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 two-dimensional NMR study.

The non-exchangeable proton resonances of the hexadeoxynucleoside pentakisphosphates d(m5C-G)3 and d(br5C-G)3 in the B form as well as in the Z form were assigned by means of two-dimensional correlated spectroscopy and two-dimensional nuclear Overhauser enhancement spectroscopy. The complete proton NMR spectrum of the B form of the methylated compound was assigned in a pure 2H2O solution as well as in a 2H2O/C2H3O2H mixed solvent, containing 5 mM MgCl2. In the latter solvent the B form occurs in slow equilibrium (on the NMR time scale) with the Z form, the resonances of which also were fully assigned. The proton resonances of the B and Z forms of the brominated fragment were assigned in a 2H2O/C2H3O2H solution containing 5 mM MgCl2. A new and general method is described for the sequential assignment of the non-exchangeable proton resonances of oligonucleotides in the Z form.