Conformation and Dynamics of Poly(oxyethylene) in Benzene Solution: Solvent Effect from Molecular Dynamics Simulation

Spectroscopic and Theoretical Studies on Conformational Stability of Benzyl Methyl Ether

Conformer-selected electronic and vibrational spectra of benzyl methyl ether and its terminal methyl group-substituted derivative in a supersonic jet have been measured using ultraviolet (UV)-UV hole burning and fluorescence-detected infrared spectroscopy to investigate the conformational stability of flexible molecules. Various quantum chemical calculations as well as experimental observations reveal the coexistence of three conformers with different CCOC dihedral angles and side-chain orientations relative to the benzene ring plane. Vibrational analysis in the excited state with time-dependent density functional theory and IR simulations containing anharmonic coupling sufficiently reproduce the experimental results, suggesting that these three conformers can be distinguished into one gauche-conformer and two trans-ones with respect to the CCOC dihedral angle. We also observe that the gauche conformer exhibits higher-frequency CH₂ modes. The natural bond orbital analysis indicates that this phenomenon is attributed to the electron delocalization from the non-bonding orbitals and the C-O orbitals associated with the neighboring oxygen atom, which leads to a conformer dependence of the methylene C-H bond strength.

Molecular Dynamics Simulation of Conformational Transition and Frictional Performance Modulation of Densely Packed Self-Assembled Monolayers Based on Electrostatic Stimulation

Self-assembled monolayers (SAMs) terminated with functional end groups such as polyethylene glycols (PEG) have attracted considerable attention because of their unique and flexible structure that exhibits conformational transition under electrostatic stimulation. Molecular dynamics simulations are used to investigate the conformational transition and associated modulation of frictional performance of densely packed PEG-terminated SAMs subjected to electrical field stimulation. Previously reported empirical potentials and atomic charges were used to model the intrachain bonds and electrostatic and interchain interactions. Simulation results indicate that significant conformational transition is generated because of the electrostatic forces. Under positive electrical fields, PEG groups are compressed and twisted into the helical form, "gauche" state, whereas under negative electrical fields, PEG groups are stretched into the straight form, "all-trans" state. Such conformational transition may lead to substantial alteration of frictional response upon SAMs. By shallow penetration and sliding using a repulsive indenter, the SAMs under positive electrical fields exhibit a level of frictional response that is comparatively lower than those under zero and negative potentials, which may be attributed to reduced interchain space for deformation, limited conformational transition, and less energy absorption. The simulation results demonstrate that with appropriate selection of functional end groups attached to SAM backbone chains it is possible to modulate frictional performance of densely packed SAMs via electrostatic stimuli.

Molecular simulation studies of protein interactions with zwitterionic phosphorylcholine self-assembled monolayers in the presence of water

Molecular simulations were performed to study the interactions between a protein (lysozyme, LYZ) and phosphorylcholine-terminated self-assembled monolayers (PC-SAMs) in the presence of explicit water molecules and ions. The results show that the water molecules above the PC-SAM surface create a strong repulsive force on the protein as it approaches the surface. The structural and dynamic properties of the water molecules above the PC-SAM surface were analyzed to provide information regarding the role of hydration in surface resistance to protein adsorption. It can be seen from residence time dynamics that the water molecules immediately above the PC-SAM surface are significantly slowed down as compared to bulk water, suggesting that the PC-SAM surface generates a tightly bound, structured water layer around its head groups. Moreover, the orientational distribution and reorientational dynamics of the interfacial water molecules near the PC-SAM surface were found to have the ionic solvation nature of the PC head groups. These properties were also compared to those obtained previously for an oligo(ethylene glycol) (OEG) SAM system and bulk water.

Molecular dynamics simulations of amphiphilic graft copolymer molecules at a water/air interface

Fully atomistic molecular dynamics simulations of amphiphilic graft copolymer molecules have been performed at a range of surface concentrations at a water/air interface. These simulations are compared to experimental results from a corresponding system over a similar range of surface concentrations. Neutron reflectivity data calculated from the simulation trajectories agrees well with experimentally acquired profiles. In particular, excellent agreement in neutron reflectivity is found for lower surface concentration simulations. A simulation of a poly(ethylene oxide) (PEO) chain in aqueous solution has also been performed. This simulation allows the conformational behavior of the free PEO chain and those tethered to the interface in the previous simulations to be compared.

Detailed Structures and Mechanism of Polymer Solvation

In the present study, we simulated a model system, PE in biphenyl, to gain the insight into the detailed solvation structures and the molecular mechanism of polymer chain solvation. Using atomistic molecular dynamics (MD) simulation, it was found that when the biphenyl is far from PE chain or in the bulk, the dihedral angle of the two rings in the solvent molecule are approximately 32 degrees. But, the dihedral angel is about 27 degrees when the biphenyls are very close to the PE chain. In the first solvation shell, the orientation angle of the biphenyl long axis to the chain segment backbone was found to be enhanced around two values: approximately 0 and approximately 60 degrees. The detailed solvation structures found here include all dyad conformations (TT, TG, TG', GT, GG, GG', G'T, G'G, and G'G') and vary as a function of the distance between PE chain and biphenyls in the first solvation shell. The closer the the solvent molecule to the PE segment, the higher the TT conformation fraction response is. The other dyad conformations such as TG, GG', etc. undergo different decreases, respectively. This study shows that the solvation even in the Theta condition makes the overall size expansion or the chain stretched. Such a cooperative change was examined here and found not due to generating or losing a conformational state but due to a change in conformational distribution. This change occurs in the middle location of the chain instead of the chain end locations.

Structuring Effects and Hydration Phenomena in Poly(Ethylene Glycol)/Water Mixtures Investigated by Brillouin Scattering

Aqueous solutions of poly(ethylene glycol) (PEG) of mean molecular mass of 600 g/mol (PEG600) are investigated by Brillouin scattering technique. At high PEG content, a relaxation phenomenon is observed, which is related to a local rearrangement of the polymer structure where the interaction, via hydrogen bonding, with the solvent molecules plays a role. The obtained values of the relaxation times match the literature data very well for a fast relaxation time revealed by dielectric relaxation measurements in very similar mixtures. The calculated concentration behaviors of the excess adiabatic compressibility turns out in good agreement with the previous findings from ultrasonic measurements at 3 MHz. The observed minimum in the adiabatic compressibility is interpreted as the result of the interaction between water and the EO units of the PEG chain, which results in a structure tighter then that typical of bulk water and of pure PEG600. Such a hypothesis is supported by the observation that volume fraction value of about 0.3 coincides with the concentration value at which full hydration of EO units takes place. The observation that at the same concentration, the polymer coils start to overlap each other further supports the idea that the adiabatic compressibility behavior is monitoring the structural evolution of the mixture. However, similar results are obtained for largely different binary mixture which suggests caution in taking this conclusion too literally. In particular, the hypothesis that the occurrence of an extreme in the excess adiabatic compressibility could be simply originated by statistical effects and that further work is required for disentangling entropic contribution from effects of hetero-association and self-aggregation of one or both the components.

Evidences of Nonideal Mixing in Poly(ethylene Glycol)/Organic Solvent Mixtures by Brillouin Scattering

The concentration dependence of the hypersonic properties of solutions of poly(ethylene glycol) of mean molecular mass 600 g/mol (PEG600) in benzene and toluene has been investigated by Brillouin scattering. The two solvents are very similar in structure and chemical properties, but while benzene is nonpolar, toluene possess a modest dipole. In both solvents a high-frequency relaxation process has been observed at high concentrations which has been assigned to conformational rearrangements of the polymeric chains, triggered by reorientation of the side groups. In both cases, the concentration dependence of the adiabatic compressibility deviates significantly from linearity, indicating the existence of nonideal mixing phenomena driven by aggregation processes taking place in the systems. However, there is no temperature dependence for solutions of PEG600 in benzene; on the contrary, the results obtained for solutions of PEG600 in toluene are noticeably dependent on the temperature. The comparison of the experimental data with the results of previous experiments on similar systems allows a general picture for weakly interacting mixtures of hydrogen-bonded systems and organic solvents to be developed. In particular, in the presence of a nonpolar solvent molecule the local structure of the mixture is dominated by solute self-association processes and any resulting solute-solvent correlation is barely induced by excluded volume effects. At high enough dilution the self-aggregation of solute molecules produces a variety of new local topologies that cannot be observed in bulk solute, and as a consequence, the concentration evolution of the system is too rich to be described in terms of a linear combination of a few components over the whole concentration range. The situation seems to be simpler for the polar toluene solvent molecules, where a three-component model seems able to fit the experimental concentration dependence of the hypersonic velocity. This result is interpreted to imply that the interaction between the solvent dipoles and the active sites of the solute produces a relatively stable heterocoordination, while the relevance of self-association is partially reduced.

Strong repulsive forces between protein and oligo (ethylene glycol) self-assembled monolayers: a molecular simulation study

Restrained molecular dynamics simulations were performed to study the interaction forces of a protein with the self-assembled monolayers (SAMs) of S(CH2)4(EG)4OH, S(CH2)11OH, and S(CH2)11CH3 in the presence of water molecules. The force-distance curves were calculated by fixing the center of mass of the protein at several separation distances from the SAM surface. Simulation results show that the relative strength of repulsive force acting on the protein is in the decreasing order of OEG-SAMs > OH-SAMs > CH3-SAMs. The force contributions from SAMs and water molecules, the structural and dynamic behavior of hydration water, and the flexibility and conformation state of SAMs were also examined to study how water structure at the interface and SAM flexibility affect the forces exerted on the protein. Results show that a tightly bound water layer adjacent to the OEG-SAMs is mainly responsible for the large repulsive hydration force.

Molecular simulation study of water interactions with oligo (ethylene glycol)-terminated alkanethiol self-assembled monolayers

Molecular simulations were performed to study a system consisting of protein (e.g., lysozyme) and self-assembled monolayers (SAMs) terminating with different chemical groups in the presence of explicit water molecules and ions. Mixed SAMs of oligo (ethylene glycol) [S(CH2)4(OCH2CH2)4OH, (OEG)] and hydroxyl-terminated SAMs [S(CH2)4OH] with a mole fraction of OEG at chiOEG = 0.2, 0.5, 0.8, and 1.0 were used in this study. In addition, methyl-terminated SAMs [S(CH2)11CH3] were also studied for comparison. The structural and dynamic behavior of hydration water, the flexibility and conformation state of SAMs, and the orientation and conformation of protein were examined. Simulation results were compared with those of experiments. It appears that there is a correlation between OEG surface resistance to protein adsorption and the surface density of OEG chains, which leads to a large number of tightly bound water molecules around OEG chains and the rapid mobility of hydrated SAM chains.