Poly(ethylene glycol)/β-Cyclodextrin Crystalline Inclusion Complexes

Graduate #38
Discipline: Chemistry and Chemical Sciences
Subcategory: Chemistry (not Biochemistry)

Amal Al-Abdulrahman - Clark Atlanta University

In literature, there is a disagreement regarding whether or not βeta-cyclodextrin (β-CD) forms inclusion complexes with polyethylene glycol (PEG). Harada et al. (1993) reported the complexation between PEG with α-CD and γ-CD; however, PEG does not form such complexes with β-CD.1 Researchers still agree and cite this article to continue the suggestion that PEG is not included within a β-CD cavity. However, there is one study that reported the formation of the inclusion complex between PEG and β-CD and demonstrated this via the preparation of a single crystal.2 The lack of existing consensus created a need for this study to test the hypothesis that PEG and β-CD form inclusion complexes. In this study, the slightly water-soluble β-CD and its crystalline inclusion complexes with different molecular weights of poly(ethylene glycol) were prepared and characterized using the method of freeze-drying. The results show that the β-CD forms crystalline inclusion complexes with PEG (MW 600) and PEG (MW 1500) in the solid state. In the solution state, 2D NMR spectroscopy shows that the PEG is present in the cavity of the β-CD. Coherent cross peaks were observed in both 2D NMR NOESY and ROESY showing correlation between the inner and outer protons of β-CD with the repeating unit of PEG. Scanning electron microscopy (SEM) shows the formation of soluble crystalline complexes between the β-CD with the amorphous PEG (MW 600) and PEG (MW 1500). Crystal formation was supported by wide angle X-Ray studies, WAXD. WAXD patterns for the β-CD/PEG crystalline complexes show new peaks indicative of formation of structures different from the crystalline β-CD. It is extremely interesting to note that even though PEG 600 is totally amorphous and a liquid at room temperature, the inclusion complex is crystalline and large crystals were observed by SEM. In the solid state the inclusion complexes most likely forms psuedorotaxane i.e. the PEG chains thread multiple β-CD. Future work calls for examining the implications of PEG/β -CD inclusion complexes for drug delivery systems.
References: 1- Harada, K.; Li, J.; and Karmachi, M. (1993). Preparation and Properties of Inclusion Complexes of Poly(ethylene glycol) with α-Cyclodextrin. Macromolecules., 26 (21), pp 5698–5703.
2-Udachin, K. A.; Wilson, L. D.; and Ripmeester, J. A. (2000). Solid Polyrotaxanes of Polyethylene Glycol and Cyclodextrins: The Single Crystal X-ray Structure of PEG-α-cyclodextrin. J. Am. Chem. Soc. 122, 12375-12376.

Funder Acknowledgement(s): The support of this study NSF CREST HRD-1137751 is gratefully acknowledged. I am grateful for fellowship support from King Khalid University.

Faculty Advisor: Ishrat Khan, IKhan@cau.edu

Role: I did all the work with Dr. Khan as my advisor. The original work on β-Cyclodextrin was successfully achieved by Janet Cowins on β-Sitosterol/β-Cyclodextrin Crystalline Inclusion Complexes.