Difluoromethylcobalamin (CF(2)Cbl), a vitamin B-12 analogue with CHF2 replacing CN, can be synthesized in a two-step procedure from aquocobalamin and CHClF2. Its crystal structure has been determined by X-ray diffraction. The compound crystallizes in the orthorhombic space group P2(1)2(1)2(1) with Z = 4 and 17 water molecules per formula unit. The unit cell dimensions are a = 24.08(1) Angstrom, b = 21.143(3) Angstrom, and c = 15.981(3) Angstrom. The refinement model was kept as simple as possible with no restraints, and with isotropic displacement parameters for all nonhydrogen atoms except for Co, P, and F. The agreement factors obtained this way are: R-1 = 0.072 for 5675 reflections with F-o > 4 sigma(F-o) and wR(2) = 0.194 for all 11844 reflections. The packing motif of CF(2)Cbl is very similar to that described for wet vitamin B-12, a distorted hexagonal close packing of the cobalamin, with channels of water running parallel to the crystallographic c axis through the crystal at x = 1/4, y = 0, and x = 3/4, y = 1/2, respectively. An analysis of interactions involving water molecules and amide groups revealed a discontinuity between oxygen-oxygen distances (which are found to be less than 2.8 Angstrom) and oxygen-nitrogen distances (which are found to be greater than 2.8 Angstrom); this provides a useful criterion for distinguishing between oxygen and nitrogen atoms in amide groups. A superposition of the crystal structures of vitamin B-12 and CF(2)Cbl shows a significant change at the molecular level. In CF(2)Cbl, the c side chain of ring B takes on a conformation that brings its terminal amide group near to the CHS group. This results in both a relatively short contact (3.11 Angstrom) between F2 and O39 of the c amide and a weak C1F-H1F ... O39 interaction.