Conformation and dynamic structure of poly(inosinic acid) in neutral aqueous solution by ¹H‐, ²H‐, ¹³C‐, and ³¹P‐NMR spectroscopy

The conformation and dynamic structure of single‐stranded poly(inosinic acid), poly(I), in aqueous solution at neutral pH have been investigated by nmr of four nuclei at different frequencies: 1 H (90 and 250 MHz), 2 H (13.8 MHz), 13 C (75.4 MHz), and 31 P (36.4 and 111.6 MHz). Measurements of the proton‐proton coupling constants and of the 1 H and 13 C chemical shifts versus temperature show that the ribose is flexible and that base‐base stacking is not very significant for concentrations varying from 0.04 to 0.10 M in the monomer unit. On the other hand, the proton T 1 ratios between the sugar protons, T 1 (H 1 ′)/ T 1 (H 3 ′), indicate a predominance of the anti orientation of the base around the glycosidic bond. The local motions of the ribose and the base were studied at different temperatures by measurements of nuclear Overhauser enhancement (NOE) of protonated carbons, the ratio of the proton relaxation times measured at two frequencies (90 and 250 MHz), and the deuterium quadrupolar transverse relaxation time T 2 . For a given temperature between 22 and 62°C, the 13 C‐{ 1 H} NOE value is practically the same for seven protonated carbons (C 2 , C 8 , C 1′ , C 2′ , C 3′ , C 4′ , C 5′ ). This is also true for the T 1 ratio of the corresponding protons. Thus, the motion of the ribose–base unit can be considered as isotropic and characterized by a single correlation time, τ c , for all protons and carbons. The τ c values determined from either the 13 C‐{ 1 H} NOE or proton T 1 ratios, T 1 (90 MHz)/ T 1 (250 MHz), and/or deuterium transverse relaxation time T 2 agree well. The molecular motion of the sugar‐phosphate backbone (O‐P‐O) and the chemical‐shift anisotropy (CSA) were deduced from T 1 ( 31 P) and 31 P‐{ 1 H} NOE measurements at two frequencies. The CSA contribution to the phosphorus relaxation is about 12% at 36.4 MHz and 72% at 111.6 MHz, corresponding to a value of 118 ppm for the CSA (σ = σ∥ − σ⟂). Activation energies of 2–6 kcal/mol for the motion of the ribose–base unit and the sugarphosphate backbone were evaluated from the proton and phosphorus relaxation data.