Separation of Enantiomers and Racemate Formation in Two-Dimensional Crystals at the Water Surface from Racemic R-Amino Acid Amphiphiles: Design and Structure Isabelle Weissbuch,* ,² Maria Berfeld, ² Wim Bouwman, ‡ Kristian Kjaer, ‡ Jens Als-Nielsen, § Meir Lahav,* ,² and Leslie Leiserowitz* ,² Contribution from the Department of Materials & Interfaces, The Weizmann Institute of Science, 76100 RehoVot, Israel, Department of Solid State Physics, Risø National Laboratory, DK 4000, Roskilde, Denmark, and Niels Bohr Institute, H. C. Ørsted Laboratory, DK 2100, Copenhagen, Denmark ReceiVed April 29, 1996. ReVised Manuscript ReceiVed NoVember 27, 1996 X Abstract: Studies are presented on the two-dimensional (2-D) crystalline packing arrangements of enantiomerically pure and racemic R-amino acid RHC(NH 3 + )CO 2 - 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 H 2n+1 -, n ) 10, 12, 16) due to the tendency for herringbone chain arrangements via glide symmetry or homochiral (for R ) C n H 2n+1 CONH(CH 2 ) 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 R-glycine by these monolayers. The relevance of the present results to the possibility of ordering and spontaneous segregation of racemates of the natural hydrophobic R-amino acids at the air-solution interface is discussed. Introduction The routes by which basic units of living systems, such as the R-amino acids, have adopted only one sense of chirality remains an unsolved mystery of nature. 1 Spontaneous segrega- tion of enantiomers from a racemic mixture might have played an important role in an abiotic process proposed to explain the tranformation from a racemic chemistry to a chiral biology. Thus much focus has been placed on the use of three-dimensional (3-D) crystals for inducing spontaneous resolution of left- and right-handed molecules. 2-6 In two-dimensions, such a separa- tion at interfaces may be no less important. On simple symmetry grounds, it should in fact be easier to bring about a segregation of enantiomers in two-dimensional (2-D) crystalline domains formed by chiral amphiphilic molecules at an air- liquid or air-solid interface, since the inversion symmetry element, so prevalent in 3-D crystals, is absent in the 2-D counterpart. However, the glide symmetry element is still available for packing molecules of opposite handedness in such crystalline monolayers. Various studies for the detection of chiral segregation of amphiphiles at the air-water interface have been reported. 7-15 Recently, chiral separation in monolayers of racemic myristoyl- alanine has been inferred from grazing incidence X-ray dif- fraction. 16 Monolayer domains of mirror-image structures, and therefore of opposite chirality, have been observed on mica support by scanning force microscopy 17 and on graphite by scanning tunneling microscopy. 18 Natural Hydrophobic r-Amino Acids at the Air-Solution Interface Several years ago, it was reported that addition of naturally occurring hydrophobic R-amino acids to supersaturated glycine aqueous solutions induced the formation of floating R-glycine crystals which exposed their (010) or (01 h0) faces to air, depending upon whether the absolute configuration of the ² The Weizmann Institute of Science. ‡ Risø National Laboratory. § H. C. Ørsted Laboratory. X Abstract published in AdVance ACS Abstracts, January 15, 1997. 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Engl. 1996, 35, 900. 933 J. Am. Chem. Soc. 1997, 119, 933-942 S0002-7863(96)01392-3 CCC: $14.00 © 1997 American Chemical Society