We here show an electrostatic polar-pi interaction from the first to the third aglycon, via the second aglycon, in the ground state in two single stranded trimeric RNAs, 5'-GpA'pA -3' (3) and 5'-GpApC-3' (4), as a result of intramolecular nearest neighbor offset-stacking. The experimental evidence in support of this conclusion has been obtained by comparing the pK(a)s of each aglycone in the two trimers with those of guanosine T-ethyl phosphate, GpEt (1) and 5'-GpA-3' (2): Thus, the pK(a) of N-1-H of guanin-9-yl of 5'-PpA(1)pA(2)-3' (3) could be measured by pH titration (pH 7.3-11.6) of its own deltaH8G (pK(a) 9.75 +/- 0.02) as well as from deltaH8A(1) (pK(a) 9.72 +/- 0.02) and deltaH2A(1) (pK(a) 9.83 +/- 0.04) of the neighboring pA(1)p moiety and the deltaH8A(2) (pK(a) 9.83 +/- 0.02) of the terminal pA(2) Moiety. Similarly, the pH titration of GpApC (4) shows the pK(a) of N-1-H of guanin-9-yl from its own deltaH8G (pK(a) 9.88 +/- 0.03) as well as from deltaH8A (pK(a) 9.87 +/- 0.01) of the neighboring pAp moiety, and deltaH5/H6C (pK(a) 9.88 +/- 0.01 and 9.90 +/- 0.01 respectively) of the 3'-terminal cytosin-1-yl. This intramolecular nearest neighbor electrostatic interaction in the single-stranded RNA modulates the pseudoaromaticity of the nearest neighbors by almost total transmission of DeltaGdegrees(pKa). because they constitute an extended array of offset-stacked coupled aromatic heterocycles within a polyanionic sugar-phosphate backbone at the ground state. The enhanced basicity of Gp residue by ca. 0.6 pKa unit in the trimers compared to that of the dinner is a result of the change in the electrostatic microenvironment owing to the nearest neighbors in the former (the nucleobases as well as the phosphates). Thus, the DeltaGdegrees(pKa) from the 5'-guanylate ion to the 3'-end aglycon via the central adenin-9-yl is 55 to 56 kJ mol(-1) in each step through a distance spanning similar to6.8 A in an unfolded state. As a result, the pKa of guanin-9-yl moiety has become 9.25 +/- 0.02 in GpEt (1), 9.17 +/- 0.02 in GpA (2), 9.75 +/- 0.02 in GpApA (3), and 9.88 +/- 0.03 in GpApC (4). This means that guanin-9-yl moiety of trimers 3 and 4 is more basic than in the monomer or the dimer. The net outcome of this electrostatic cross-talk between the two neighboring heterocycles is creation of new hybrid aglycones in an oligo or polynucleoticle, whose physicochemical property and the pseudoaromatic character are completely dependent both upon the nearest neighbors, and whether they are stacked or unstacked. Thus, this tunable physicochemical property of an aglycon (tin array of the extended genetic code) may have considerable implication in our understanding of the specific ligand binding ability of an aptamer, the pKa and the hydrogen bonding ability of nucleic acids in a microenvironment, or in the triplet usage by the anticodon-codon interaction in the protein biosynthesis in the ribosome.