  Certain sequences of peptoid polymers (synthetic analogs of peptides)
assemble into bilayer nanosheets via a nonequilibrium assembly pathway of
adsorption, compression, and collapse at an air-water interface. As with other
large-scale dynamic processes in biology and materials science, understanding
the details of this supramolecular assembly process requires a modeling
approach that captures behavior on a wide range of length and time scales, from
those on which individual sidechains fluctuate to those on which assemblies of
polymers evolve. Here we demonstrate that a new coarse-grained modeling
approach is accurate and computationally efficient enough to do so. Our
approach uses only a minimal number of coarse-grained sites, but retains
independently fluctuating orientational degrees of freedom for each site. These
orientational degrees of freedom allow us to accurately parameterize both
bonded and nonbonded interactions, and to generate all-atom configurations with
sufficient accuracy to perform atomic scattering calculations and to interface
with all-atom simulations. We have used this approach to reproduce all
available experimental X-ray scattering spectra (for stacked nanosheets, and
for peptoids adsorbed at air-water interfaces and in solution), in order to
resolve the microscopic, real-space structures responsible for these
Fourier-space features. By interfacing with all-atom simulations, we have also
laid the foundations for future multiscale simulations of sequence-specific
polymers that communicate in both directions across scales.
