Capillary electrophoresis for diastereomers of (R,S)- tetrahydroisoquinoline-3-carboxylic acid derivatized with (R)-4-nitro-7-(3- aminopyrrolidin-1-yl)-2,1,3-benzoxadiazole: Effect of molecular geometries

Abstract

Diastereomers derived from (R,S)-tetrahydroisoquinoline-3-carboxylic acid (Tic), a potential neurotoxin with a chiral fluorescence tagging reagent, (R)-4-nitro-7-(3-aminopyrrolidin-1-yl)-2,1,3-benzoxadiazole (NBD-APy), are well resolved by capillary electrophoresis (CE). For a better understanding of the separation mechanism, a semiempirical computational method (i.e., AM1 method) is used to study the molecular geometry, relative energy, and size of the derivatives. The molecular sizes are estimated to be 216.3 and 240.6 cm 3/mol for (R)-NBD-APy-(R)-Tic and (R)-NBD-APy-(S)-Tic, respectively. The CE elution order of the diastereomeric derivatives confirms the AM1 computational results: (R)-NBD-APy-(R)-Tic elutes before (R)-NBD-APy-(S)-Tic. The effects of running buffer pH and the addition of a chiral selector, β-cyclodextrin (β-CD), on the separation are studied. In the presence of β-CD, the migration behavior of the diastereomers is changed because of the formation of CD inclusion complexes. Study of the space-filling models for optimized conformations of the diastereomeric derivatives and β-CD suggests that the geometries of the diastereomers decides that the diastereomers are incorporated into the CD cavity to form CD inclusion complexes with different volumes. Experimental results from CE separations conclude the same.