Use of Diastereomeric Interactions To Probe the Inplane Attachment of Water-Soluble Molecules to the Polar Head Groups of Langmuir Films of Cu−α-Amino Acid Complexes

Abstract

To provide more direct information on the role played by “tailor-made” auxiliary molecules in the early stages of crystal nucleation, the interplay between clusters of polar headgroups of monolayers of the copper complexes S−Cu−S‘ and S−Cu−R‘ and water-soluble copper complexes S‘−Cu−S‘ and R‘−Cu−R‘ were investigated [where S represents enantiomerically pure (S)-palmitoyl-Nε-lysine, and S‘ and R‘ represent chiral resolved (S) and (R) forms of alanine, serine, or valine]. The different monolayers were formed by spreading the amphiphilic (R) or (S) α-amino acid on an aqueous solution of copper acetate followed by injection of the water-soluble (S‘) or (R‘) α-amino acid into the subphase. The surface pressure−molecular area isotherms of the Langmuir monolayers of the two type of complexes (S−Cu−S‘ and S−Cu−R‘) are different, the former being substantially more expanded. The polar headgroups of the S−Cu−S‘ and of the S−Cu−R‘ monolayers transferred onto a solid support assume a trans and cis configuration, respectively, according to comparative X-ray photoelectron spectroscopy (XPS) studies with appropriate cis and trans three-dimensional (3-D) α-amino acid Cu complexes. A grazing incidence X-ray diffraction (GIXD) analysis demonstrated that the S−Cu−S‘ and S−Cu−R‘ monolayers have different 2-D crystal structures, in keeping with the XPS results. A model is presented suggesting that the water-soluble S‘−Cu−S‘ copper complexes are enantioselectively bound to the periphery of the domains of the cis S−Cu−S‘ monolayers, but not to the domains of the trans S−Cu−R‘ monolayers. By symmetry, the same principal holds for the monolayers and water soluble copper complexes of α-amino acids of the opposite handedness.