Bis-indole derivatives for polysaccharide compositional analysis and chiral resolution of D-, L-monosaccharides by ligand exchange capillary electrophoresis using borate-cyclodextrin as a chiral selector

Abstract

A series of aldo-bis-indole derivatives (aldo-BINs) was prepared by aromatic C-alkylation reactions of aldoses and indole in acetic acid solution. Common monosaccharides such as glucose, mannose, galactose, fucose, xylose, rhamnose, ribose, arabinose and N-acetylglucosamine were smoothly derivatized to form the UV absorbing aldo-BINs. The use of a capillary electrophoretic method to separate these novel aldo-BIN derivatives was established. The capillary electrophoresis conditions were set by using borate buffer (100 mM) at high pH (pH 9.0). The limit of determination was assessed to be 25 nM. The enantioseparation of D, L-pairs of aldo-BINs based on chiral ligand-exchange capillary electrophoresis technology was also achieved by using modified hydroxypropyl-β-cyclodextrin as the chiral selector in the presence of borate buffer. This aldose labeling method was applied successfully to the compositional and configurational analysis of saccharides, exemplified by a rapid and efficient method to simultaneously analyze the composition and configuration of saccharides from the medicinal herbs Cordyceps sinensis and Dendrobium huoshanense.

Scheme 1

Synthesis of bis(3’-indolyl)alkanes by the C-glycosylation of indole with unprotected aldoses [8].

Figure 1

Electrophorogram of aldo-BINs in CE.Peaks: 1 = Rib-BIN; 2 = Rha-BIN; 3 = GlclNAc-BIN; 4 = Xyl-BIN; 5 = Glc-BIN; 6 = Man-BIN; 7 = Ara-BIN; 8 = Gal-BIN; 9 = Fuc-BIN. CE conditions: buffer, 100 mM borate (pH 9.0) contains hydroxypropyl-β-CD (10 mg/mL); applied voltage, 20 kV; uncoated fused-silica capillary, 50/60.2 cm × 50 μm I.D.; sample injection, 3s by pressure (0.5 psi); wavelength, 254 nm; system temperature, 15 °C.

Figure 2

Enantioseparation of seven enantiomeric pairs of indole derivated monosaccharides (aldo-BINs). Conditions: borate buffer (100 mM, pH 9.0) contains hydroxypropyl-β-CD (10 mg/mL); applied voltage, 20 kV (detector at cathode side); uncoated fused-silica capillary, 50 cm (effective length) × 50 μm I.D.; wavelength, 254 nm; system temperature, 15 °C.

Figure 3

Simultaneous enantioseparation of seven enantiomeric pairs of indole derivatized monosaccharides. Peaks: 1 = D-Rib-BIN; 2 = L-Rib-BIN; 3 = D-Xyl-Bin; 4 = L-Man-BIN + L-Ara-BIN; 5 = L-Xyl-BIN; 6 = D-Glc-BIN; 7 = D-Man-BIN; 8 = D-Ara-BIN; 9 = L-Glc-BIN + D-Fuc-BIN; 10 = D-Gal-BIN; 11 = L-Fuc-BIN; 12 = L-Gal-BIN. CE conditions: borate buffer (100 mM, pH 9.0) contains hydroxypropyl-β-CD (10 mg/mL); applied voltage, 20 kV (detector at cathode side); uncoated fused-silica capillary, 50 cm (effective length) × 50 μm I.D.; wavelength, 254 nm; system temperature, 15 °C.

Figure 4

Electrophorograms of composition and configuration analysis of polysaccharide hydrolysate from medicinal herbs. (A) Enantioseparation of D, L pairs of seven kinds of aldo-BINs. (B) Standards of nine D-aldo-BINs for monosaccharide compositional analysis. (C) The polysaccharide hydrolysate from Dendrobium huoshanen. (D) The polysaccharide hydrolysate from Cordyceps sinensis. Peaks: 1 = D-Glc-BIN; 2 = D-Man-BIN; 3 = D-Gal-BIN; 4 = L-Glc-BIN; 5 = L-Man-BIN; 6 = L-Gal-BIN. CE conditions: buffer, 100 mM borate (pH 9.0) contains hydroxypropyl-β-CD (10 mg/mL); applied voltage, 20 kV (detector at cathode side); uncoated fused-silica capillary, 50 cm (effective length) × 50 μm I.D.; sample injection, 3s by pressure (0.5 psi); wavelength, 254 nm; system temperature, 15 °C.

Figure 5

The chemical structures of D-, L-aldo-BIN.