Conformation and Dynamics of Poly(N-isopropyl acrylamide) Trimers in Water: A Molecular Dynamics and Metadynamics Simulation Study

Abstract

Conformational features, structure, and dynamic properties of the trimer of poly(N-isopropyl acrylamide), (NIPAAm)(3), in aqueous solution at 293 and 323 K were investigated by a double simulation approach. The free energy behavior as a function of backbone conformation was obtained by metadynamics-umbrella sampling simulations. The structural characteristics, the intramolecular and water hydrogen bonding, and the torsional dynamics were analyzed by molecular dynamics simulations. The four stereoisomers of (NIPAAm)(3), representing syndiotactic, isotactic, and atactic sequences, were studied to highlight the tacticity effect on the system properties. The simulation results indicate that the experimentally observed lower hydrophilicity of isotactic poly(N-isopropyl acrylamide), in comparison with the syndiotactic one, is related to a lower conformational entropy. The atactic stereoisomers display the highest intramolecular hydrogen bond capability, at both studied temperatures, due to formation of hydrogen bonds between external amide groups. The mobility of the backbone in the syndiotactic trimer is more homogeneous than in other stereoisomers. The temperature increase was found mainly to affect the conformation of N-isopropyl amide side chains, and a structural rearrangement was observed for the atactic stereoisomers, in agreement with their experimental solution behavior. Simulation results are discussed in relation with available experimental data on solution properties and reactivity of poly(N-isopropyl acrylamide).

Conformation change of an isotactic poly (N-isopropylacrylamide) membrane: Molecular dynamics

“…We have found an average of 2.3 hydrogen bonds per configuration for the isotactic chain, which does not seem to be enough to explain its stabilization by rigidization. Moreover, we have found an average of 5.2 hydrogen bonds per frame for the syndiotactic chain (atactic trimers display the highest intramolecular hydrogen bond capability as compared to the syndiotactic and isotactic ones 26 ). Naively, this result seems odd since one expects isotactic chains to have consecutive amide groups closer than the syndiotactic chains.…”

“…5). This produces a gain of self-entropy, since large isotactic groups on the same side of the backbone have little space to move without overlapping (syndiotactic trimers own a more favorable conformational entropy than the isotactic ones 26 ). Conversely, the syndiotactic chain exposes its backbone while letting the side groups at the same side, since this arrangement is not sterically hindered.…”

“…Indeed, it has been shown that this property strongly affects the PNIPAM behavior. 23,25,26 On the one hand, short oligomers (∼37-mers) with high isotacticity are water insoluble at room temperature and slowly disperse at higher temperatures. 27 This insolubility suggests isotactic oligomers should show a globule structure at room temperature.…”

Photo and Thermo Dual‐Responsive Copolymer Surfaces for Efficient Cell Capture and Release

“…As demonstrated above, poly(SP‐ co ‐NiPAAm)‐modified surfaces exhibit enhanced efficiencies of cell capture and release in our design, probably on account of binary cooperative effect of two responsive units ,. It has been known that too much hydrophobic additives to poly(NiPAAm) may vanish its thermo responsiveness,, partially because the additives hinder the phase transition of those separated small poly(NiPAAm) pieces . Considering the SP/NiPAAm ratios may influence the enhancement, we prepared a series of smart surfaces modified by different SP/NiPAAm ratios from 0.5 % up to 7 %.…”

“…[29,30] It has been known that too much hydrophobic additives to poly(NiPAAm) may vanish its thermo responsiveness, [31,32] partially because the additives hinder the phase transition of those separated small poly(NiPAAm) pieces. [33] Considering the SP/NiPAAm ratios may influence the enhancement, we prepared a series of smart surfaces modified by different SP/NiPAAm ratios from 0.5 % up to 7 %. As shown in Figure 3, 1 % SP modified surfaces yield the maximum number of captured cells and also of released cells after UV/ 23 8C treatment.…”