Surface recognition and chirality

The substitution of chiral molecules onto the crystal surfaces has been investigated by Mein Lahav, Lia Addadi, Leslie Leiserowitz and co-workers.The chiral molecule is recognized by the molecules already oriented in the crystal lattice. This can be true for either an enantiomorphous crystal composed of molecules with only one chirality sense, or for a centrosymmetric racemic crystal in which a face that is composed only of one enantiomer interacts with a chiral agent. 

The molecules in the original crystal need not be chiral they can be pro-chiral and still give an analogous effect. Glycine, which lacks an asymmetric carbon atom, crystallizes in the a form as bipyramids with well-developed [110] and [Oil] sets of faces.These crystals are mono- 

FIGURE 17.16. The crystal packing of RS serine is shown, viewed down the a axis. R- and 5-threonine and -serine ate D- and L-threonine and -serine, respectively. Threonine impurity molecules (with their methyl groups represented by large filled circles) are added stereospecifically. They then prevent further addition of serine molecules on the labelled crystal faces. Remember, from Chapter 2, that the faces that are observed on a crystal are those that have grown most slowly. Therefore, when threonine adds to a growing crystal, the face to which it adds becomes prominent. 

Crystals of a-glycine float on the surface of the solution, with one of the two flat (010) or (010) faces exposed to solution. If one enantiomer of a chiral hydrophobic amino acid (leucine or phenylalanine, for example) is now added to the solution, the crystal will become oriented so that the face that occludes amino acids of opposite chitrality is exposed to solution. D-amino acids are only incorporated in crystals with the 010 face exposed to solution, while L-amino acids only interact with the exposed (010) face. Therefore, if a floating crystal with the (010) face exposed to solution is presented with a solution of DL amino acids, the crystal will bind D-amino acids and will enrich the solution with respect to L-amino acids. If, for example, the underside of a film of a resolved amino acid with a long hydrophobic side chain [such as (5)-a-aminooctanoic acid] is exposed to a solution of a racemic mixture of amino acids, some optical resolution will occur by the principles just described (see Reference 64). 

In guest—host chemistry the shapes of the participating molecules are fundamental to the process of co-aggregation. Charles J. Pedersen  

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