Abstract
Lignins are hydrophobic, branched polymers that regulate water conduction and provide
protection against chemical and biological degradation in plant cell walls. Lignins also form
a residual barrier to effective hydrolysis of plant biomass pretreated at elevated temperatures
in cellulosic ethanol production. Here, the temperature-dependent structure and dynamics of
individual softwood lignin polymers in aqueous solution are examined using extensive (17ms)
molecular dynamics simulations. With decreasing temperature the lignins are found to transition
from mobile, extended to glassy, compact states. The polymers are comprised of blobs,
inside which the radius of gyration of a polymer segment is a power-law function of the number
of monomers comprising it. In the low temperature states the blobs are inter-permeable, the
polymer does not conform to Zimm/Stockmayer theory, and branching does not lead to reduction
of the polymer size, the radius of gyration being instead determined by shape anisotropy.
At high temperatures the blobs become spatially separated leading to a fractal crumpled globule
form. The low-temperature collapse is thermodynamically driven by the increase of the
translational entropy and density fluctuations of water molecules removed from the hydration
shell , thus distinguishing lignin collapse from enthalpically driven coil-globule polymer transitions
and providing a thermodynamic role of hydration water density fluctuations in driving
hydrophobic polymer collapse. Although hydrophobic, lignin is wetted, leading to locallyenhanced
chain dynamics of solvent-exposed monomers. The detailed characterization obtained
here provides insight at atomic detail into processes relevant to biomass pretreatment
for cellulosic ethanol production and general polymer coil-globule transition phenomena.
