Separation of Enantiomers and Racemate Formation in Two-Dimensional Crystals at the Water Surface from Racemic α-Amino Acid Amphiphiles:  Design and Structure

Abstract

Studies are presented on the two-dimensional (2-D) crystalline packing arrangements of enantiomerically pure and racemic α-amino acid RHC(NH3 +)CO2 - monolayers on water and on glycine aqueous solutions, as determined by synchrotron grazing incidence X-ray diffraction. The amphiphiles have been designed such that their racemic mixtures form 2-D crystals which are either heterochiral (for R = C n H2 n +1−, n = 10, 12, 16) due to the tendency for herringbone chain arrangements via glide symmetry or homochiral (for R = C n H2 n +1CONH(CH2)4−, n = 11, 17, 21) by virtue of hydrogen bonding by translation of the amide group in the chains leading to a spontaneous separation into islands of opposite chirality. The two different crystalline motifs led to a correlation between their packing arrangements and induced oriented nucleation of 3-D crystals of α-glycine by these monolayers. The relevance of the present results to the possibility of ordering and spontaneous segregation of racemates of the natural hydrophobic α-amino acids at the air-solution interface is discussed.

Oriented Crystalline Thin Films of Tetracosanedioic Acid and Its Metal Salts at the Air-Aqueous Solution Interface

“…These very small crystals are generally unstable due to their high surface tension, and thus have a tendency to transform to larger particles by Oswald ripening. In recent years, quantum dots were prepared mainly in matrices and also as self-assembled structures, such as reverse micelles, [10] zeolites, [11] polymers, [12] self-assembled monolayers, [13] biopolymers, [14] layered metal phosphonates, [15] and Langmuir±Blodgett (LB) films. [16,17] In the preceding communication, [18] we reported that spreading of tetracosanedioic acid HOOC(CH 2 ) 22 COOH (C 24 diacid) on surfaces of aqueous solutions containing di- …”

Metal Ion Complexation: A Route to 2 D Templates?

“…[23] Tailoring the chemical composition along the alkyl chain allows the patterning of organic monolayers on the nanometer scale, [24] and this has also been shown for systems at the liquid/air interface, where the molecules lie with their long axes parallel to the substrate. [25,26] Self-assembly has also been used to directly form a nanotemplate that does not require further manipulation by an SPM probe. Based on metal complexation, examples have been reported of the formation of welldefined coordination arrays on surfaces (metallo-grids), which were found to possess interesting electronic, magnetic, and structural properties.…”

Aspects of spontaneous separation of enantiomers in two- and three-dimensional crystals

“…Such an approach has recently been applied for achieving spontaneous separation in two-dimensional crystals. 42 The most important symmetry elements by which organic molecules are related in three-dimensional crystals are, of course, translation, center of inversion, twofold screw axis, and glide plane. In the two-dimensional counterpart at a liquid surface for amphiphilic molecules that are specifically oriented vis-à-vis the water surface, the situation is simpler.…”

“…43 Recently a focus was put on the design of ␣-amino acid amphiphiles, that their racemates would spontaneously separate or form a racemic compound. 42 Thus monolayers of amphiphiles of the type C n H 2n+1 CH(NH 3 + )CO 2 − , labelled C n -gly, n = 10,12,16, have been studied by grazing incidence X-ray diffraction (GID) (Fig. 3).…”