The oligonucleotides containing N7-regioisomer of guanosine: influence on thermodynamic properties and structure of RNA duplexes.

During the chemical synthesis of the purine riboside, N7-regioisomer is kinetically formed, whereas N9-regioisomer is a thermodynamically formed product. We have studied the effect of substituting N9-regioisomer of guanosine with its N7-regioisomer (N7-guanosine, 7G) at a central position of several RNA duplexes. We found that this single substitution by 7G severely diminished their thermodynamic stabilities when 7G paired with C and U, but remarkably, led to a significant amount of stabilization in most of the duplexes when forming mismatches with G and A. The extent of stabilization was observed to be dependent on the sequence and orientation of neighboring base pairs of N7-guanosine. 1D and 2D NMR studies on the duplexes along with extensive molecular dynamics simulations revealed the conformational differences occurring due to the substitution of G by 7G, and it was observed that the thermodynamic results were largely explainable by considering the formation of stable noncanonical hydrogen bonding interactions, although other interactions such as stacking and electrostatic interactions could also play a role. These observations can have important applications in the design of RNA-based disease diagnostics and therapeutics.