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)aminomethane] 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.