26 K. LINDERSTR0M-LANG [2 



PDLA was first deuterated completely and the rate of back-exchange with 

 ordinary water was followed. 



A volume of 150 /m1. of a 2% PDLA solution, pH 2-8, was lyophilized 

 and dried in a vacuum over P2O5. Next 150 jul, of 99-7% DgO was added 

 and allowed to exchange with the peptide at 38-6°C for 4-15 hr. The deu- 

 terated peptide was freed from D2O by cryosublimation and drying for 4 hr. 

 at 68°C,*-' after which time 150 [A. of 01 m citrate buffer of the desired 

 pH and temperature were added, and samples of 15 /nl. were taken out at 

 intervals for measurement of the deuterium exchange (cryosublimation and 

 determination of the density of the water in the gradient tube.^-*-') 



It should be noted that back-exchange is measured here in buffers in 

 order to fix pH in the otherwise poorly buffered PDLA solutions. We also 

 call attention to the fact that the solid, the dissolution of which is the initial 

 step in the kinetic experiment, is the same in all cases (obtained by lyo- 

 philization of 2% PDLA solutions at pH 2-8), and that consequently pH 

 is determined by the composition of the buffers added. The value obtained 

 for the total exchange should therefore be the same in all experiments 

 («»=33). 



4. RESULTS OF THE EXCHANGE EXPERIMENTS 

 In each kinetic experiment the total exchange, w„, at the given conditions 

 was determined by placing from one to two 15 jul. samples at 38-6°C for 

 4-15 hr. and determining the deuterium content of the water. As the mean 

 of 24 determinations under widely different conditions, a value of 33-2 was 

 found with a standard deviation of 0-4, a result which is in good agreement 

 with the above assumptions. Nevertheless the «-values of a given kinetic 

 run were corrected by multiplying them by the ratio 33/(«„)a;, where (n„)x 

 is the value for n^ determined in this particular run. A rough correction 

 for losses during lyophilization and incomplete removal of D2O is obtained 

 in this way. 



Figs. 1 and 2 show the kinetic results. The curves drawn are theoretical 

 ones and will be explained in the following section. 



5. THEORY 

 It is obvious that the curves in Figs. 1-2 may be satisfactorily described by 

 Eq. 1 (see (4)) if the number and values of ßi can be adjusted arbitrarily 

 to fit the curves. The variations of ßi with a given parameter X (pH) may 

 be most adequately determined by the variation of t at constant extension 

 of the reaction (constant n). From Eq. (1) we get 



yS^t _0.t 



/8lnt\ ^ "hX^ _ 8ßil8X ,. 



\8xln~ ^iSiC-^»' ~ ßi ^ ^ 



If now the ßi may be regarded as products of a general factor, B, which 



