Synthesis, crystal structure, and spectroscopic properties of 8-(8-guanosyl)guanosine

Abstract

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 χ_CN[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.