Structural properties of α- and β-cyclodextrin and of a β-cyclodextrin-2-naphthol complex. A case study

Abstract Cyclodextrins (CDx) and their complexes are good model systems to study the structural complexity of macro-molecular systems in general and, in particular, unspecific molecular recognition processes in host-guest systems. In this paper the conformational space of α-CDx, β-CDx and of a 2-naphthol-β-CDx complex are studied by molecular mechanics methods and are discussed in adapted symmetry coordinates. In contrast to highly symmetrical α-CDx, symmetry breaking due to a strong relative tilt of the glucose subunits and to an elliptical squeeze of the macrocycle is demonstrated for β-CDx. The formation of symmetrical fragments within globally unsymmetrical β-CDx is shown. The conformational landscapes of β-CDx and the β-CDx-2-naphthol complex are then converted into correlational maps between individual geometry variables to discuss dynamic aspects of the macromolecules. The severe impact of complexation on the conformational space of the host is demonstrated as correlational maps simplify drastically and the geometrical variables defining the complex geometry become highly correlated. The impact of complexation on β-CDx symmetry is demonstrated, as two distinct types of complex structure can be characterized.

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