Conformational flexibility of the 3' acceptor end of transfer ribonucleic acid

The intimate details of the conformational features and dynamics of the trinucleoside diphosphates CpCpA and ApCpC in aqueous solution have been arrived at by the complete analysis of their proton magnetic resonance spectra. In addition to the right-handed stacked species in which the phosphodiester torsions conform to the gauche-gauche domains, sugar puckers 3E, C4'-C5 approximately equal to 60 degrees, C5'-O5' approximately 180 degrees, C3'-O3' approximately equal to 205 degrees, and chi CN approximately equal to 40 degrees, the trimers display a variety of spatial configurations, an important one being a bulged configuration in which the central nucleotide unit is bulged out, enabling stacking interactions between the end units. It is further shown that the 3' acceptor end of tRNA, CpCpA, displays considerable flexibility for the terminal adenine nucleotide unit. Theoretical NMR calculations demonstrate that the predominant solution conformation does not conform to the CCA terminus of tRNA as reported by four independent crystallographic studies of tRNAPhe. It is shown that the preferred intramolecular order of CCA in solution is such that chi 1 = chi 2 = chi 3 = 40 degrees, all the three sugars are in 3E, psi 1 = psi 2 = psi 3 = 60 degrees, phi 2 and phi 3 = 170 and 180 degrees, respectively, phi 1 = phi 2' = 205 degrees, and omega 1/omega 1 and omega 2/omega 2' = 240/205 degrees and 295/265 degrees, respectively.