Quenched phosphorescence as a detection method in capillary electrophoretic chiral separations. Monitoring the stereoselective biodegradation of camphorquinone by yeast

Quenched phosphorescence detection of camphorquinone in cyclodextrin-based electrokinetic chromatography provides very favorable detection limits, i.e., 7 x 10(-)(7) M, 3 orders of magnitude lower than conventional UV absorption detection at 200 nm. The detection is based on the dynamic quenching by the analyte of the strong phosphorescence emission of brominated naphthalenesulfonate, under deoxygenated buffer solution conditions. This approach has been used to detect (1S)-(+)- and (1R)-(-)-camphorquinone after enantiomeric separation by CE. Although the use of the negatively charged carboxymethyl beta-cyclodextrin (CM-beta-CD) alone was not successful, the addition of a second, neutral cyclodextrin, alpha-CD, provided an adequate enantiomeric separation of camphorquinone. Using 25 mM borate buffer (pH 8.5) with 10 mM CM-beta-CD and 20 mM alpha-CD (applied voltage 20 kV, ambient temperature), the enantiomeric separation was performed in approximately 14 min. The chiral method was applied to monitor the stereoselectivity of the biotransformation of a racemic mixture of camphorquinone by yeast. It was found that the enantiomeric ratio calculated from the peak areas in the electropherogram (RSD = 5%) after 24 h of incubation decreased from 0.92 for the control solution (culture medium without yeast) to 0.24 for the culture medium; a similar ratio of 0.25 was observed for cell extract solutions. Therefore, racemic camphorquinone is enantioselectively degraded by yeast, the biodegradation of (1S)-(+)-camphorquinone being faster than that of the (1R)-(-)-enantiomer.