Circular dichroism of oligosaccharides containing N-acetyl amino sugars

Abstract

The circular dichroism (CD) spectra of N-acetyl amino monosaccharides and some of their methoxy derivatives, N-acetyl disaccharides, the chitin series of oligosaccharides (disaccharide through hexasaccharide), and a glycopeptide linkage compound 2-acetamido-1- N- l-aspartyl-2-deoxy-β- d-glucopyranosyl-amine (GlcNAc-Asn) were measured in order to determine which structural features are influential in determining the CD in the 190–220 nm region. The CD of the α anomers of 2-acetamido-2-deoxy- d-glucose and -galactose (GlcNAc and GalNAc, respectively) are identical to those of the corresponding equilibrium mixture whereas, for 2-acetamido-2-deoxy- d-mannose, the α anomer has a significantly more positive CD in the n-π ∗ (208–210 nm) region than does the β anomer. The CD of the α- and β-methoxy glycosides of GlcNAc and GalNAc show negative CD bands of comparable magnitude in the n-π ∗ region (209 nm) and positive CD bands of differing magnitudes in the π-π ∗ (190 nm) region. The CD spectra of β-1–4 and β-1–6 linked 2,2′-diacetamido-2,2′-dideoxyglucosyl glucoses both show a negative CD band in the n-π ∗ region and a positive band in the π-π ∗ region. The CD curves of the chitin series of oligosaccharides all exhibit a negative CD band at about 210 nm, followed by a positive band at about 192 nm. A small progressive increase in molar ellipticity per sugar residue for both the positive and negative bands with an increase in chain length is observed. The CD spectra of GlcNAc-Asn at three different pH's differ greatly from those of N-acetyl mono- and disaccharides. The CD spectrum of GlcNAc-Asn indicates a strong interaction of the exciton type between chromophores. It is concluded from the CD data that the n-π ∗ band for these compounds is not very sensitive to anomeric configuration or to disaccharide linkage of the 1–4 and 1–6 types. In contrast, the CD in the π-π ∗ region does show variation with anomeric configuration, substituent position, and linkage position. In particular, substituents at the 3 position lead to a small negative CD at 190 nm while substituents at the 4 position lead to enhanced positive CD at 190 nm.