A carbon-13 magnetic resonance study of the helix-coil transition in polyuridylic acid

Abstract

The temperature-dependent conformations of poly(U) in 0.5M CsC1 have been studied by carbon-13 nuclear magnetic resonance. The transition from random coil to an ordered structure results in broadening of lines in the ¹³C spectra, due to intramolecular ¹H-¹³C dipolar interactions and restricted motions in the ordered state. Changes in the chemical shifts suggest that the bases are interacting below the transition temperature. The random coil form shows conformation preferences for internal rotation about C4'-C5', C5'-O5', and C3'-O 3' bonds. The statistical randomness of the coil arises mainly because of flexibility about O-P bonds. The results are analyzed in conjunction with theoretical calculations and light-scattering data.