Conformational Analysis of the Nucleotides A2'-5'A, A2'-5'A2'-5'A and A2'-5'U from Nuclear Magnetic Resonance and Circular Dichroism Studies

In recent publications A2'-5'A2'-5'A was found to be an inhibitor of protein synthesis. In this research conformational analysis of the 2'-5'-linked nucleotides A2'-5'A, A2'-5'A2'-5'A and A2'-5'U is reported. The complete 1H-NMR assignment of the three compounds is given. The degree and mode of base-base stacking is extracted from coupling constant data and circular dichroic (CD) spectra at various temperatures. The 2'-5' nucleotides surprisingly show a much stronger tendency to stack than the 3'-5' compounds. At 85 degrees C A2'-5'A occurs for about 50% in stacked states. The mode of stacking is different from 3'-5'ribonucleotides where the sugar rings predominantly adopt an N conformation. A2'-5'U displays an A(S)2'-5'U(N) stacked state. In A2'-5'A 'mixed' modes of stacking, i.e. NN, NS, SN and SS, are proposed to account for the CD and NMR observations.