2Ј,5Ј-Linked oligoadenylates (2-5A) are involved in the antiviral action of interferon. The 2-5A binds and activates 2-5A dependent RNase (RNase L), which degrades viral mRNA, resulting in the inhibition of protein synthesis. 2Ј,5Ј-Linked phosphorothioate oligoadenylates with an Rp configuration bind to and activate the RNase L. On the other hand, 2Ј,5Ј phosphorothioate oligoadenylate with an Sp configuration weakly binds to the RNase L and is devoid of the RNase L activation ability. Comparative circular dichroism (CD) and NMR studies are carried out to characterize the difference in properties between the two configurations of the 2Ј,5Ј phosphorothioate oligoadenylates. 2Ј,5Ј Rp-Phosphorothioate oligoadenylates showed CD spectra similar to those of the corresponding native 2Ј,5Ј oligoadenylates, while the 2Ј,5Ј Sp-phosphorothioate oligoadenylates exhibited a weaker CD band compared to the former two, indicating the weaker base-stacking interaction of the 2Ј,5Ј Sp-phosphorothioate oligoadenylates. The temperature-dependent change in the CD revealed that 2Ј,5Ј phosphorothioate oligoadenylates showed larger ⌬H°and ⌬S°values for the thermal transition of the conformation than the corresponding native 2Ј,5Ј oligoadenylates. The NMR spectral assignment was accomplished by several NMR measuring techniques. The 2Ј-H of the ribose ring linked to the 2Ј,5Ј Sp-phosphorothioate showed a higher field chemical shift of the proton NMR than that linked to the corresponding 2Ј,5Ј Rp-phosphorothioate. 2Ј,5Ј Rp-and Sp-phosphorothioate oligoadenylates possess a sugar conformation similar to that of the corresponding native 2Ј,5Ј oligoadenylates.

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