The Molecular Structure of 6,7‐Dimethyllumazine‐ N ‐1‐β‐ d ‐ribofuranoside, a Uridine Derivative in the syn ‐Conformation

The molecular structure of the uridine analog 6,7‐dimethyllumazine‐ N ‐1‐β‐ d ‐ribofuranoside has been determined from 4114 three‐dimensional X‐ray data. The substance crystallized in the monoclinic space group P2 1 with two crystallographically independent molecules per asymmetric unit. The intensity data were measured diffractometrically, and the positions of the 46 non‐hydrogen atoms were located by direct methods. The atomic parameters were refined by full‐matrix least‐squares methods to a discrepancy index of 5.9%. Both molecules are in the unusual syn conformation with C(2′)‐ endo sugar puckering and a gauche, gauche conformation about the C(4′)‐C(5′) bond; thus an intramolecular O(5′)‐H … O(2) hydrogen bond can be formed. The molecules are packed such that the “hydrophobic” pteridine heterocycles are stacked in endless ribbons which are separated by regions containing the sugar residues. The lumazine substituents at O(2), O(4) and N(3)‐H but not N(5) and N(8) are involved in hydrogen bonding. A discussion of the possible biochemical consequences are given.