Chiroptical detection during liquid chromatography: Deconvolution of overlapping peaks of enantiomers and its applications

Abstract

The peaks of enantiomers in liquid chromatography (LC) frequently overlap for different reasons. The experimental curve can be deconvolved, i.e., transformed into the two curves of the enantiomers, without any assumption concerning their peak shapes. Besides the usual photometric UV detection, resulting in absorbance A, polarimetric or circular dichroic detection is required, providing the rotation 90 degree angle alpha or the differential absorbance DeltaA, respectively. The accuracy of the ratio alpha/A or DeltaA/A for the pure enantiomers is essential for the quality of the deconvolution. The determination of these ratios, using the overlapping peaks, and the subsequent computer deconvolution of the latter are discussed in more detail than in the earlier publications, e.g. Ref. 1 concerning this particular method. The computer program developed for this purpose is characterized. A condition is given which limits the availability of ratios and, therefore, the possibility of deconvolution. Several novel examples are described which stem from the following fields of application of deconvolved peaks: actual optical purities during LC (on-line analysis), overall optical purity of a sample, purities of chromatographic peaks, and, finally, enantiomerization during LC.

Chiroptical fingerprints to characterize lavender and lavandin essential oils

“…polarimetric and circular dichroism (CD) detectors [2][3][4] , were developed [5] and are nowadays routinely used in HPLC on chiral support to identify and differentiate chiral molecules [6] . Moreover, chiroptical detectors can be applied to the determination of enantiomeric ratios on chiral stationary phases (CSPs) with baseline [7] or partial separation [8,9] , but also on achiral support [10][11][12][13] , to preparative purposes [14,15] , and to the assignment of absolute configurations [16][17][18] . They have sometimes been used to characterize complex mixtures focusing on chiral components [19] , because the analysis is simplified by hiding achiral molecules, which are not sensed by chiroptical detection.…”

9.08 Use of Industrial Scale Chromatography in Pharmaceutical Manufacturing

“…Such a controller has been developed at ETH, Zurich. 43,69 and shows great promise. However, certainly because of quality concerns (validation of the process, calibration, etc.)…”

“…By coupling two detectors (UV+polarimeter for chiral separations) and using mathematical deconvolution of the two signals, the purity of the stream can be measured in real time and adjustment of the separation parameters or the collection can be done. 43,69 It is conceivable that, once the equipment is calibrated, a controller can take over the piloting of the process automatically. Such a controller has been developed at ETH, Zurich.…”

Unusual thermo(photo)chromic properties of some mononitro- and dinitro- substituted 3′-alkyl indolospirobenzopyrans

“…This is perhaps unexpected since ring-opening around the highly sterically crowded spirocyclic centre of 2, additionally containing a spirocyclic cyclohexyl-group, would result in a greater relief of steric strain e a thermodynamic driving force for the ring-opening reaction ('steric acceleration') e than for the relatively less sterically hindered gem-methylsubstituted compound. Further, ring-opening must involve initial cleavage of the spirocyclic CeO heteratomic bond (the energy barrier to enantiomerization in spirobenzopyrans is reported [18] to be approximately 86 kJ mol À1 , with the activation energies [19] of thermal interconversion ranging between 80 and 130 kJ mol À1 ) to produce a 'distorted' planar iminium ion containing structure. However, large differences in the rates of spirocyclic ring-opening for compounds 1 and 2 are likely to arise in a strongly polar solvent such as methanol (Polarity (P) ¼ 5.1: as a reference the most polar solvent is H 2 O with a value of 10.2) due to the relatively greater thermodynamic solvation stabilization, through both dipolar and hydrogen bonding interactions with the resultant open-form (eight zwitterionic isomeric conformers are theoretically possible, although only the four trans-merocycanine conformers are thermodynamically likely to significantly contribute to the isomeric equilibrium mixture): Solvation differences would be particularly dominant around the relatively less sterically hindered 'planar' iminium ion of compound 1, being flanked by the gemmethyl group, as opposed to the significantly more bulky 3 0 -spirocyclohexyl-substituent of compound 2; thus representing a possible explanation for the relatively greater rate of merocyanine formation for this compound.…”