Hydrogen‐tritium exchange of the random chain polypeptide

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Abstract
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 Kw 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: