Tritium–hydrogen exchange of poly‐ L ‐glutamic acid in aqueous solutions

To investigate the flexibility or motility of the secondary structure of poly‐ L ‐glutamic acid, a simple model molecule of proteins, the kinetics of the tritium–hydrogen exchange of this polymer in aqueous solutions was followed at various pH values by use of a freeze‐drying technique and a liquid scintillation counter. The most essential point of the experimental results is that all of the protons (or isotopes) attached to peptide nitrogens were exchanged according to a simple first‐order reaction and the rate was proportional to the fraction of random coil conformation determined by optical rotatory dispersion measurements. This means that the tritium attached to a peptide unit is exchangeable with a proton in water only when this unit is contained in the random coil structure and that the interchange of each unit between helical and random coil conformations takes place as a result of thermal fluctuation at very high frequency, although the average helical content of each molecule has a definite value.