SUMMARY AND DISCUSSION 193 



methane hydrate, (CH4)8 (H20)46 • JSIethane hydrate contains all of the hydro- 

 gen bonds that ice does and its melting point is above 0°C. The water molecules 

 form hydrogen bonds in such a way that there is a big enough hole for the 

 methane molecule to tit into. 



Dr. Pressman: In connection with these last remarks, there is the experi- 

 ment in which increased pressure will reverse an antigen-antibody reaction. 

 Presumably, the antigen and antibody separately occupy a smaller space than 

 the complex itself, plus the water which has been squeezed out during reaction. 

 I was wondering if this might well not be pertinent to this argument in that the 

 water which is squeezed out is low density. 



Chairman Pauling: I think that this effect is due to electrostriction. The 

 positive and negative charges on ionic groups compress the water around them 

 because they attract the water molecules so strongly and if the positive and 

 negative charges combine with one another during the reaction the electro- 

 striction compression disappears and the water expands. 



Professor Kirkwood: This brings up another point which is not new at all; 

 it is not an idea that I have proposed but it has been floating around a long 

 time. It has not been brought up in this discussion, although I am sure many 

 of you are aware of it, certainly Dr. Pressman and Dr. Pauling. 



Let us consider the forces between protein molecules or protein and a small 

 molecule, — Dr. Pauling really hinted at this when he spoke about electrostric- 

 tions. Of course, one is interested in the change in free energy when one brings 

 two molecules together and one knows that in the neighborhood of a protein 

 molecule, having a number of polar side chains, one has a very severe hindrance 

 of rotation of peripheral water molecules. This influence may extend out to 

 several layers of water molecules. 



These water molecules possess less entropy per molecule than water in bulk 

 possesses, and when you bring up another molecule to the surface of a protein 

 you free some water molecules whose rotation has been frozen or partially 

 frozen and therefore you gain a considerable amount of entropy, and, this can 

 contribute very appreciably to binding. Some interesting examples have come 

 up in, I think, rather spectacular form from some of Dr. Sturtevant's thermo- 

 chemical studies of interactions of proteins and in particular enzymes and sub- 

 strates. Another instance in which this occurs is in, say, the formation of the 

 mercury mercaptalbumin dimer. 



Chairman Pauling: Thank you, Professor Kirkwood, for discussing this 

 important matter. 



Professor Pitzer: I wish to come back to the matter of restricted rotation 

 in the polypeptides. If you define an angle expressing internal rotation and then 

 expand the appropriate potential, you get a three-fold term and a six-fold term. 

 I believe we have shown that the six-fold term is negligible. 



Ch.airman Pauling: That is in ethane. 



