No abstract

Cyclodextrins (CDs) are often used to separate the enantiomers of chiral drugs by HPLC, GC, and capillary electrophoresis (CE). For dipeptides containing one aromatic amino acid, a reversal of the migration order is observed in CE upon increasing the pH value of the background electrolyte from 2.5 to 3.5. In order to understand the mechanism of chiral recognition of this phenomenon, NMR experiments, namely complexation‐induced chemical shifts (CICS) and ROESY, were performed. Comparing the CICSs obtained for the pairs Diac‐β‐CD/Ala‐Phe and HDAS‐β‐CD/Ala‐Phe and Diac‐β‐CD/Ala‐Tyr and HDAS‐β‐CD/Ala‐Tyr revealed very small changes, thereby indicating a rather weak interaction of the respective guest molecules with their host. The CICS for β‐CD and HS‐β‐CD confirmed an inclusion of the aromatic moiety into the CD cavity. It is shown that, at pH 2.5, the D,D‐enantiomer of Ala‐Tyr immerses deeper into the cavity of β‐cyclodextrin than the L,L‐enantiomer, and this was subsequently confirmed by molecular dynamics simulations. Furthermore, at pH 3.5 the immersion is shallower, as shown by the 1H and ROESY NMR findings and confirmed by the molecular dynamics simulations. The immersion depths of the dipeptides were calculated by defining a plane in the cavity to gain information about the different behaviour of the studied chemical species. The employed procedure is easily generalised to other host‐guest complexes and is expected to improve data evaluation in other cases, too. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

The antibiotic agent gentamicin sulfate is a mixture of several structurally very similar aminoglycosides. The major components of gentamicin are gentamicin C1, C1a, C2 and C2a. These major components were separated by means of column chromatography. The 1 H and 13 C NMR spectra of the free bases and the corresponding sulfates were assigned using HMQC, HMBC and selective TOCSY experiments. Additionally, the 1 H and 13 C NMR spectra of the sulfates and hydrochlorides, respectively, of the minor components were assigned. The full assignment of all components of gentamicin is necessary for the quantitative analysis of batches of gentamicin. By means of coupled liquid chromatography-NMR, several different batches were studied, revealing different compositions of batches from various sources.

No abstract

High-resolution NMR spectroscopy is a powerful tool in the elucidation of a structure with respect to constitution, configuration and conformation. In recent years, NMR has been increasingly used in quantitative analysis. In this study, we show the ability of 1 H NMR to determine the isomeric composition of the antidepressant drug fluvoxamine. The activity of fluvoxamine resides on the E-isomer, and the current British Pharmacopoeia limits the content of the Z-isomer to 0.5%. The NMR method described here is able to determine the content of the Z-isomer down to the 0.2% level on a total amount of 15 mg of the substance.