We synthesized four optical isomers [D-(ApA), ADpAL, ALpAD, L-(ApA)] of adenylyl-(3'-5')adenosine (ApA) and investigated the chemical and helical structures of the dimers by means of enzymatic digestion, circular dichroism (CD) and UV melting experiments. The results of enzymatic digestion experiments with nuclease P1, snake venom phosphodiesterase (SVPD) and RNase T 2 confirmed the chemical structures of the dimers. It is known that D-(ApA) and L-(ApA) form right-and left-handed helical

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... res, respectively [P. O. P. Ts'o et al. Biochemistry, 9, 3499-3514 (1970)]. The CD spectra of the heterochiral dimers suggested that ALpAD has a right-handed helical sense whereas ADpAL has a left-handed helical sense. This result was also confirmed by UV melting experiments of the triple helices formed by the dimers with D-poly(U), which showed that the thermal stability of D-(ApA)•2poly(U) and ALpAD•2poly(U) is much higher than that of L-(ApA)•2poly(U) and ADpAL•2poly(U). Thus, the propensity of ALpAD to form the right-handed helical structure is similar to that of D-(ApA), whereas L-(ApA) and ADpAL have the similar propensity of resisting the formation of the right-handed helical structure. These results indicate that the chirality of the 3'-end residue is the primary factor for determining the helical sense of ApA. On the basis of the above results, the chemical evolution of RNA and the origin of the homochirality of RNA were discussed. (Keyword) homochirality of RNA, adenylyl-(3'Q5')-adenosine (ApA), structure of heterochiral RNA, chemical evolution of RNA, RNA world