poly(N-isopropylacrylamide)
(PNIPA) is a thermosensitive hydrogel, that is highly swollen in water at a
temperature bellow the lower critical solution temperature (LCST) (ca.
32 oC), and shrinks drasticaly upon heating above it. This volume
transition is associated with an increases in the gel hydrophobicity above the
LCST. PNIPA-based sorbent have been successfully used in temperature-modulated
chromatography of biochemicals, in which the resolution and selectivity are
controlled via the column temperature. To obtain further insight into
the mechanisms governing this process, we have
measured the binding enthalpy between three model amino acids (valine, aspartic
acid and phenyl alanin) and PNIPA microgels. Equilibrium binding isotherms of
aspartic acid and valin showed enhanced binding with a temperature elevation
from 25oC to 37oC. On the other hand, phenyl alanin which
is more hydrophobic than valine showed a decrease in binding with a temperature
elevation. Isothermal Titration Calorimetry (ITC) measurements showed that the
binding of valine and phenyl alanin was endothermic, which implies an
entropy-driven binding mechanism. Contrary, the binding of aspartic acid was
exothermic, suggesting a mechanism based on interactions such as hydrogen
bonding. Differential Scanning Calorimetry (DSC) measurements detected changes
in the gel phase transition temperature. Our results show that a direct
measurment of the binding enthalpy gives further insight into the balance of
binding mechanisms between biochemicals and
PNIPA gel.
