Chiral separation in modified silica nanotube membranes: A molecular simulation study

Novel functionalized nanotube membranes were recently developed and used to efficiently separate a chiral drug from its racemic mixture [S.B. Lee, D.T. Mitchell, L. Trofin, T.K. Nevanen, H. Soderlund, C.R. Martin, Science 296 (2002) 2198]. Enantiomeric separation in these materials strongly relates to modifier choices and the interplay with the nanopore confinement and substrate-modifier interactions. By means of molecular simulations we propose that the enantioselectivity of such membranes can be improved in a bio-inspired way. We use molecular dynamics simulations to evaluate the capability of a modified silica nanotube for enantiomeric separation of two amino acids, R- and S-2-phenylglycine. This smart nanotube is functionalized as an artificial protein channel in cell membranes. The biomimicry is performed through attaching functional residues (Arg, Glu, Asp) into the nanotube. Simulations indicate that the selective transport of one of the enantiomers (S-) inside the modified channel is strongly affected by presence of a special electrostatic field inside the channel. The mechanism of enantioselective passage depends on the internal degrees of freedom of the attached residues and interactions of phenylglycine molecules with these residues. The translational-rotational motion of chiral molecules as well as their average dipole orientation is responsible for selective chiral transport inside the nanotube. It is remarkable how configuration of the immobilized residues enhances the enantiomeric separation of the functionalized nanotube. As an immediate application, this study would help us design more efficient, nature-inspired selective chiral membranes that are able to separate enantiomers of chiral species. (C) 2007 Elsevier B.V. All rights reserved.