Abstract
To investigate the molecular motility and rigidity of polypeptides, the kinetics of tritium-hydrogen exchange was followed in several copolymers of glutamic acid and alanine at various helical content determined by pH. It was found that all tritiums attached to peptide nitrogens of copolymer in the intermediate pH range of the helix-coil transition were exchanged according to a simple first-order reaction and the rate was proportional to the fraction of random-coil conformation. This indicates that the interchange between the helical and random-coil conformations takes place at high frequency in all parts of each molecule even in a copolymer containing more than 70% alanine residues. In the low pH range of the complete helical conformation, however, these polymers consist of two kinds of helical conformations, rigid and flexible, different from each other in kinetic behaviour although they are not distinguishable by optical rotatory dispersion. The very slow rate of exchange in the rigid helix can not be attributed to the intra- or inter-aggregation of helices, for no such aggregation was detected by measurements of the hydrodynamic properties of the solution.
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