X-Ray analysis of d(CGCGAATTXGCG)(2) containing a 2' or minute-deoxy-N(4)-methoxycytosine residue at X: a characteristic pattern of sugar puckers in the crystalline state of the Dickerson-Drew type DNA dodecamers.

In a series of structural studies on damaged DNA, a modified Dickerson-Drew dodecamer with the sequence d(CGCGAATTmo(4)CGCG), where mo(4)C is 2'-deoxy-N(4)-methoxycytidine, was synthesized and its structure in a new crystal form has been determined by the X-ray diffraction method. The two dodecamers form a B-form duplex, in which the two mo(4)C residues, respectively, form a wobble pair and a Watson-Crick type pair with the guanine residues of the opposite strand. A comparison of the sugar conformations with those of the other related Dickerson-Drew dodecamers indicates a common feature of their puckering patterns. The sugar pucker of the third residue always adopts an intermediate state (C4'-exo-O4'-endo) between the A-form and B-form. This deviation is ascribed to the stacking interaction of the ribose ring at the third residue with the guanine base at the 12th residue, which is brought about by an extra G12:G2 interaction between two duplexes related by a crystallographic 2(1) symmetry.