Synthesis, crystal structure, and spectroscopic properties of 8-(8-guanosyl)guanosine
The title compound (3) has been prepared by u.v. irradiation of a mixture of guanosine and 8-bromoguanosine. Its tetrahydrate, C20H24N10O10·4H2O, crystallises as monoclinic needles with a= 26.69(1), b= 8.425(5), c= 12.286(7)Å, β= 104.9(2)°, Z= 4, space group C2. Its structure was solved by direct methods and refined to R 0.068 for 1 071 diffractometer reflections. Two independent molecules (A) and (B) are found in the crystal with a crystallographic two-fold axis bisecting the C(8)–C(8) bonds. The conformation about the glycosidic bonds is syn for both molecules with the torsion angles χCN[O(1′)–C(1′)–N(9)–C(4)] equal to 35.7° in molecule (A) and 47.6° in molecule (B). The orientation about the C(4′)–C(5′) bonds is gauche–gauche in both molecules. The puckering of the ribose rings is C(3′)-endo for molecule (A) and C(2′)-endo for molecule (B). Owing to steric crowding, the ribose moieties in 8-(8-guanosyl)guanosine must adopt the syn-orientation in solution as well as in the solid state; its u.v., 1H n.m.r., and c.d. spectra are discussed in the context of its molecular geometry. On excitation at 330 nm, (3) exhibits intense fluorescence emission in the region of 405 nm.