2] DEUTERIUM EXCHANGE 31 



reaction is faster at pH 5-6 than at pH 3, but that even in the latter case 

 about 85% of the 2 ND-groups have reacted with water in the course of 

 2 minutes. This finding is compatible with a half-time of 0-5 min. for the 

 reaction of these groups at pH 3, but as an argument in favour of our value 

 for ^2[QH] it is not very strong from an experimental point of view, and 

 is further considerably weakened by the fact that about 15% of ND-groups 

 apparently exchange at a lower rate. This phenomenon requires further 

 study but may be due to association of GGG in solution. 



If, however, the value 530 cal. is adopted for AFr, it is possible to cal- 

 culate the degree of unwinding of PDLA in aqueous solution. Some of the 

 calculations and results are summarized in Table 2, where the double in- 

 dices refer to the two ends of the peptide (cxy is the concentration of a 

 molecule with x bonds opened at one end, and y bonds at the other). 



Table 2 

 COMPOSITION OF PDLA-SOLUTION 



For explanation see text 



Although these figures should be accepted with proper reservation con- 

 sidering their highly approximate character, they suggest an interesting 

 quahtative picture of the state of PDLA in aqueous solution. 99% of the 

 molecules have more than 21 intact hydrogen bonds out of 26, and PDLA 

 may therefore be regarded as being essentially in the helical form. It is also 

 inherent in the considerations leading to the value suggested for AFr that 

 the rate of opening and closing of the hydrogen bonds by unwinding the 

 helix is very high, corresponding to a low free energy of activation. A mole- 

 cular structure of this kind may be said to possess a high motihty (a term 

 suggested to the author at the Gordon Conference 1957) and any qualita- 

 tive distinction between a native and a denatured state in aqueous solution 

 is meaningless. 



In a previous paper* on the deuterium exchange of insulin, a different 



