2] DEUTERIUM EXCHANGE 33 



aqueous solution, the stabilizing factor must be an intrinsic one. It is there- 

 fore natural to think of the methyl side groups which, if there is a random 

 distribution of d and l forms along the chain, will protrude in two different 

 directions from the core of the helix, and so that a quarter of them may 

 form close pairs over the gap between two turns of the helix. The distance 

 between such methyl groups, one reaching down from a given a-carbon 

 of the hehx, the other reaching up from an a-carbon in the next turn 

 (separated from the first a-carbon by 3 peptide groups and 2 a-carbons), 

 is 3-6 A. reckoned from center to center of the /3-carbon atoms. Since the 

 average distance between the molecules in liquid methane at — 164°C is of 

 the order of 4 Â., there is reason to believe that a hydrophobic or nonpolar 

 bond may be formed. 



If we assume that AFr for the naked helix is zero, we shall have to adopt 

 the value of 4x(-f530)^ — f2000 cal. for the contribution of each nonpolar 

 bond to the set of AFr of PDLA (since only one-fourth of the group form 

 pairs). With a random distribution of the pairs, it may be permissible as 

 a first approximation to spht the contribution in four parts of +530 assigned 

 equally to four hydrogen bonds. An exact treatment would involve a com- 

 plete statistical analysis and is therefore extremely complex. 



It is difiicult to say whether the value +2000 cal. is a reasonable one or 

 not. Due to the small additional entropy change involved in the formation 

 of the nonpolar bond in the helix, we may here have to do with a AH value. 

 Figures of the order 1 to 2 kcal, have been suggested for nonpolar bond 

 strengths of CH2 groups, ^^^ 2^'^* but it seems that very little has been done 

 to determine experimentally the thermodynamic functions for the forma- 

 tion of nonpolar bonds in water. We shall therefore leave the question open. 



The assumption concerning the internal stabilization of PDLA by non- 

 polar bonds is supported by the fact that poly-DL-serine of similar chain 

 lengths (25 residues) exchanges 'instantaneously'. It is apparently contra- 

 dicted by the recent findings^*-^^'^^ that polypeptides of L-amino acids have 

 a tendency to form righthanded helices, the stability of which decreases 

 with increasing admixture of D-amino acids. Similarly D-amino acids have 

 a preference for lefthanded hehces. However, these important results were 

 obtained with poly amino acids having more bulky sidechains than PDLA, 

 so the question is not quite settled. It is clear, however, that our preparation 

 must contain equal quantities of righthanded and lefthanded helices; the 

 question is only whether D-alanine predominates in the lefthanded helices 

 and L-alanine in the righthanded ones, in which case the frequency of methyl 

 pairs along the hehces may be greatly reduced. This question may be solved 

 by digesting our sample with leucylaminopeptidase or carboxypeptidase, 

 the action of which will stop whenever a D-alanyl group is encountered. 

 In the extreme case, in which the preparation consists of pure poly-L-alanine 

 (righthanded helix) and pure poly-D-alanine (lefthanded helix), these en- 

 zymes should spUt half of the peptide bonds (14-15 per mole). If, however, 



Cps 



