Gentiobiose has the molecular formula C12H22O11 and has been isolated from gentian root and by hydrolysis of amygdalin. Gentiobiose exists in two different forms, one melting at 86°C and the other at 190°C. The lower melting form is dextrorotatory ([a] 22/D + 16°), the higher melting one is levorotatory ([a] 22/D -6°). The rotation of an aqueous solution of either form, however, gradually changes until a final value of ([a] 22/D + 9.6° is observed. Hydrolysis of gentiobiose is efficiently catalyzed by emulsin and produces two moles of D-glucose per mole of gentiobiose. Gentiobiose forms an octamethyl ether, which on hydrolysis in dilute acid yields 2, 3, 4, 6-tetra-Omethyl-D-glucose and 2, 3, 4-tri-O-methyl-D-glucose. What is the structure of gentiobiose?