Chiral Crystallization of Glutamic Acid on Self Assembled Films of Cysteine DAVID H. DRESSLER AND YITZHAK MASTAI * Department of Chemistry, Bar-Ilan University, Ramat-Gan, Israel ABSTRACT In this article, we describe the preparation and use of chiral surfaces derived from enantiomerically pure crystals of amino acids. For this purpose, we chose to employ a self-assembly process to grow nanoscale chiral films of (þ)-L or ()-D cyste- ine, onto gold surfaces. We utilized those chiral films as resolving auxiliaries in the crys- tallization of enantiomers from solutions. To demonstrate the chiral discriminating abil- ity of the chiral surfaces in crystallization processes, we investigated the crystallization of rac-glutamic acid onto the chiral films. Our study demonstrates the potential applica- tion of chiral films to control chirality throughout crystallization, where one enantiomer crystallizes on the chiral surfaces with relatively high enantiomeric excess. In addition, crystallization of pure glutamic acid enantiomers, and its racemic compound on to chiral films resulted in crystal morphology modification with preferred crystal orientation, which assists in the interpretation of the ability of our chiral surfaces to function as chi- ral selectors. Chirality 19:358–365, 2007. V V C 2007 Wiley-Liss, Inc. KEY WORDS: chirality; crystallization; self-assembly; thin films; chiral surfaces; amino acids INTRODUCTION In the last few years, the adsorption and assembly of chiral molecules on solid surfaces has become a particu- larly fruitful area of research. Organized organic films of chiral molecules are of interest in various applications such as stereoselective chemical synthesis and catalysis. Overall chiral surfaces can be prepared by three main approaches. First, the most direct way to shape chiral sur- faces is to apply naturally chiral bulk crystalline struc- tures. 1,2 One of the most common chiral surfaces of this type is quartz, and in some studies it was shown that amino acids e.g. (þ)-L and ()-D-alanine 3 show a preferen- tial enantioselective adsorption onto quartz powder sam- ples. Further example for the chiral nature of bulk crystal- line structure was recently reported by Hazen et al. 4 who demonstrated that calcite (CaCO 3 ) crystals immersed in a racemic aspartic acid solution, display significant adsorp- tion and chiral selectivity of ()-D and (þ)-L enantiomers on pairs of mirror-related crystal surfaces. Second, chiral surfaces can also be formed from crystal- line materials of achiral bulk structures. These types of chiral surfaces are created by exposing high Miller indexes of metal single crystal surfaces. 5,6 Most of these surfaces have structures formed from terraces, steps, and kinks. The chirality in such surfaces is due to the lack of symmetry of the kink sites. Therefore it is possible to pre- pare single crystal surfaces which possess kink sites with intrinsic left or right handedness. Some of these surfaces can act as enantioselective heterogeneous catalysts. 7,8 For instance, Hovrath and Gellman 9 showed that Cu (643) chi- ral surfaces revealed enantiospecific adsorption of (þ)-R- propylene oxide and ()-S-propylene oxide. Third, the most common approach to construct chiral surfaces is based on self-assembled chiral monolayers onto metallic surfaces. Such surfaces can be used for enan- tioselective heterogeneous catalysis 10,11 or enantioselec- tive chromatography. 12,13 Chiral surfaces of this kind dem- onstrated high enantioselectivite catalysis, the develop- ment of catalysts based on chiraly modified metals, chiral polymers, and heterogenized chiral metal complexes has been reviewed extensively by Baiker and coworkers. 14,15 Chiral surfaces based on self-assembled monolayers (SAM) and Langmuir–Blodgett 16 films have been used for many years to study the influence of well-defined function- alized surfaces on nucleation, polymorphism, and selective orientation of crystals. For instance, crystals of many dif- ferent materials have been grown on SAM, including pro- teins, 17,18 enantiomerically pure amino acids, 19–21 semicon- ductors, 22 and biominerals. 23–25 SAMs have also been employed to control crystal morphology and orientation, for example Swift and coworkers described that SAMs can be used to direct the nucleation and subsequent orienta- tion of crystals 26,27 and to control crystal polymorphism. 28 Generally chiral surfaces can provide an effective environ- ment for chiral molecular discrimination. In the past dec- ade, chiral surfaces 29 have received attention for their potential applications, 1,30–32 such as stereoselective chemi- *Correspondence to: Dr. Y. Mastai, Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel. E-mail: mastai@mail.biu.ac.il Received for publication 26 October 2006; Accepted 13 January 2007 DOI: 10.1002/chir.20389 Published online 12 March 2007 in Wiley InterScience (www.interscience.wiley.com). CHIRALITY 19:358–365 (2007) V V C 2007 Wiley-Liss, Inc.