USE OF H-1-NMR SPECTROSCOPY TO DETERMINE THE ENANTIOSELECTIVE MECHANISM OF NEUTRAL AND ANIONIC CYCLODEXTRINS IN CAPILLARY ELECTROPHORESIS

One-dimensional (1D) and two-dimensional (2D) H-1 nuclear magnetic res onance (NMR) techniques have been used to investigate the chiral recog nition process in capillary electrophoresis (CE) for seven different c yclodextrins (CDs) with the calcium channel blocker amlodipine as a mo del compound. These include five neutral CDs (alpha-CD, beta-CD, gamma -CD, hydroxypropyl-beta-CD and hydroxyethyl-beta-CD) and two anionic C Ds (sulphobutyl-ether-beta-CD and carboxymethyl-beta-CD) where mixture s of amlodipine with each of the seven CDs were examined by 1D NMR in deuterated phosphate buffer at pD 3.4. The resonance shift of signals with added CD, relative to the CD-free position (shift displacement, D elta delta) and shift non-equivalence (Delta delta) of enantiomeric s ignals shifted relative to each other after addition of CD were examin ed for non-overlapped protons of amlodipine. The possible correlations of NMR shift non-equivalence data with chiral separation in CE for am lodipine have been critically assessed. Qualitative differences in the 1D NMR shifts and enhanced enantioselectivity in CE were observed for amlodipine with sulphobutyl-ether-beta-CD. Further experiments on the through-space interactions using 2D rotating frame nuclear Overhauser effect spectroscopy (ROESY) indicated that there was no association b etween internal glucopyranose hydrogen atoms and the aromatic hydrogen s of amlodipine. This gives evidence for the aromatic ring not being i ncluded in this CD. Moreover, data from spin-lattice relaxation times (T-1) measured for amlodipine in the free state and after addition of the anionic sulphobutyl-ether-beta-CD indicate that the aromatic moiet y of amlodipine is not included into the sulphobutyl-ether-beta-CD cav ity. There is evidence that it interacts with the sulphobutyl side cha ins, and may adopt a preferred orientation outside the sulphobutyl-eth er-beta-CD toroid itself. (C) 1997 Elsevier Science B.V.