Hydrogen‐tritium exchange of the random chain polypeptide

The Hydrogen–tritium exchange character of poly‐ D , L ‐alanine was studied in detail as a model for the hydrogen exchange behavior of the unhindered, polymeric peptide group. The random chain nature of poly‐ D , L ‐alanine was evident in the uniformity of exchange rate of all its hydrogens and in the similarity between this rate and that of random chain poly‐ D , L ‐lysine and other known, unhindered secondary amide groups. An equilibrium isotope effect favoring the binding of tritium over protium to the extent of 21% was measured. Specific acid and base catalysis of the exchange and the absence of detectable general catalysis were demonstrated. Apparent energy of activation is 17 kcal/mole for deprotonation, largely due to dependence of K w on temperature, and 15 kcal/mole for protonation, which correlates with the extreme apparent pK. The hydrogen –tritium exchange half‐time rate; of poly‐ D , L ‐alamine at any pH and temperature (T: °C) is given by the equation:\documentclass{article}\pagestyle{empty}\begin{document}$$ \[t_{\frac{1}{2}} \,(\min )\, = \,200\,\, \times \,10^{0.05{\rm T}} /\,[10^{{\rm pH} - 3} \, + \,10^{3 - {\rm pH}} ]\] $$\end{document}