24 K. LINDERSTR0M-LANG [2 



later. In a few cases, where the side chains are engaged in hydrogen bond- 

 ing, lower reaction rates might be observed, but we have been unable to 

 distinguish such cases from those discussed in the following paragraph. 



3. The deuterium atoms bound to nitrogen in the backbone of the peptide 

 chain show a highly varying rate of exchange. Half-times from less than 

 0-5 min. to more than 24 hours have been observed at pH 3 and 0°. Since 

 the fast reactions are primarily found in unfolded, denatured, proteins and 

 peptides, whilst the slow reactions are predominant in native molecules, it 

 is natural to ascribe the sluggishness of the reaction in the latter case to 

 internal hydrogen bonding in the secondary structure of these native mole- 

 cules, viz. to the presence of groups of the type 



I I 



C=0- -D— N (i) 



I I 



where the deuterium atom is less available for exchange. It is to be expected 

 that the stronger the hydrogen bond in question, the slower the observed 

 exchange, i.e. the smaller ßi. 



4. The rate of deuterium exchange is strongly dependent upon tempera- 

 ture and generally the more so, the lower the rate. 



5. The rate of deuterium exchange varies with pH, the most general 

 feature being that it increases with increasing pH from a pH about 3. In 

 certain cases this may be due to a change in the stabiHty of the hydrogen 

 bonds caused by ionization or desionization of the protein molecule but 

 in the best investigated case, which I shall discuss in a moment, the deter- 

 mining factor seems to be the rate of the exchange process proper which is 

 strongly catalysed by hydroxyl ions. 



6. Addition of denaturing agents generally increases the rate of exchange 

 in agreement with the fact that deuterium atoms released from internal 

 hydrogen bonds of the secondary structure may react faster with the solvent. 



The rapidly growing literature on the secondary structure of proteins and 

 polypeptides has made it likely that the a-helix of Pauling and Corey is an 

 essential element of this structure^"-^^-^^-^^-^^'^^, and the bonds of type 

 (i) may therefore in many cases be regarded as situated in a-helices. One 

 of these cases is represented by poly-DL-alanine (PDLA) which in its soluble 

 form, according to Elliott, ^°-^^ shows an infrared band at 1662 cm~^, con- 

 sidered to be characteristic of the internal CO-HN-bond. The question 

 whether this polypeptide in aqueous solution maintains its helical configura- 

 tion is of course of considerable interest, and since PDLA contains no ex- 

 changeable hydrogen atoms in its side chains and furthermore is unassociated 

 in aqueous solution, it appears to be an ideal object for the study of the 

 stability of helical hydrogen bonds by means of the deuterium exchange 

 method. 



