2] DEUTERIUM EXCHANGE 25 



In the following a discussion of some of the results obtained with PDLA 

 will therefore be given. 



2. THE PDLA PREPARATION 



The preparation of PDLA used was synthesized in Katchalski's laboratory 

 and was reported to contain an average of 30 residues per molecule. The 

 material, which contained pyridine, was purified by dissolution in dilute 

 NaOH, lyophilization over P2O5, redissolution in an equivalent amount of 

 HCl, and, finally, lyophilization after removal of a small quantity of undis- 

 solved gel. The purified PDLA dissolved in water to a clear solution. The 

 pH of the solution was 2-8, and the peptide was therefore present mainly 

 as a positive charged ion. 



A simple PDLA molecule containing 30 residues has a molecular weight 

 of 2151 and a nitrogen content of 19-53%. The ratio of a-amino groups to 

 total nitrogen atoms should be 1 : 30. The present preparation when dis- 

 solved in 0-lN-KCl at 22°C gave a titration curve which showed the presence 

 of one amino group (pK 8-1) per 36-7 nitrogen atom, a figure which is 

 compatible with the assumed average chain length, if end groups of the type 



H00CCH(CH3)NHC0NH— 



(see Sela and Berger^^) replace a-amino groups in about 20% of the mole- 

 cules. PDLA molecules with such end groups have a molecular weight of 

 2266 and a nitrogen content of 19-16%, and the average nitrogen content 

 of our preparation should therefore be 19-46%. The experimental value 

 19-0% (corrected for NaCl and acid content) is in fair agreement with the 

 theoretical one. The molar concentration, Cs, of PDLA in a given solution 

 was calculated from the nitrogen content, and the number of exchangeable 

 hydrogen atoms per molecule was assumed to be 29 (backbone) +4 (end 

 groups) at pH 2-8, i.e. a total of 33. 



Since the hydrogen exchange was measured in 0-1 M citrate buffers which 

 were 0-1m with respect to glycerol (cf. '), the sedimentation constant of 

 PDLA was determined at pH 3 in this medium. The sedimentation con- 

 stant corrected to water at 20°C was found to be represented by the equation 



^20. 10i3=0-4-0-07 x(% PDLA) 



in the concentration range 0-5 to 2%, which is compatible with a molecular 

 weight of 2000 (D^IOAQ-'', F~0-74), Hence there is no indication of 

 significant association. The question of the distribution of molecular sizes 

 in our preparation is unsettled but, since the boundary spreading in the 

 centrifuge appeared to be normal, it may be inferred that the size distribution 

 is within rather narrow limits. 



3. EXCHANGE METHOD 

 The method used in all cases was Method 2 described in (4) and (7); i.e. 



