Separation of Enantiomers through Local Vorticity: A Screw Model Mechanism

We present a model to explain the
mechanism behind enantiomeric
separation under either shear flow or local rotational motion in a
fluid. Local vorticity of the fluid imparts molecular rotation that
couples to translational motion, sending enantiomers in opposite directions.
Translation–rotation coupling of enantiomers is explored using
the molecular hydrodynamic resistance tensor, and a molecular equivalent
of the pitch of a screw is introduced to describe the degree of translation–rotation
coupling. Molecular pitch is a structural feature of the molecules
and can be easily computed, allowing rapid estimation of the pitch
of 85 druglike molecules. Simulations of model enantiomers in a range
of fluids such as Λ- and Δ-[Ru­(bpy)3]­Cl2 in water and (R, R)- and
(S, S)-atorvastatin in methanol
support predictions made using molecular pitch values. A competition
model and continuum drift–diffusion equations are developed
to predict separation of realistic racemic mixtures. We find that
enantiomeric separation on a centimeter length scale can be achieved
in hours, using experimentally achievable vorticities. Additionally,
we find that certain achiral objects can also exhibit a nonzero molecular
pitch.