(a)-(b): Superposition of d(G-G) and d(G-C+) pairs to the reference d(A-T) one in parallel DNA. (c)-(d): Superposition of d(G·C-G) and d(G·C-C+) triads to the reference d(A·T-T) one in parallel triplex. In all the cases the central purine atoms were used as reference in the fitting.
