Predicted conformations of middle base pairs in (A) A-U, (B) 7A-U, (C) 7A^L-U, (D) A-C, (E) 7A-C, (F) 7A^L-C, (G) A-A, (H) 7A-A, (I) 7A^L-A, (J) A-G, (K) 7A-G and (L) 7A^L-G. Red, blue and green are used to highlight base pairs formed by A, 7A and 7A^L, respectively. Base pairs highlighted in black represent the closing GC base pairs with transparent ones illustrating the top GC base pairs. Dashed blue lines represent the hydrogen bonds. Note that due to inversion of adenine in 7A and 7A^L, functional groups are re-located, which either can destabilize the system as observed in B and C by losing a hydrogen bond or stabilize the system as observed in H, I, K and L by forming extra hydrogen bonds (Table 2).