[Studies on the crystal structure of ivermectin (H2B1a)].

The crystal structure of ivermectin (H2B1a) was determined by X-ray diffraction. Ivermectin was found to be composed of more than 80% H2B1a and less than 20% H2B1b. The crystal of H2B1a belongs to the space group C2 with a = 40.736(6)A, b = 9.295(3)A, c = 14.966(3)A, beta = 106.93(1) degree. After structure refinement using least-squares method, the final R factor of H2B1a is 0.0658. The space configuration of H2B1a is a long chain along the C-axis. The molecular aggregates are extended into pairs of long chains parallel to each other, some distance apart. The distance between chains is the same, but their direction is opposite. This structure of H2B1a is similar to some biomacromolecules.