**1. Introduction**

(2′,5′)Oligoadenylates represent one of the elements of cellular endogenous antiviral defense which is induced by interferon in response to RNA molecules synthesized in virally infected cells [1, 2]. The key role of 5′-triphosphorylated (2′-5′) oligoadenylates, 2-5A [ppp(A2′p5′)nA, n = 2÷15; mainly, trimer, n = 2] (**Figure 1**), in the antiviral effect of interferon is widely recognized. Oligoadenylates bind and then activate 2-5A-dependent endoribonuclease (RNase L) contributing to the hydrolysis

#### **Figure 1.**

*Structures of 5*′*-phosphates of the trimer 2*′*,5*′*-oligoadenylic acid [pppA2*′*p5*′*A2*′*p5*′*A (***2-5A***)] and dephosphorylated analog [***Core***].*

of viral mRNA and, consequently, the inhibition of viral protein synthesis [3–5]. Low molecular weight oligomers are present in nanomolar (10−9 M) concentration in living organisms and play a crucial role in the antiviral effects of interferon, maintaining early pregnancy, endochondral ossification, myogenesis, neuronal differentiation, apoptosis, heat-shock response, etc. Findings obtained from biochemical examinations of the cellular endogenous antiviral defense aroused great interest in analyzing of the 2-5A system general functioning mechanisms, as well as biochemical role played by its specific components [6–10].

However, 2-5A is rapidly cleaved by 2′-5′-phosphodiesterase [11, 12]. Moreover, the molecule has a large negative charge and is incapable to penetrate the cell membrane. These shortcomings can be eliminated by modifying the molecule while retaining the ability to bind to RNase L. Modifications can be made to the base, sugar, or phosphate fragments. But the development of new modified oligonucleotides is a challenge due to chemical, electronic, and steric problems. Additionally, the scope of application of oligonucleotides can be expanded through their structure chemical modification. Therefore, synthetic oligonucleotides are currently used for a wide range of purposes including biotechnology, molecular biology, diagnostics, and therapy.

In this review, the main focus will be on ppp(A2′p5′)2A and dephosphorylated trimers modified in the nucleotide part, and the analysis of effects of structural and stereochemical characteristics on some biological properties including immunotropic activity.
