Guest-responsive structural changes in cholic acid intercalation crystals

Abstract

CHOLIC acid forms multimolecular inclusion compounds with a variety of organic substances¹. Some of these inclusion compounds have crystal structures containing channels², which can perform efficient chiral recognition of, for example, lactones³; others, with ethanol⁴ or water⁵ as guest molecules for example, have no such channels. Although these two types of structure seem at first to be very different, we report here results that indicate that they are closely related. In particular we present the novel finding that guests can be added, removed or exchanged without the intervention of an amorphous state. We suggest that cholic acid crystals have a dynamic layered structure which can be modified to incorporate guests by rearrangement of the intermolecular hydrogen-bonded networks. To our knowledge, this is the first example of intercalation in organic crystals accompanied by guest-responsive on-off control of channels, although many similar inorganic intercalation compounds (of graphite and clay, for example) are known6,7. These observations suggest that small molecules can express significant molecular information when assembled into crystalline structures.