The hydrogen-deuterium exchange of macromolecules
An infra red spectroscopic technique has been developed which allows
the hydrogen-deuterium exchange between a polymer and absorbed heavy
water to be monitored with the polymer in contact with liquid heavy
water. This method extends the existing methods to the highest
possible internal water concentration.
The results of Moore167,168 on the hydrogen-deuterium exchange of
poly (N-t-butlyacrylamide) have been extended to the lower homologues
of the series. The rate and extent of the exchange of these polymers
is dependent upon the pH of the conditioning solution in which the
polymer is exposed prior to the exchange study. The exchange is acid
and base catalysed with the latter only apparent in the lower homologues.
The features of the acid catalysed exchange have been tentatively
explained in terms of the mobility of the hydrogen ion in the polymer
matrix, whilst the restricted base exchange is attributed to the lack
of hydration stabilisation of the intermediate ion complex of the base
catalysed mechanism. .
The hydrogen-deuterium exchange properties of nylon, silk and cellulose
have also been studied and these polymers show far less sensitivity to
the pH of the conditioning treatment. The differences in exchange
characteristics between the poly(N-alkylacrylamides) and nylon, which
both involve the amide group, is explained in terms of the modified
absorption and diffusion of acid by the nylons resulting from the
presence of acidic and basic end groups in the polymer. The exchange
of the hydroxyl groups of cellulose is much more rapid than that of
the amide group, and the exchange is governed by the rate of diffusion
of 020 into and H20 out of the polymer, rather than by the rate of the
exchange reaction itself.