We synthesized four diastereomeric isomers [D-d(ApA), d(ADpAL), d(ALpAD), L-d(ApA)] of 2’-deoxyadenylyl-(3’-5’)-2’-deoxyadenosine and investigated the chemical and helical structures of the dimers by means of enzymatic digestion, circular dichroism (CD) and UV melting experiments of their complexes with poly(U). The results of enzymatic digestion experiments with nuclease P1 and snake venom phosphodiesterase (SVPD) confirmed the chemical structures of the dimers. The CD spectra of the dimers suggested that heterochiral d(ALpAD) has a right-handed helical sense as well as natural D-d(ApA), whereas d(ADpAL) has a left-handed helical sense as well as L-d(ApA), an unnatural enantiomer of D-d(ApA). The right-handed helix-forming ability of the dimers in the presence of the complementary strand was also investigated by UV melting experiments of the triple helices formed by the dimers with D-poly(U). The results showed that D-(ApA), d(ALpAD) and d(ADpAL) form relatively stable triple helices with poly(U), whereas L-d(ApA) is unable to hybridize with poly(U). Thus, the propensities of D-(ApA), d(ALpAD) and d(ADpAL) to form the right-handed helical structure with complementary strands are not much different from each other, in contrast, that of L-d(ApA) is quite low. On the basis of these results, the chemical evolution of RNA and the origin of the homochirality of RNA were discussed.