The conformation and dynamic stucture of single-stranded poly(inosinic acid), poly(I), in aqueous solution a t neutral pH have been investigated by nmr of four nuclei a t different, trequencies: 'H (90 and 250 MHz), 'H (13.8 MHz), '"C (75.4 MHz), and 31P (36.4 and 111.6 MHz). Measurements of the proton-proton coupling constants and of the 'H and 13C 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 0.10M in the monomer unit.
On the other hand, the proton T I ratios between the sugar protons, T1 (Hl')/Tl (H:(), 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 a t different temperatures by measurements of nuclear Overhauser enhancement (NOE) of protonated carbons, the ratio of the proton relaxation times measured a t two frequencies (90 and 250 MHz), and the deuterium quadrupolar transverse relaxation time Tz. For a given temperature between 22 and 62"C, the ]:'C-{ 'Hi NOE value is practically the same for seven protonated carbons (C2, Cg, CI,, C y , C:y, C4,, C.5,). This is also true for the T1 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, T,, for all protons and carbons. The 7,. values determined from either the '"C-{'HI NOE or proton 7'1 ratios, Tl(90 MHz)/T1(250 MHz), and/or deuterium transverse relaxation time 7'2 agree well. The molecular motion of the sugar-phosphate backbone (0-P-0) and the chemical-shift anisotropy (CSA) were deduced from 7' 1 (:"P) and :31P-{LHJ NOE measurements a t two frequencies. The CSA contribution to the phosphorus relaxation is about 12% a t 36.4 MHz and 72% a t 111.6 MHz, corresponding to a value of 118 ppm for the CSA (a = q -g1). 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.