A 270 mhz 1H-NMR study of two “branched” ribonucleotides A 3'p5'u2'p5'g and A 3'p5'u2'p5'g which are products of pre-mRNA processing (“splicing”) reactions

Abstract

A 270 MHz 1H-NMR study of two branched trinucleotides 2 and 3 have been carried out by comparing their temperature-dependent chemical shifts with A2'p5'G ( 4 ). It has emerged from this study that the adenine and guanine residues in 2 and 3 are quite strongly stacked as in A2'p5'G (4). Appropriate ID NOE experiments have shown that the pyrimidine residue in 2 and 3 is In an anti conformation with respect to its own sugar moiety white the adenine and guanine residues are in syn conformation. The pyrimidine residue should be coplanar with the adenine' residue in order to accomodate the observed deshielding of H8A and HI'A in 2 and 3 as compared to 4 . The latter rationale also suggests a spatial proximity of H8A/H1'A and H5C(H5U)/H6C(H6U) in compounds 2 and 3 . The proposed conformation of the branched trinucleotides 2 and 3 Is thus comprised of a stacked state between adenine and guanine moieties while the pyrimidine residue Is apart and should adopt a coplanar state with the adenine part. Finally, the analysis of the conformational states of the branched nudeotides 2 and 3 , has led to the development of a suitable rationale for the conformatibn-driven-energy-pump for the self-catalyzed splicing reactions.