Methyl β-lactoside [methyl β-D-galactopyranosyl-(1→4)-β-D-glucopyranoside] monohydrate: a solvomorphism study

Methyl β-lactoside [methyl β-D-galactopyranosyl-(1→4)-β-D-glucopyranoside] monohydrate, C₁₃H₂₄O₁₁·H₂O, (I), was obtained via spontaneous transformation of methyl β-lactoside methanol solvate, (II), during air-drying. Cremer-Pople puckering parameters indicate that the β-D-Galp (β-D-galactopyranosyl) and β-D-Glcp (β-D-glucopyranosyl) rings in (I) adopt slightly distorted ⁴C₁ chair conformations, with the former distorted towards a boat form (B_C1,C4) and the latter towards a twist-boat form (^O5S_C2). Puckering parameters for (I) and (II) indicate that the conformation of the βGalp ring is slightly more affected than the βGlcp ring by the solvomorphism. Conformations of the terminal O-glycosidic linkages in (I) and (II) are virtually identical, whereas those of the internal O-glycosidic linkage show torsion-angle changes of 6° in both C-O bonds. The exocyclic hydroxymethyl group in the βGalp residue adopts a gt conformation (C4' anti to O6') in both (I) and (II), whereas that in the βGlcp residue adopts a gg (gauche-gauche) conformation (H5 anti to O6) in (II) and a gt (gauche-trans) conformation (C4 anti to O6) in (I). The latter conformational change is critical to the solvomorphism in that it allows water to participate in three hydrogen bonds in (I) as opposed to only two hydrogen bonds in (II), potentially producing a more energetically stable structure for (I) than for (II). Visual inspection of the crystalline lattice of (II) reveals channels in which methanol solvent resides and through which solvent might exchange during solvomorphism. These channels are less apparent in the crystalline lattice of (I).