Theoretical study of α-CD based [3] pseudorotaxanes: the role played by threadlike polymer on the stability of Cyclodextrin dimers

Abstract

As interações entre ciclodextrinas e seus agregados com cadeias poliméricas, têm atraído a atenção de pesquisadores em diferentes subáreas da química supramolecular. Esses compostos, conhecidos como "necklaces", podem ser empregados na formulação de fios e nanotubos moleculares. No presente artigo a formação de dímeros de -CD foi estudada teoricamente considerando as três possíveis orientações relativas, denominadas "head-to-head" (HH), "tail-totail" (TT) e "head-to-tail" (HT). A influência da cadeia polimérica na estabilidade relativa das diferentes associações foi avaliada através do estudo dos compostos de inclusão de ( -CD) 2 com oligo(etilenoglicol) (OEG). Os resultados mostram que a orientação relativa das CDs é definida, primeiramente, pelas interações intermoleculares entre as unidades de -CD, tendo a cadeia polimérica um papel secundário no processo de agregação.Cyclodextrins (CDs) and polymeric chains have attracted considerable attention, being addressed in the literature as novel molecular assembly. The so-called "Molecular Necklace" synthesized by the inclusion of a polymeric chain inside CDs cavity has been employed in the formulation of molecular wires and nanotubes. In this paper we applied our previous reported mixed basis set approach in order to investigate theoretically the -CD inclusion complexes formed by two CD units and an oligo(ethylene glycol) (OEG). In attempt to analyze the role played by the OEG in the formation of pseudo-rotaxane in gas-phase, DFT calculations were performed for six possible dimer associations of CDs, named head-to-head (HH), tail-to-tail (TT) and head-to-tail (HT) with or without an OEG threadlike molecule included in the cavity formed by the -CD dimer. The comparison between relative energies of empty (HH, TT and HT) and filled associations (HH-OEG, TT-OEG and HT-OEG) shows that the OEG chain does not interfere significantly in the relative stabilization energies of the supramolecular systems, therefore the relative arrangements of CDs in the necklace structures should be primarily driven by interactions between cyclodextrin units. Keywords: DFT, alpha cyclodextrin, dimer, OEG, inclusion complex IntroductionCyclodextrin (CD) is a cyclic oligomer of -D-glucose obtained by the action of certain enzymes on starch. Generally described as shallow truncated cones, this class of carbohydrate presents a hydrophobic cavity of different sizes, depending on the number of elementary glucose units. In addition, the structure of the CD molecule possess two different rims, a wider (head) containing all secondary hydroxyl groups and a narrower (tail) containing all primary hydroxyl groups. There are three natural cyclodextrins readily available having six, seven or eight glucose units named -CD, -CD and -CD, respectively.The applications of CDs in supramolecular chemistry have been widely addressed in the literature and the applicability of this class of carbohydrate is closely related to its ability to form inclusion compounds with a very wide range of guest molecules in aqueo...

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Theoretical study of spectroscopic properties of insulated molecular wires formed by substituted oligothiophenes and cross‐linked α‐cyclodextrin

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