2-Acetamido-2-deoxy-3-O-β-d-galactopyranosyl-d-glucose dihydrate

In the title compound, C14H25NO11·2H2O, the primary hydroxyl group connected to the anomeric C atom of the N-acetyl-β-d-glucopyranose residue exhibits positional disorder, with occupancy factors for the α and β anomers of 0.77 and 0.23, respectively. The two torsion angles (Φ and Ψ) and the bridge angle (τ) that describe conformation of the glycosidic linkage between the galactopyranose and glucopyranose rings are Φ = −81.6 (3)°, Ψ = 118.1 (2)° and τ = 115.2 (2)°. Two water molecules stabilize the molecular packing by forming hydrogen bonds with the saccharide residues.

In the title compound, C 14 H 25 NO 11 Á2H 2 O, the primary hydroxyl group connected to the anomeric C atom of the Nacetyl--d-glucopyranose residue exhibits positional disorder, with occupancy factors for the and anomers of 0.77 and 0.23, respectively. The two torsion angles (È and É) and the bridge angle () that describe conformation of the glycosidic linkage between the galactopyranose and glucopyranose rings are È = À81.6 (3) , É = 118.1 (2) and = 115.2 (2) . Two water molecules stabilize the molecular packing by forming hydrogen bonds with the saccharide residues.
This study was supported in part by a grant from the Promotion of Basic Research Activities for Innovative Biosciences (PROBRAIN) of Japan.

Comment
It is widely accepted that oligosaccharides other than lactose in human milk (human milk oligosaccharides, HMOs) play a key role in the growth of Bifidobacteria in the gut. Bifidobacteria, Gram-positive anaerobes, are considered to be beneficial for human health. Recently, a unique metabolic pathway specific for lacto-N-biose I (Gal-β1→3GlcNAc, LNB) was found using Bifidobacteria (Kitaoka et al., 2005;Nishimoto & Kitaoka, 2007a). LNB is one of the basic core disaccharides of HMOs and is suggested to be a bifidus factor.
The molecular structure of compound (I) is shown in Fig. 1. There are two water molecules per LNB molecule in the crystal lattice. The primary hydroxyl group connected to the anomeric carbon atom of the GlcNAc residue exhibits disorder, with occupancy factors of O71 (α anomer) and O72 (β anomer) of 0.77 and 0.23, respectively.
The Gal ring is close to the ideal 4 C 1 chair conformation with ring puckering parameters (Cremer & Pople, 1975)  Both saccharide rings lie approximately parallel to the bc plane and the intermolecular hydrogen bonds were only along the a-axis (Table 2). Two water molecules stabilize the molecular packing by forming hydrogen bonds with sugar molecules in three dimensions.

Experimental
Compound (I) was synthesized from sucrose and GlcNAc by the concurrent action of four enzymes: sucrose phosphorylase, UDP-glucose-hexose-1-phosphate uridylyltransferase, UDP-glucose 4-epimerase, and lacto-N-biose phosphorylase (Nishimoto & Kitaoka, 2007b). Single crystals suitable for X-ray analysis were obtained by slow diffusion of ethanol into an aqueous solution.