Intramolecular triplexes. A) Schematic representation of an intramolecular triplex. The third strand (gray) is shown residing in the major groove of duplex DNA. B) Schematic representations of H-DNA isomers H-y3 (1) and H-y5 (2), and H'-DNA isomers H-r3 (3) and H-r5 (4). In H(H') DNA nomenclature, the nature of the third strand is identified as either pyrimidine (y) or purine (r) and originating from either the 3' or 5' end of the corresponding oligopyrimidine or oligopurine strand. The purine-rich strand is shown in black, while the pyrimidine-rich strand is shown in light gray.