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-N ε-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.

From Nucleation to Engineering of Crystalline Architectures at Air−Liquid Interfaces

“…Another example of supercell formation, as a result of binding of Cu 2+ ions to glycine headgroups, and thus indicative of an ordered arrangement of the Cu 2+ ions, was found in the monolayer structures of some enantiomerically pure R-amino acid amphiphiles on copper acetate solution. 62,63 The absence of a supercell in monolayer structures of longchain carboxylic acids and amino acids bound to metal ions at the air-solution interface does not imply that the metal ions are disordered. Bragg rod data analyses appear to reveal the presence of ordered binding of K + to the amino acid, 57 and Ca 2+ -ion to arachidic acid.…”

“…Refined Cd positions in the supercell (Figure b) gave a very good fit to the observed GIXD data (Figure c). Another example of supercell formation, as a result of binding of Cu 2+ ions to glycine headgroups, and thus indicative of an ordered arrangement of the Cu 2+ ions, was found in the monolayer structures of some enantiomerically pure α-amino acid amphiphiles on copper acetate solution. ,

6 Arachidic acid, C 19 H 39 COOH, monolayer over a CdCl 2 subphase (pH 8.8) at 9 °C. (a) GIXD peaks and ( h , k ) reflection assignment.

…”

Stereochemical studies on chiral systems in two dimensions

“…14 Reasoning on the above conclusions, the diastereoisomeric interactions between long-chain copper-a-amino acid complexes, at the air-solution interface, and watersoluble a-amino acid complexes present in the aqueous subphase were investigated. 33 The Cu 2 ions bind both to an a-amino acid head-group of the insoluble amphiphile and to the water-soluble a-amino acid molecules from the aqueous subphase. Langmuir films of enantiomerically pure (S)-or (R)-and racemic (R,S)-N e -palmitoyllysine and a-aminostearic were investigated on copper acetate solutions without and in the presence of enantiomerically pure (S)-or (R)-alanine,-serine and-valine [hereafter labeled R or S].…”

“…The p-A isotherms for the systems of the same absolute configuration, say amphiphile S and solute S, are always more expanded as compared to those of the diastereoisomeric molecules S and R. These diastereoisomers assume different packing arrangements, as demonstrated by GIXD measurements of the films directly at the air-solution interface. 33 Furthermore, on the basis of comparative x-ray photoelectron spectroscopic (XPS) studies of the films of the two types of hetero-complexes and 3-D crystals of (S)-serine and (R,S)-serine Cu complexes, the configuration of the a-amino acid ligands around the Cu 2 ions in these complexes indicated a structural similarity. In the thin films the S-Cu-S complex displays a syn configuration whereas the S-Cu-R complex adopts an anti configuration (Scheme 1).…”

Guest Intercalation at Corrugated Surface of Host Monolayer Crystal on Water: Cholesteryl-l-Glutamate and Water-Soluble Amino Acids

“…This amphiphile was expected to self-assemble as a monolayer at the air−water interface, generating a corrugated surface with hydrophobic chiral grooves exposed to the water, appropriate for stereospecific inclusion of water-soluble hydrophobic amino acids. Furthermore, these architectures might induce thin film crystallization of the water-soluble amino acids at the periphery of the crystalline clusters by line epitaxy 1 (a) Cholesteryl- l - glutamate (CLG), (b) π − A isotherms at 5 °C for a film of CLG spread on different subphases, water, and saturated solutions of amino acids at 0 °C of dl- Leu (8 g/L), d- Leu (22 g/L), l - Leu (22 g/L), l - Phe (20 g/L), and l - Met (18 g/L), (c) GIXD patterns obtained in the uncompressed state from a monolayer of CLG spread on water (black), l - Met (yellow), l - Leu (blue), l - Phe (green).…”