Xianghong Qian

Effects of Salt on the Lower Critical Solution Temperature of Poly (N-Isopropylacrylamide)

Classical molecular dynamics simulations were performed to investigate the effects of salt on the lower critical solution temperature (LCST) of Poly (N-isopropylacrylamide) (PNIPAM). PNIPAM is often studied as a protein proxy due to the presence of a peptide bond in its monomer unit. PNIPAM is a temperature sensitive polymer which exhibits hydrophobic-hydrophilic phase transition at its LCST. The presence of salt in the solution will shift its LCST, typically to a lower temperature. This LCST shift follows the so-called Hofmeister series. Molecular dynamics (MD) simulations of PNIPAM in 1 M of NaCl, NaBr, NaI, and KCl were carried out to elucidate the effects of different salt on LCST and protein stability. Our results suggest that direct interactions between the salt cations and the polymer play a critical role in the shift of LCST and subsequently on protein stability. Further, cations have a much stronger affinity with the polymer, whereas anions bind weakly with the polymer. Moreover, the cation-polymer binding affinity is inversely correlated with the cation-anion contact pair association constant in solution.

Magnetically Activated Micromixers for Separation Membranes

Presented here is a radically novel approach to reduce concentration polarization and, potentially, also fouling by colloids present in aqueous feeds: magnetically responsive micromixing membranes. Hydrophilic polymer chains, poly(2-hydroxyethyl methacrylate) (PHEMA), were grafted via controlled surface-initiated atom transfer radical polymerization (SI-ATRP) on the surface of polyamide composite nanofiltration (NF) membranes and then end-capped with superparamagnetic iron oxide magnetite (Fe(3)O(4)) nanoparticles. The results of all functionalization steps, that is, bromide ATRP initiator immobilization, SI-ATRP, conversion of PHEMA end groups from bromide to amine, and carboxyl-functional Fe(3)O(4) nanoparticle immobilization via peptide coupling, have been confirmed by X-ray photoelectron spectroscopy (XPS) and field emission scanning electron microscopy (FESEM). These nanoparticles experience a magnetic force as well as a torque under an oscillating external magnetic field. It has been shown, using particle image velocimetry (PIV), that the resulting movement of the polymer brushes at certain magnetic field frequencies induces mixing directly above the membrane surface. Furthermore, it was demonstrated that with such membranes the NF performance could significantly be improved (increase of flux and salt rejection) by an oscillating magnetic field, which can be explained by a reduced concentration polarization in the boundary layer. However, the proof-of-concept presented here for the active alteration of macroscopic flow via surface-anchored micromixers based on polymer-nanoparticle conjugates has much broader implications.

Mechanisms and Energetics for Brønsted Acid-Catalyzed Glucose Condensation, Dehydration and Isomerization Reactions

The mechanisms and energetics for Brønsted acid-catalyzed glucose condensation, dehydration and isomerization reactions were discussed based on our earlier CPMD–MTD simulation results. Glucose condensation reaction is initiated by the protonation of C1–OH, whereas both dehydration and isomerization reactions are initiated by the protonation of C2–OH to form a common 5-member ring intermediate. Glucose dehydration to form HMF occurs via the direct cyclic mechanism, rather than via the open chain mechanism converting glucose to fructose then to HMF. Fructose is formed via a 1,2 hydride shift process following the formation of 5-member ring intermediate. The barriers for Brønsted acid-catalyzed glucose reactions are largely solvent induced due to the competition for proton from the solvent molecules.

Acidic Sugar Degradation Pathways

Ab initio molecular dynamics (MD) simulations were employed to elucidate xylose and glucose degradation pathways. In the case of xylose, a 2,5-anhydride intermediate was observed leading to the formation of furfural through elimination of water. This pathway agrees with one of the mechanisms proposed in the literature in that no open chain intermediates were found. In the case of glucose, a series of intermediates were observed before forming the 2,5-anhydride intermediate that eventually leads to hydroxymethylfurfural (HMF). One of these intermediates was a very short-lived open-chain form. Furthermore, two novel side-reaction pathways were identified, which lead to degradation products other than HMF.

Acidic sugar degradation pathways : An ab initio molecular dynamics study

Ab initio molecular dynamics (MD) simulations were employed to elucidate xylose and glucose degradation pathways. In the case of xylose, a 2,5-anhydride intermediate was observed leading to the formation of furfural through elimination of water. This pathway agrees with one of the mechanisms proposed in the literature in that no open chain intermediates were found. In the case of glucose, a series of intermediates were observed before forming the 2,5-anhydride intermediate that eventually leads to hydroxymethylfurfural (HMF). One of these intermediates was a very short-lived open-chain form. Furthermore, two novel side-reaction pathways were identified, which lead to degradation products other than HMF.

The effects of acetate anion on cellulose dissolution and reaction in imidazolium ionic liquids.

Quantum mechanical calculations were carried out to determine the mechanisms for the superiority of the imidazolium acetate-based ionic liquids to the corresponding chloride-based ionic liquids. Our results indicate that the imidazolium cation can react with the acetate anion to generate a carbene, a highly reactive intermediate. The carbene produced then reacts with cellulose to facilitate its dissolution in the ionic liquid solvents in addition to the stronger hydrogen bonds formed between the acetate anion and the hydroxyl groups on cellulose. The mechanisms for the imidazolium cation and acetate anion reactions involve the initial ion pairing of the cation and anion via hydrogen bonding and electrostatic interactions. The hydrogen bond formed between the C2-H on the imidazolium cation and COO(-) of the anion facilitates the transfer of the H(+) to the anion to form a carbene intermediate.

Free energy landscape for glucose condensation reactions.

Ab initio molecular dynamics and metadynamics simulations were used to determine the free energy surfaces (FES) for the acid catalyzed β-D-glucose condensation reaction. Protonation of C1-OH on the β-D-glucose, breakage of the C1-O1 bond, and the formation of C1 carbocation is the rate-limiting step. The effects of solvent on the reaction were investigated by determining the FES both in the absence and presence of solvent water. It was found that water played a critical role in these reactions. The reaction barrier for the proton-catalyzed glucose condensation reaction is solvent induced because of protons high affinity for water. During these simulations, β-D-glucose conversion to α-d-glucose process via the C1 carbocation was also observed. The associated free energy change and activation barrier for this reaction were determined.

The Effects of Water on β-d-Xylose Condensation Reactions

Car-Parrinello-based ab initio molecular dynamics simulations (CPMD) combined with metadynamics (MTD) simulations were used to determine the reaction energetics for the beta-D-xylose condensation reaction to form beta-1,4-linked xylobiose in a dilute acid solution. Protonation of the hydroxyl group on the xylose molecule and the subsequent breaking of the C-O bond were found to be the rate-limiting step during the xylose condensation reaction. Water and water structure was found to play a critical role in these reactions due to the protons high affinity for water molecules. The reaction free energy and reaction barrier were determined using CPMD-MTD. We found that solvent reorganization due to proton partial desolvation must be taken into account in order to obtain the correct reaction activation energy. Our calculated reaction free energy and reaction activation energy compare well with available experimental results.

Specificity in cationic interaction with poly(N-isopropylacrylamide).

Classical molecular dynamics (MD) simulations were conducted for PNIPAM in 1 M monovalent alkali chloride salt solutions as well as in 0.5 M divalent Mg(2+) and Ca(2+) chloride salt solutions. It was found that the strength for the direct alkali ion-amide O binding is strongly correlated with the size of the ionic radius. The smallest Li(+) ion binds strongest to amide O, and the largest Cs(+) ion has the weakest interaction with the amide bond. For the divalent Mg(2+) and Ca(2+) ions, their interactions with the amide bond are weak and appear to be mediated by the water molecules, particularly in the case of Mg(2+), resulting from their strong hydration. The direct binding between the cations and amide O requires partial desovlation of the ions that is energetically unfavorable for Mg(2+) and also to a great extent for Ca(2+). The higher cation charge makes the electrostatic interaction more favorable but the dehydration process less favorable. This competition between electrostatic interaction and the dehydration process largely dictates whether the direct binding between the cation and amide O is energetically preferred or not. For monovalent alkali ions, it is energetically preferred to bind directly with the amide O. Moreover, Li(+) ion is also found to associate strongly with the hydrophobic residues on PNIPAM.

Glucose isomerization to fructose from ab initio molecular dynamics simulations.

Car-Parrinello molecular dynamics simulations (CPMD) coupled with metadynamics (MTD) simulations were conducted to investigate glucose isomerization to fructose in acidic aqueous solution. Glucose to fructose isomerization is initiated by protonation of the C2-OH and the formation of a furanose aldehyde intermediate. Fructose is produced via a hydride transfer from C2 to C1 on the furanose aldehyde followed by the rehydration of the C2 carbocation. Hydride 1,2 shift to form a C2 carbocation is an energetically favorable process but the barrier is relatively high at around 35 kcal/mol. The final step during glucose to fructose isomerization involves the rehydration of the C2 carbocation with an estimated barrier of 25 kcal/mol from our CPMD-MTD simulations.