Structural mapping of an unsymmetrical chemically modified cyclodextrin by high-field nuclear magnetic resonance spectroscopy

Abstract

Samples of 2,6-per-O-methyl-ß-cyclodextrin (DMßCD), prepared according to literature procedures, have been shown to be less than 70% pure. The major impurity has been identified as the unsymmetrical over-methylated cyclodextrin derivative (DM+1)ßCD. Samples of pure DMßCD and (DM+1)ßCD have been prepared by (i) benzoylating a chromatographically homogeneous mixture obtained after methylating ßCD with dimethyl sulphate, (ii) separating chromatographically the resulting DMßCD heptabenzoate (DMßCD-B₇) and (DM+1)ßCD hexabenzoate [(DM+1)ßCD-B₆], and then (iii) subjecting the pure perbenzoates to de-O-benzoylation. Both DMßCD-B₇ and (DM+1)ßCD-B₆ have been fully characterised. High-resolution ¹H and ¹³C n.m.r. spectroscopy, with use of modern pulse techniques (homonuclear double resonance difference spectroscopy, COSY, J-resolved, JMOD, XHCORRD, and CHORTLE) for signal assignment and n.O.e. difference spectroscopy for residue sequencing, has been used to assign individually (i) 41 out of the 49 heterotopic protons and (ii) 29 out of the 42 heterotopic carbon atoms of the unsymmetrical (DM+1)ßCD-B₆(nuclei associated with O-methyl and O-benzoyl groups were excluded from consideration). The complete spectroscopic characterisation of unsymmetrical chemically modified cyclodextrins is important in the investigation of these compounds as potential enzyme models. The necessity of preparing pure chemically modified cyclodextrin derivatives cannot be over-emphasised.