82 JERRY DONOHUE 



This is the point you were making, is it not, that the molecules do an extra- 

 ordinary job in finding a way to satisfy all structural features? 



The results of X-ray analysis of substances such as amino acids and peptides 

 have shown that molecules conform extraordinarily well to structural chemical 

 principles. There is some conjugation in an amide group such as to make a 

 planar molecule more stable than a non-planar one twisted around the C — N 

 bond. You can calculate by simple theory how much strain is involved in 

 twisting this by, say, 6 degrees. The amount of strain is about one tenth of a 

 kilocalorie per mole, much less than kT, much less than the equipartition en- 

 ergy. Yet in general, peptides retain the planar configuration to within 6 de- 

 grees, and the other structural features, such as the bending of the hydrogen 

 bond, are, in general, ideal; that is, they do not involve strain to an amount 

 greater than strain energy around a tenth of a kilocalorie per mole for each 

 structural feature. 



These structures show a closer approximation to ideal structures than one 

 would anticipate. In the process of crystallization, the forces between molecules 

 are much larger. The heat of sublimation of one of these substances may be 10 

 kilocalories per mole, or 20, for a molecule as big as threonine, and one would 

 think that the crystallization process might strain the molecule. It is surpris- 

 ing that serious strain occurs so rarely. 



Dr. Pressman: I would like to ask about the situation in aqueous solution 

 because that is where so many of the biological reactions occur. 



How much deviation form a straight line might be tolerated under such con- 

 ditions? Would you expect bifurcated hydrogen bonds, or is this just something 

 which you fit into the crystal? 



Professor Donohue: Obviously a crystallographer shouldn't answer this 

 question, but I will go ahead and try. If, as Dr. Pauling said, a complicated 

 molecule like threonine can set everything up around it so that the situation 

 is satisfactory in a solid where we have symmetry elements to deal with and 

 the molecules are in fixed relationships to each other, then it seems to me that 

 in a solution, where the molecules are moving back and forth all the time, this 

 linearity N — H • • • O or OH • • • O will hold, and also there will not be any 

 bifurcation. 



jSIr. Lippincott: I would like to ask about the difference between the 

 N • • • • O distance in hydrogen bonds involving zwitter-ions and the ones which 

 do not. Is there a significant difference in the N • • • • distance between these 

 two different types? 



Professor Donohue: We have not noticed any real correlation here. One 

 would think, of course, that for the NHs"^ case the distance would be shorter, but 

 this has not turned up. The only thing that comes to mind immediately is the 

 case of urea, where the distances are longer than is observed in the amino acids. 

 But, on the other hand, in the urea molecule there are four hydrogen atoms and 



