1235
Stereochemistry of Nucleic Acids and Their Constituents. XV.' Crystal and Molecular Structure of 2-Thiocytidine Dihydrate, a Minor Constituent of Transfer Ribonucleic Acid George Hung-Yin Lin, M. Sundaralingam," and S. K. Arora
Contribution f r o m the Department of Biochemistry, University of Wisconsin, Madison, Wisconsin 53706. Received M a y 28, 1970 Abstract: 2-Thiocytidine crystallized from aqueous etbanol solutio? as the dihyGrate, C9HI3N3O4S. 2Hz0, in the form of triclinic crystals, space group P1 with a = 9.595 A, b = 7.135 A, c = 5.030 A, a = 95.58", /3 = 99.81 and y = 104.65'; Z = 1. The crystal structure has been determined from 1103three-dimensional X-ray intensity data (Cu Ka: radiation) obtained by the use of a four-circle diffractometer. The structure was solved by the heavy-atom method and refined by [ull-matrix least squares to an R of 0.039. The average estimated standard deviations in bond distances is 0.005 A and in bond angles 0.4" for nonhydrogen atoms. The glycosidic torsional angle, 20.3', is anti. The glycosidic C(1')-N bond distances in the pyrimidine derivatives show a tendency to increase with a decrease in this angle. An analysis of the deviations of the base atoms from the least-squares plane of the base reveals that the base is flexible. The mode of puckering of the base is regulated by the molecular environment. The sugar conformation is C(3 ')-endo-C(2')-exo (T2). The conformation about the C(4')-C(5 ') bond is gauchegauche. The sulfur atom is hydrogen bonded both to a water molecule and the amino group of an adjacent molecule. The latler hydrogen bond is the only interbase hydrogen bond. There is also a water-water hydrogen bond of 2.14 A. O,
T
he thio derivatives of pyrimidine and purine bases and nucleosides have been a subject of considerable interest in recent years. U p to date, the crystal structure of 2-thioura~i1,~ 2,4-dithiouracil, 2-thio~ytosine,~ 4 - t h i o ~ r i d i n e , 4-thiouridine ~ disulfide,6 2,4-dithiouridine,? 6 - m e r c a p t o p ~ r i n e , ~ > 2-mercapto-6-methylpu~ rine, lo 6-thioguanine, l 1 and 6-thioinosine l 2 have been reported. Moreover, the thio derivatives such as 4-thiouridine, l 3 - I 5 5-methylaminomethy1-2-thiouraci1, l6 N 6 (3-methy1-2-b~tenyl)-2-methylthioadenosine,~~ and 2thiocytidine16 are found as minor constituents in several E. coli transfer ribonucleic acids (tRNAs), while 6-thioinosine and 6-thioguanosine are effective as enzyme inhibitors and antitumor agents.lS The function of the thio analogs of pyrimidine and purine nucleosides in tRNAs is not yet known. The present investigation of the molecular structure of 2-thiocytidine dihydrate is ( I ) Part XIV: G. H.-Y. Lin and M . Sundaralingam, Acta Crystallogr., in press. (2) D. Tsernoglou, Ph.D. Dissertation, Yale University, New Haven,
Conn., 1966. (3) E. Shefter and H. G. Mautner, J . Amer. Chem. Soc... 89.. 1249 ( 1967). (4) S . Furburg and L. H. Jensen, private communication. ( 5 ) W. Saenger and I
