Chiral capillary electrophoresis–mass spectrometry of tetrahydroisoquinoline-derived neurotoxins: Observation of complex stereoisomerism

Abstract

Previous studies have shown that certain 1,2,3,4-tetrahydroisoquinoline derivatives (TIQs) are neurotoxins inducing Parkinsonism. Further, individual enantiomers of these toxins such as ( R/ S)-N-methylsalsolinol (( R/ S)-NMSal) possess distinct neurotoxicological properties. In this work, a chiral capillary electrophoresis (CE) method with electrospray ionization-tandem mass spectrometric (ESI-MS/MS) detection was developed for the quantification of TIQ enantiomers. Enantioseparation was achieved with sulfated β-cyclodextrin (sulfated β-CD) as chiral selector. To avoid any potential contamination of MS ionization source by the non-volatile chiral selector, partial filling technique was deployed in the CE separation. TIQ derivatives, including ( R/ S)-6,7-dihydroxy-1-methy-TIQ (salsolinol, Sal), ( R/ S)-1-benzyl-TIQ (BTIQ), and ( R/ S)-NMSal, were base-line resolved with resolution values ( R) ranging from 3 (for Sal) to 4.5 (for BTIQ), which were much better than those reported previously by HPLC methods. ESI-MS/MS detection of the resolved TIQ enantiomers was specific and sensitive (LOD = 1.2 μM for Sal enantiomers). The proposed chiral CE–MS/MS method was used to study in vitro formation of ( R/ S)-NMSal. It was found that NMSal was formed from the incubation of epinine (a dopamine metabolite) with acetaldehyde (a metabolite of alcohol). More interestingly, four isomers of NMSal were separated and detected in the incubation solution. They were identified as ( R)-e.e-NMSal, ( R)-e.a-NMSal, ( S)-e.e-NMSal, and ( S)-e.a-NMSal. This was the first lab evidence that this Parkinsonian neurotoxin exists in multiple isomeric forms.