Thermal Behavior and Local Structural Organization in Curcumin Polymorphs’ Bulk Phase: A Molecular Dynamics Simulation Analysis

Curcumin (CUR), a
bioactive compound with known polymorphism,
exhibits
distinct conformational and thermophysical properties across its three
crystalline forms. In this study, we employ molecular dynamics simulations
to investigate the thermal behavior, local structural organization,
and polymorph-specific stability of CUR in the bulk phase. We first
evaluate four widely used classical force fields (OPLS-AA, CGENFF,
GAFF2, and GROMOS) against experimental melting points, densities,
and conformational preferences, identifying OPLS-AA as the most suitable
one. Through targeted reparametrization of intramolecular dihedral
angles based on DFT benchmarks, we significantly improve the ability
of this force field to reproduce conformational distributions and
melting transitions. Using the refined model, we characterize the
temperature dependence of several structural observables, including
local density (via Voronoi tessellation), nearest-neighbor distributions,
pair interaction energies, hydrogen bonding, and molecular orientation.
Our results reveal that conformational transitions, packing rearrangements,
and fluctuations occur cooperatively near polymorph-specific temperatures,
often beginning with disrupted π–π stacking and
propagating through the lattice. Notably, cooling simulations fail
to induce recrystallization, resulting in amorphous states. This comprehensive
analysis highlights the critical interplay between molecular conformation,
packing, and directional interactions in determining CUR’s
polymorphic behavior, providing a mechanistic foundation for controlling
phase transitions in flexible molecular solids.