Interaction energies in hydrogen-bonded purine-pyrimidine triplets.

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Abstract
Quantum-mechanical calculations have been performed on the interaction energies in hydrogen-bonded purine-pyrimidine triplets. In the cyclic homo-triplet of hypoxanthines as in poly I [inosinic acid], the formation of triplets represents a more advantageous organization than that of a dimer by about 1.2 kcal per base. In the open hetero-triplets A [adenine] + 2 U [uridine] and A + 21 [inosine], the interaction between the nonlinked terminal bases introduces a repulsion, which may be significant for the geometry of the adopted configuration. The interaction energy is relatively small in A + 21. It is, on the contrary, relatively very great in G [guanine] + 2C [cytosine]. The involvement of a protonated cytosine in hydrogen bonding with guanine (or with a nonprotonated cytosine) greatly increases the energy of interaction.