**1.2 Types of aptamers**

a molecular weight ranging from 5 to 15 kDa [4]. Multiple aptamers have been generated so far, successfully binding to a wide variety of different objects such as small molecules, proteins, and cells. Similar to the antibody-antigen interaction, the

Recently aptamers are capable of binding different targets such as large protein complexes, simple inorganic molecules, and total cells [5]. Thus, aptamers can be regarded as nucleotide analogues of antibodies [6]. However, the production of

Their binding properties are similar to those found for antibodies, being in the nanomolar to the picomolar range [7], and aptamers have been identified to distinguish between members of a protein family, as they recognize target structures in an epitope-specific manner [8]. Compared with antibodies, nucleic acid aptamers have many advantages in their suitability for clinical application and industrializa-

*Schematic representation of binding of an aptamer to a target protein. After binding, the aptamer interacts with a target molecule such as protein to fold into a 3D structure, which forms a stable target aptamer complex [3].*

tion, including almost no immunogenicity, efficient penetration, less batch variation, easy modification, cost-effectiveness, and short production times [9]

recognition between aptamers and their target is very specific.

aptamers is easier and less expensive than antibodies.

*Neuroprotection - New Approaches and Prospects*

(**Table 1**).

**Figure 1.**

**Table 1.**

**274**

*Comparison between aptamer and antibody.*

Nucleic acid aptamers are short single-stranded DNA or RNA molecules and can be selected from complex libraries by a technique called "systematic evolution of ligands by exponential enrichment" (SELEX). These aptamers are capable of binding to the target molecule with high affinity and specificity. Nucleic acid aptamers are functionally similar but have some differences in their stability and accessibility (**Table 2**). DNA aptamers are less reactive and relatively stable because of the C▬H bonds at the 21st position of the deoxyribose sugar of DNA nucleotides. This chemical difference gives DNA aptamers an inherent advantage in stability over RNA aptamers [10].

RNA aptamers are defined as RNA oligonucleotides that bind to a specific target with high affinity and specificity, similar to how an antibody binds to an antigen [11]. RNA aptamers are less stable than DNA aptamers due to the presence of a reactive hydroxyl group (▬OH) in the 21st position of the ribose sugar in the RNA nucleotides. This ▬OH group is especially deprotonated, particularly in alkali solutions. The resulting anionic 21-O can be nucleophilically attached to the phosphorus atom of the phosphodiester bond, leading to hydrolysis of RNA molecules. It was found that the nuclease resistance of RNA aptamers increased when the 21 hydroxyl group was removed from RNA sugars [10, 12]. Because of the C▬H bonds of the DNA nucleotides at position 21 of the deoxyribose sugar, DNA aptamers are less reactive and relatively stable. Due to this chemical difference, DNA aptamers are more stable than RNA aptamers.

Another type of aptamer developed around 1996 was peptide aptamers. The concept, originally introduced by Roger Brent, proposed a short amino acid sequence embedded ("double constrained") within the context of a small and very stable protein backbone ("scaffold") [13, 14]. Peptide aptamers are conjugated protein molecules with specific binding affinity to target proteins formed under intracellular conditions. The typical structure of peptide aptamers is a short peptide region inserted within a scaffold protein. The short peptide region is responsible for binding with the target protein. The scaffold protein helps to increase binding affinity and specificity by a restriction on the conformation of the binding peptide [15]. Thus, peptide aptamer applications range from in vitro detection of various proteins in a complex mixture to in vivo modulation. In addition, peptide aptamers


#### **Table 2.**

*Comparison of RNA, DNA, and peptide aptamers.*

have been recognized as therapeutic molecules due to their anticancer and antivirus activities.
