Abstract
The base analogue N 4-methoxycytosine (mo 4C) is ambivalent in its hydrogen-bonding potential, since it forms stable base-pairs with both adenine and guanine in oligomer duplexes. To investigate the base-pair geometry, the structure of d(CGCGmo 4CG) has been determined by single-crystal X-ray diffraction techniques. The d(CGCGmo 4CG) 2 crystallized in a left-handed double helical structure ( Z-type). Refinement using 2559 reflections between 10 and 1.7 Å converged with a final R = 0.181 ( Rw = 0.130) including 68 solvent molecules. The orthorhombic crystals are in the space group P2 12 12 1, with cell dimensions a= 18.17 A ̊ , b = 30.36 A ̊ , c = 43.93 A ̊ . The mo 4C · G base-pair is of the wobble type, with mo 4C in the imino form, and the methoxy group in the syn configuration.
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