Molecular recognition of l-leucyl-l-alanine: enantioselective inclusion of alkyl methyl sulfoxides

A simple aliphatic dipeptide, l-leucyl-l-alanine (Leu-Ala), includes several alkyl methyl sulfoxides enantioselectively to form inclusion crystals. From single-crystal X-ray analyses of three inclusion compounds of dimethyl sulfoxide (DMSO), isobutyl methyl sulfoxide, and benzyl methyl sulfoxide, it was elucidated that Leu-Ala molecules self-assemble to form layer structures and the sulfoxides are included via hydrogen bonding in a cavity between these layers. The inclusion cavity has methyl group and isobutyl group at its each side, and the guest sulfoxide is placed in such a manner that its methyl group faces toward the methyl of the Leu-Ala cavity. When the alkyl group of the sulfoxide is comparably large, it is located in the residual space of the cavity to attain effective crystal packing. Thus, the sulfoxides having a comparably large group such as isobutyl, butyl, and benzyl are included with a high (R)-enantioselectivity in Leu-Ala crystals.