NMR Studies of Thermo-responsive Behavior of an Amphiphilic Poly(asparagine) Derivative in Water.

The thermo-responsive behavior of a unique biocompatible polymer, poly(-substituted α/β-asparagine) derivative (PAD), has been studied with several NMR methods. The H and C solution NMR measurements of the PAD in DMSO- were used to investigate the isolated polymer and perform spectral assignments. By systematic addition of DO we have tracked structural changes due to aggregation and observed contraction of hydrophilic side chains. Solution and cross polarization / magic angle spinning (CP/MAS) C NMR approaches were implemented to investigate the aggregates of the PAD aqueous solution during the liquid to gel transition as the temperature was increased. At temperatures near 20 °C, all of the peaks from the PAD were observed in the C CP/MAS and C solution NMR spectra, indicating the presence of polymer chain nodes. Increasing the temperature to 40 °C resulted in a partial disentanglement of the nodes due to thermal agitation and further heating resulted in little to no additional structural changes. Deuterium and two-dimensional relaxation spectroscopies using an inverse Laplace transform, were also implemented to monitor the water-PAD interaction during the phase transition. At temperatures near 20 °C the dynamical characteristics of water were manifested into one peak in the deuterium map. Increasing the temperature to 40 °C resulted in several distinguishable reservoirs of water with different dynamical characteristics. The observation of several reservoirs of water at the temperature of gel formation at 40 °C is consistent with a physical picture of a gel involving a network of interconnected polymer chains trapping a fluid. Further increase in temperature to 70 °C resulted in two non-exchanging water reservoirs probed by deuterium measurements.