Differences in pressure and temperature transitions of proteins and polymer gels

Abstract

Pressure-driven and temperature-driven transitions of two thermoresponsive polymers, poly(N-isopropylacrylamide) (pNIPAM) and poly(N-vinylisobutyramide) (pNVIBA)), in both a soluble linear polymer form and a cross-linked hydro-gel form, were examined by a dynamic light-scattering method and direct microscopic observation, respectively. Their behavior was compared with that of protein systems. Changes in some characteristic parameters in the time-intensity correlation functions of dynamic light-scattering measurement of aqueous solutions of pNIPAM at various pressures and temperatures showed no essential differences during temperature and pressure scanning and, as a whole, the motions of polymers in aqueous solutions were similar in two types of transitions until chain shrinkage occurred. The gels (cross-linked polymer gels) prepared from the thermoresponsive polymers also showed similar volume transitions responding to the pressure and temperature increase. In temperature transitions, however, gels showed drastic volume shrinkage with loss of transparency, while pressure-induced transition showed a slow recovery of transparency while keeping the size, after first transient drastic volume shrinkage with loss of transparency. At a temperature slightly higher than the transition under atmospheric temperature, socalled reentry of the volume change and recovery of the transparency were observed during the pressure-increasing process, which implies much smaller aggregation or non-aggregated collapsed polymer chains in the gel at higher pressures, indicating a certain mechanistic difference of the dehydration processes induced by temperature and pressure.

Molecular insights on poly(N-isopropylacrylamide) coil-to-globule transition induced by pressure

“…39,40 The effect of size re-increasing by applying pressure is found to be independent of the polymer architecture. 41 Since pressure seems to always favor the formation of more hydrated states, 40 the presence of a genuine coil-to-globule transition has been questioned at high pressures. 42 Altogether these findings show that pressure increases the water affinity of PNIPAM and plays an antagonistic effect with respect to temperature, 39 with a balance that depends on the specific (P,T) conditions.…”

Characterization of free, restricted, and entrapped water environments in poly(N‐isopropyl acrylamide) hydrogels via1H HRMAS PFG NMRspectroscopy

“…Stimuli‐responsive hydrogels are capable of changing properties when exposed to external stimuli including variations in temperature, pressure, solvent composition, pH, ionic strength, electrical current, or exposure to light . Typically the introduction of the physiochemical stimuli produce changes in the degree of swelling within the gel by either increasing/decreasing the amount of water or solvent incorporated in the polymer matrix, or by inducing a complete phase transition from a fully hydrated or soluble polymer phase to a dehydrated or insoluble micro‐phase separated state .…”

Circular dichroism and UV‐resonance Raman investigation of the temperature dependence of the conformations of linear and cyclic elastin

“…Over the years, considerable effort has been expended to elucidate the mechanism of elastin's unique elasticity 3–13, 16–25. The elastic properties are thought to be associated with or a consequence of elastin's unusual phase transition behavior, which closely resembles that of the thermal volume phase transition behavior of poly( N ‐isopropylacryamide) (PNIPAM) 26–38. PNIPAM polymers contain both hydrophilic and hydrophobic regions, whose exposure to the aqueous medium may change as the polymer collapses at high temperature.…”