently 

 thought to be due to a change in binding which occurs as a result of inac- 

 curate protein synthesis and poor binding to its lipid. 



Whereas Coulombic forces are fairly long-range forces (al/a' 2 ) the 

 London-van der Waals forces are very short-range ( « \/d 7 ) but become im- 

 portant when the particles approach very close to one another (see Table 

 2-2). 



