Unusual Structures in Single-Stranded Ribonucleic Acid: Proton Nuclear Magnetic Resonance of AUCCA in Deuterium Oxide

Conformational features of the oligoribonucleic acid (oligo-RNA) A¹-U²-C³-C⁴-A⁵ are explored by proton nuclear magnetic resonance (NMR). The sequence is a molecular cognate of a portion of the TɳC loop and stem regions of yeast tRNA^phe. The molecule forms at least two classes of flexible yet ordered structures. Class I states are similar in spectral properties to the component oligomers, AU, AUC, and AUCC, and are likely to be standard right-helical structures. Class II states are characterized by a 2'-endo pucker at A¹ and unusually large shielding of several C³ and U² protons. Most of these features are consistent with identifying the class II solution structures with the “arch” conformation for the TɳC region determined by X-ray crystallography of yeast tRNAPhe.