The Crystal and Molecular Structure of β-Lactose

Abstract

Abstract The crystal structure of β-lactose (C12H22O11) was solved by direct phasing methods with integrated photographic intensity data. The structure was refined by the anisotropic least-squares method to give a final R value of 0.068 for the observed reflections. The space group is P21 with Z=2 and with unit cell dimensions of a=10.839 (6), b=13.349 (6), c=4.954 (5) Å, and β=91.31 (9)°. All the hydrogen atoms were located on different syntheses. The crystals are nearly isostructural with those of β-cellobiose except for the axial O(4) atom. The exocyclic C(5)–C(6) and C(5′)–C(6′) bonds are significantly shorter than the other C–C bonds. The lengths of the two ring C–O bonds are unequal in the galactose unit and equal in the glucose unit. The C(1)–O(1) and C(1′)–O(1′) bond lengths are shorter by 0.015 Å and 0.030 Å than the mean exocyclic C–O length of 1.417 Å. The structure contains an intramolecular hydrogen bond and exhibits an asymmetrical twist about the bridge bond. All the oxygen atoms, except the bridge oxygen atom, are involved in the hydrogen-bonding network, which comprises two terminating chains.