A Molecular Dynamics Simulation Study of Two Dipeptide Based Molecular Micelles: Effect of Amino Acid Order.

Molecular dynamics (MD) simulations were used to compare the structures of the chiral molecular micelles (MM) poly-(sodium undecyl-(L,L)-leucine-valine) (poly(SULV)) and poly-(sodium undecyl-(L,L)-valine-leucine) (poly (SUVL)). Both MM contained polymerized surfactant monomers tenninated by chiral dipeptide headgroups. The study was undertaken to investigate why poly(SULV) is generally a better chiral selector compared to poly(SUVL) in electrokinetic chromatography separations. When comparing poly(SULV) to poly(SUVL), poly(SULV) had the more conformational flexible dipeptide headgroup and hydrogen bond analyses revealed that the poly(SULV) headgroup conformation allowed a larger number of intramolecular hydrogen bonds to form between monomer chains. In addition, a larger number of water molecules surrounded the chiral centers of the poly(SULV) molecular micelle. Poly(SULV) was also found to have a larger solvent accessible surface area (SASA) than poly(SUVL) and fluctuations in the poly(SULV) SASA during the MD simulation allowed dynamic monomer chain motions expected to be important in chiral recognition to be identified. Finally, approximately 50% of the Na+ counterions were found in the first three solvation shells surrounding both MM, with the remainder located in the bulk. Overall the MD simulations point to both greater headgroup flexibility and solvent and analyte access to the chiral centers of the dipeptide headgroup as factors contributing to the enhanced chiral selectivity observed with poly(SULV).