Conformational analysis of the B and Z forms of the d(m5C-G)3 and d(br5C-G)3 hexamers in solution. A 300-MHz and 500-MHz NMR study.

The B and the Z forms of the DNA hexamers d(m5C-G)3 and d(br5C-G)3 were investigated by means of NMR spectroscopy. It is demonstrated that the low-salt form of d(m5C-G)3 is a B DNA structure. The form, which becomes increasingly predominant when increasing amounts of MgCl2 and/or methanol are added to the solution, has Z DNA characteristics. It is shown that the major geometrical features of the Z form of d(m5C-G)3 in the crystal structure are maintained in solution, with the dC residues S sugar conformation, gamma + and the base in the anti orientation and the dG residues N (except the 3'-terminal residue), gamma t and syn. Neither the Z form of the methylated nor that of the brominated compound resembles the Z' form, in which the deoxy guanosine sugar rings adopt a C1'-exo conformation. Substitution of m5C by br5C causes no perceptible conformational changes in either the B or in the Z forms.