(741c) Molecular Dynamics of Volatile Organic Compounds in the Epicuticle of Plant Epidermal Cells

Molecular
Dynamics of Volatile Organic Compounds in the Epicuticle of Plant Epidermal
Cells
Shaunak
Ray¹*, Natalia Dudareva², John A. Morgan^1,2
1Davidson School of Chemical Engineering, Purdue University,
West Lafayette, Indiana 47907
²Department of Biochemistry, Purdue University, West Lafayette,
Indiana 47907

Plants
produce a diverse set of volatile organic compounds (VOCs) that serve roles in
the attraction of pollinators and seed dispersers, defense against pathogens
and herbivores, and in plant-plant signaling. Due to substantial role of VOCs
in plant-environment interactions, the study of volatile translocation through
subcellular regimes has raised questions into the transport mechanisms
governing observed volatile emission rates. The outermost layer of the plant
epidermis is the cuticle which is the last barrier controlling the transport of
volatiles to the atmosphere. Transport of volatiles through the cuticle layer
is hypothesized to be via simple diffusion. Wax components in the epicuticular
region self-assemble into a multiphase system of crystalline and amorphous
regions, where the crystalline flux waxes exclude diffusive flux. Petal waxes are
largely composed of long chain primary alcohols and n-alkanes, and we
hypothesize that the modification of ­the composition of the waxes significantly
affects the cuticle ultrastructure and polymorphism such that volatile diffusivity
is affected. To ascertain these changes, molecular dynamics simulations were
conducted for a binary model wax system consisting of a primary alcohol,
1-docosanol (C₂₂H₄₅OH), and an n-alkane,
tetracosane (C₂₄H₅₀) in differing proportions in a
canonical (NVT) ensemble. Molecular diffusion of floral VOCs was simulated
through the model wax structures over 500 ps, and diffusion coefficients were
obtained from their mean-square displacements yielding values in agreement to
experimental values. The molecular trajectories of the model wax and volatile
compounds indicate strong correlations between volatile diffusion and model
parameters such as aliphatic chain mobility and volatile lipophilicity. Additionally,
solvation free energies are calculated to determine the partitioning of
volatile compounds from the aqueous phase into the hydrophobic wax layers. The
parameters for the VOC diffusion model are thus described at the molecular
level, and used to quantify the barrier properties of the plant epicuticle in
relation to floral scent emission. Determination of epicuticular resistance to
VOC emission results in a further developed mechanistic model of subcellular
volatile translocation and serves in the prediction of volatile emission phenotypes
in flower genetic variants with altered cuticle compositions.