B to Z transition of double-stranded poly[deoxyguanylyl(3'-5')-5-methyldeoxycytidine] in solution by phosphorus-31 and carbon-13 nuclear magnetic resonance spectroscopy

The B to Z transition of poly(dG-m5dC) . poly(dG-m5dC) has been monitored by 31P nuclear magnetic resonance (NMR) spectroscopy as a function of concentration of several salts. In 5 mM Tris- [tris(hydroxymethyl)amino-methane] HCl buffer two peaks of equal area (0.27 ppm separation) are observed at -4.2 ppm (upfield from trimethyl phosphate), indicating an alternating (right-handed) B conformation. Upon addition of NaCl, CsF, MgCl2, or Co(NH3)6Cl3, a signal arises at -3 ppm that is characteristic of the left-handed Z form. This signal is half of a doublet (1.3-2.1 ppm separation) and is tentatively assigned to the tg+ phosphodiester conformation in the Z form. The midpoints of the cooperative B to Z transitions with these salts are in good agreement with the values reported from circular dichroism (CD) studies for this copolymer. The 13C NMR spectra at natural abundance of poly(dG-m5dC) . poly(dG-m5dC) in the alternating B conformation in low salt show a broad resonance for the C2' and no clear resonance for the C3'. This is in contrast to the alternating B form of poly(dA-dT) . poly(dA-dT) under similar conditions, which exhibits a well-resolved doublet for both C2' and C3' that is presumed to derive from alternating C2'-endo and C3'-endo deoxyribose conformations. In the 13C NMR spectrum of the Z form of poly(dG-m5dC) . poly(dG-m5dC), two major peaks are observed for the C2', which may also indicate an alternating sugar conformation.