We
propose the first model of a polypeptide chain based on a hybrid-particle
field approach. The intramolecular potential is built on a two-bead
coarse grain mapping for each amino acid. We employ a combined potential
for the bending and the torsional degrees of freedom that ensures
the stabilization of secondary structure elements in the conformational
space of the polypeptide. The electrostatic dipoles associated with
the peptide bonds of the main chain are reconstructed by a topological
procedure. The intermolecular interactions comprising both the solute
and the explicit solvent are treated by a density functional-based
mean-field potential. Molecular dynamics simulations on a series of
test systems show how the model here introduced is able to capture
all the main features of polypeptides. In particular, homopolymers
of different lengths yield a complex folding phase diagram, covering
from the collapsed to swollen state. Moreover, simulations on models
of a four-helix bundle and of an alpha + beta peptide evidence how
the collapse of the hydrophobic core drives the appearance of both
folded motifs and the stabilization of tertiary or quaternary assemblies.
Finally, the polypeptide model is able to structurally respond to
the environmental changes caused by the presence of a lipid bilayer