Abstract: | Methyl β‐allolactoside [methyl β‐d ‐galactopyranosyl‐(1→6)‐β‐d ‐glucopyranoside], (II), was crystallized from water as a monohydrate, C13H24O11·H2O. The βGalp and βGlcp residues in (II) assume distorted 4C1 chair conformations, with the former more distorted than the latter. Linkage conformation is characterized by ϕ′ (C2Gal—C1Gal—O1Gal—C6Glc), ψ′ (C1Gal—O1Gal—C6Glc—C5Glc) and ω (C4Glc—C5Glc—C6Glc—O1Gal) torsion angles of 172.9 (2), −117.9 (3) and −176.2 (2)°, respectively. The ψ′ and ω values differ significantly from those found in the crystal structure of β‐gentiobiose, (III) [Rohrer et al. (1980). Acta Cryst. B 36 , 650–654]. Structural comparisons of (II) with related disaccharides bound to a mutant β‐galactosidase reveal significant differences in hydroxymethyl conformation and in the degree of ring distortion of the βGlcp residue. Structural comparisons of (II) with a DFT‐optimized structure, (IIC), suggest a link between hydrogen bonding, pyranosyl ring deformation and linkage conformation. |