192 LINUS PAULING 



are more-or-less fortuitous; I do not think that they have much relationship 

 to what is actually happening. 



However, this kind of interaction is well known. It is the kind of interaction 

 which is responsible for the formation of soap micelles, detergent micelles par- 

 ticularly, where there is a carboxylate ion attached to a massive or long hydro- 

 carbon chain. The aggregation is not really so much due to the fact that the 

 hydrocarbons are fond of each other as that they are crowded together by the 

 water and if one did not have the charged groups, of course, one knows what 

 would happen. There would be a phase separation and then the hydrocarbon 

 would simply separate as a separate phase. However, this process stops in the 

 formation of a micelle because one gets the protective layer of ionic groups 

 which interact with the water. 



I think that in the case of protein structure one should not speak just about 

 the stability of a helical structure being due to the direct hydrogen bond forma- 

 tion, although that is essential, but that the hydrogen bond can be reinforced 

 by essentially the effect of water in scjueezing side chains into contact. I think 

 Linderstr0m-Lang recently has been thinking along these lines and I believe 

 that it is well to think of the important role of the solvent in enhancing the 

 strength of the primary bond in an indirect way. 



There was one other thing I wanted to say in this connection; that the Ham- 

 macher relationship for likes always interacting more than unlikes in a binary 

 mixture is limited to non-polar mixtures and that it can lead to very serious 

 errors when one is considering one component such as water which is strongly 

 hydrogen bonded to itself or a polar solvent, as the theorem applies only to 

 mixtures of components which are interacting by van der Waals forces alone. 



Chairman Pauling: I should like to emphasize the points that Professor 

 Kirkwood has made. I think that people who have been talking about these 

 things for fifteen years, like Dr. Pressman, forget that fifteen years ago or ten 

 years ago they were careful to say that by the energy of the hydrogen bond be- 

 tween the antibody and the antigen we really mean the difference in energy 

 of that hydrogen bond plus the new hydrogen bonds formed between water 

 molecules and the energy of the hydrogen bonds that were formed with the 

 water molecules in the system in its initial state. All of this was gone into in 

 detail in the early papers. 



There is a point that I should like to make about the matter of the water 

 squeezing a hydrocarbon out. I think that this argument can be overdone, 

 because you can fit a methyl group or, say, a methane molecule into water 

 without breaking any hydrogen bonds. In the van der Waals calculation which 

 Dr. Wilson referred to and which he took from Dr. Pressman's paper, the Lon- 

 don expression was applied to the interaction energy of the group with water 

 and then of the group with the antibody molecule but with no correction for 

 the hydrogen bonds in the water. 



The fact is that a little methane helps water to form hydrogen bonds, as in 



