A novel intercalation of a water molecule between pyrimidine bases in 5‐nitro‐1‐(β‐d‐ribosyluronic acid)–uracil monohydrate [1‐(5‐nitro‐2,4‐dioxopyrimidinyl)‐β‐d‐ribofuranuronic acid monohydrate]

C 9 H 9 N 3 0 9 . H 2 0 , monoclinic, P21, a = 8 .982 (1), b = 10.245 (1), c = 6.651 (1) ,/k, fl = 92 .27 (1) ° at 22 + 3 ° C , V = 611.5 A 3, Z = 2, F W 321, 2(Cu Kt~) = 1 .54051 /~ , D m (flotation) = 1.74, D x = 1-75 g cm -3, g (Cu Ka) = 7.6 cm -~, R = 0 .056. The molecule exhibits anti conformat ion (ZCN = 53"9°) , an intramolecular C(6) -H . . . 0 ( 5 ' ) hydrogen bond and C(2')-endo, C(3')-exo (2T3) pucker. Two unusual features in this structure are (a) the intercalation of water molecules between stacked pyrimidine rings 6.55 apar t and related by the c translat ion and (b) a number of C H . . . O interactions. Introduction. In connection with a project on radiation sensitization, we have been investigating a series of nitro-substituted bases and nucleosides. Our interest in these structures was to study the hydrogen-bonding scheme of these modified nucleosides as well as to correlate the orientation of the 'g ' tensor of these nitro radicals (analysis in progress by Dr H. Box of this Institute) using spin resonance techniques. The title compound (I) was first synthesized by Fox and co-workers (Wempen, Doerr , Kaplan & Fox, 1960) in the course of their screening of nitro analogs of nucleosides for possible ant i tumour activities. Crysta ls of the title compound were obtained by the addition of conc. H N O 3 to the aqueous solution of uridine, followed by slow evaporat ion of the resulting solution. The unit-cell and other crystal lographic da ta are given * Stereochemistry of Nucleic Acid Components and Their Reaction Products. V. Part IV: Parthasarathy, Ohrt & Chheda (1976). t To whom correspondence should be addressed. in the Abstract. 1438 reflections (only nine had intensities <2tr) to the limit of 20 -165 ° for Cu Kct were collected using the s ta t ionary-c rys ta l s ta t ionarycounter procedure (Furnas & Harker , 1955) and were processed in the usual way. A detailed absorption correction depending on the shape of the crystal (Coppens, 1970) was carried out. The structure (Fig. 1) was solved by a combinat ion of M U L T A N (Germain , Main & Woolfson, 1971) and tr ial-and-error methods. One of the E maps gave two Table 1. Atomic coordinates (x l04 ) for the nonhydrogen atoms with e.s.d.'s in parentheses