[Structures of isomers of ursodeoxycholic acid: 3 beta,7 alpha- and 3 beta,7 beta-dihydroxy-5 beta-cholan-24-oic...

Isomers of ursodeoxycholic acid, 3 beta,7 alpha- and 3 beta,7 beta-dihydroxy-5 beta-cholan-24-oic acids (3 beta 7 alpha U and 3 beta 7 beta U) crystallize in the orthorhombic space group P2(1)2(1)2(1) containing one molecule and in the monoclinic group P2(1) containing two independent molecules in an asymmetric unit. The cell dimensions are a = 28.032(17), b = 9.973(5), c = 8.049(6) A for 3 beta 7 alpha U, and a = 11.771(8), b = 27.999(12), c = 6.637(2) A, beta = 90.78(6) degrees for 3 beta 7 beta U, respectively. Both structures were solved by the direct method and refined to the residual values of 0.065 (3 beta 7 alpha U) and 0.059 (3 beta 7 beta U). The conformations of the D rings of each molecule are different from each other: 3 beta 7 alpha U intermediate, 3 beta 7 beta U (A and B) half-chair form. In the crystal, 3 beta 7 alpha U molecules are connected by a hydrogen bond extended nearly parallel to the bc plane and 3 beta 7 beta U molecules are connected by the helical hydrogen bond system or by the zigzag one parallel to the ac plane.