摘要:
IntroductionCholic acid (CA), one of the steroidal bile acids, is known to form inclusion compounds with a wide variety of organic substances.1–5Its molecular structure and numbering scheme are shown below.The most striking of these inclusion compounds has a facially amphiphilic structure which is due to three hydroxy groups in the steroidal nucleus pointing α to (i.e.below) the plane of the ring, forming a hydrophilic face, and two methyl groups which are βorientated (pointing above the plane of the ring) forming a lipophilic face. XRay crystallographic studies have revealed that CA forms bilayer structuresviaan alternating stack of hydrophilic and lipophilic layers and that host cavities are formed in the lipophilic layers due to the bent molecular shape of the component CA molecules. It should be noted that there are many possibilities for the stacking of these layers but for simplicity we consider two variations on the lipophilic side. One arises through flip of the layers, parallel and antiparallel, as shown inFig. 1. The other arises through sliding (α, β and non interdigitationtype)between the layers (Fig. 2), where the type of sliding is characterized by the interdigitation patterns of the methyl groups within the lipophilic layers. The combination of flip and sliding between the layers can produce at least six types of stacking arrangement, as shown inTable 1; three of these have already been found.4e,6,7Here, we present the novel crystal structure of CA withmchloroaniline which shows a unique parallel orientation with non interdigitationtype sliding in the lipophilic layer.Schematic representation of the molecular orientation (a) antiparallel and (b) parallel in the lipophilic layer. The two ends of the molecule, that of the hydroxy group at the carbon 3position and that of the carboxylic group, are distinguished as the head and tail, respectively. The heads of the arrows represent the head of cholic acid.Schematic representation of sliding types for the lipophilic layers: (a) α, (b) β and (c) non interdigitation-type.Classification of bilayer stacking in the lipophilic layer of CAInterdigitation typeAntiparallel orientationParallel orientationαKnown6,7UnknownβKnown6UnknownNon interdigitationKnown4eThis work
ISSN:1466-8033
DOI:10.1039/b101906f
出版商:RSC
年代:2001
数据来源: RSC