Molecular Dynamics Simulations of Stimuli-Responsive Macromolecule in Various Solvents
Guofeng Wang, Ryan Martin, Daniel T. Mainz, Georgios Zamanakos,
Paul Miklis, Tahir Cagin, William A. Goddard III
Materials and Process Simulation Center, Beckman Institute, 139-74, California Institute of Technology, Pasadena, California 91125
A novel amphiphilic hybrid macromolecule has been built by Jean M. J. Frechet et al. [J. Am. Chem. Soc., 118, 3785-3786, (1996)]. This macromolecule has hydrophobic dendritic groups at the periphery of a hydrophilic polyethylene glycol (PEG) star. Light scattering experiments suggest that changing the solvent from THF (tetrahydrofuran) to methanol leads to large changes in structure. To study the response of these macromolecules to such variations in environment, we used molecular dynamics (MD) to predict structures and properties for the macromolecule in methanol and THF. These calculations used explicit solvent and periodic boundary conditions. The Frechet macromolecule has 2,761 atoms and we included also 26,892 methanol molecules or 13,140 THF molecules. We used the MSC MD program (MPSim) to carry out 200ps of NVT dynamic simulations at 300K. These results show that that in THF the Frechet macromolecule has a somewhat compact PEG core with the dendrimer extending outward into the solvent while in methanol the PEG tends to wrap around the dendrimer to bury it away from the solvent. We calculated scattering intensities expected from Small Angle Neutron Scattering (SANS) for comparisons to ongoing experiments by Diallo et al. These results validate the interpretations by Frechet et al.
The research project reported is supported by grants ARO-MURI, ARO-AASERT and ARO-DURIP.
Figure 1. Snapshot of macromolecule in THF.
Figure 2. Snapshot of macromolecule in methanol.
Figure 3. Simulated SANS