188 LINUS PAULING 



effect of replacing a methyl group by a hydrogen atom on the cholinesterase 

 enzyme he got ratios of between 8 and 30, which correspond to distances be- 

 tween positive charge and negative charge between 5.8 A and 5.0 A. These are 

 close to the minimum distance of approach of the charged groups, a trimethyl- 

 ammonium ion group and a carboxylate ion group. 



The van der Waals interactions are important. We know a great deal about 

 van der Waals interactions because of London's theory, and Professor Hirsh- 

 felder in his discussion based upon virial coefficients of gases pointed out that 

 the theories of interaction of molecules involving not only the van der Waals 

 attractions of molecules but also interactions of electric dipole moments and 

 electric quadrupole moments are in good agreement with experiment, especially 

 the experiments on transport phenomena in gases, and that we can have con- 

 fidence in simple theoretical considerations based upon these ideas. 



The differences in interaction energy that Dr. Pressman and Dr. Wilson re- 

 ported corresponding to replacing a methyl group in a place where there is a 

 hole big enough for it by a hydrogen atom, differences of about one kilocalorie 

 per mole, are reasonable differences to interpret as resulting from the van der 

 Waals attraction of the surrounding atoms for the hydrogen atom as compared 

 with that for a methyl group. 



Professor Kirkwood discussed a force due to fluctuations in electric charges 

 on two protein molecules that can be significantly large under certain circum- 

 stances and can operate over a rather long distance. This is an interesting new 

 kind of interaction of molecules that are, on the average, electrically neutral 

 but, because of charge fluctuations in the ionization of ionizable groups, can 

 show a significant electrostatic interaction, significant probably only when the 

 molecules are essentially at their isoelectric points; otherwise the force is small 

 compared with the larger forces of interaction of the permanent electric charges. 



Of course, there are other kinds of forces that also need to be considered, some 

 of which have not been discussed at all, or mentioned only very briefly. There 

 can be significant forces operating between two big protein molecules rather far 

 apart, a whole molecular diameter apart from one another in aqueous solution, 

 if their polarizabilities, specific polarizabilities, differ significantly from those of 

 the medium. 



An interesting aspect of the theory is that if both of the two protein mole- 

 cules have larger specific polarizability than the medium, they attract one an- 

 other. If they both have smaller specific polarizabilities than the medium they 

 also attract one another. That is, perhaps, not a result that one would have 

 foreseen. 



One more point about Professor Haurowitz' talk. He said that we know mat 

 the dimensions of the active region of an antibody molecule are only around 

 a couple of hundred square angstroms, and that the region might represent 

 only about four amino acid residues. If we say that each of these could be any 



