Background

Nucleic Acid Aptamers are single-stranded DNA or RNA oligonucleotides characterized by their low molecular weight, ease of synthesis and modification, lack of immunogenicity, high stability, and broad target range. Precise, controllable chemical synthesis is essential both for constructing the initial screening library and for generating and applying candidate aptamers identified through screening. This article details the complete chemical synthesis workflow for nucleic acid aptamers, covering sequence design, solid-phase synthesis, cleavage and detritylation, and purification and quality control.

Core Process of Aptamer Chemical Synthesis

Sequence Design

Aptamer sequences are typically determined based on screening results. Aptamer lengths generally range between 20 and 80 nucleotides. Synthesis difficulty, efficiency, and cost increase with nucleotide count. Sequence design often incorporates chemical modifications to enhance aptamer stability, affinity, and specificity. Special functional groups may also be introduced.

Monomer (Phosphoramidite) Preparation

The core raw materials for chemical synthesis are protected nucleoside phosphoramidite monomers. DNA synthesis employs dA, dC, dG, and dT monomers; RNA synthesis uses rA, rC, rG, and rU monomers.

To achieve specific aptamer functions, various modified monomers are available:

Backbone Modifications

Phosphorothioate (PS, enhances nuclease resistance), phosphonate (increases hydrophilicity).

Sugar Ring Modifications

2'-Fluoro (2'-F), 2'-Methoxy (2'-OMe), 2'-O-Methyl, Locked Nucleic Acid (LNA, significantly improves stability and affinity).

Base Modifications

5-Methylcytosine, inosine, etc.

Terminal Modifications

5' or 3' end Amino (-NH₂), Thiol (-SH), Biotin, Fluorescent dyes (FAM, Cy series), Polyethylene Glycol (PEG, extends half-life), Cholesterol (membrane anchoring).

Solid-Phase Synthesis

Solid-phase synthesis utilizes the Solid-Phase Phosphoramidite Chemistry in a cyclic process, adding one nucleotide per cycle.

Key steps are:

Detritylation

An acidic reagent (e.g., trichloroacetic acid/TCA) removes the 5'-terminal dimethoxytrityl (DMT) protecting group, exposing the 5'-hydroxyl.

Coupling

An activator (commonly a tetrazole derivative) activates the phosphoramidite monomer. The activated monomer reacts with the exposed 5'-hydroxyl to form a phosphite triester linkage.

Capping

An acetylating reagent (e.g., acetic anhydride/N-methylimidazole) acetylates any unreacted 5'-hydroxyl groups.

Oxidation

An iodine solution oxidizes the phosphite triester linkage to a stable pentavalent phosphate triester bond.

Note: Sulfurization reagents replace oxidation if phosphorothioate linkages are required.

Steps (1) through (4) are repeated until the full-length Aptamer Sequence is synthesized.

Post-Synthesis Processing

Cleavage

Ammonia or methylamine solution cleaves the synthesized oligonucleotide chain from the solid support.

Base Detritylation

Protecting groups on the nucleobases are removed.

Additional steps are required to remove protecting groups from any specially modified moieties.

Purification and Quality Control

Aptamer applications demand high purity. Impurities like synthesis failure sequences, deprotection reagents, and salt ions must be removed.

Primary purification techniques include:

(i) High-Performance Liquid Chromatography (HPLC: Reverse Phase (RP) or Ion Exchange (IE)).
(ii) Polyacrylamide Gel Electrophoresis (PAGE).
(iii) Affinity Purification. Desalting is typically performed post-purification to eliminate residual salts and small molecule impurities.

Critical quality control parameters include:

(i) Purity: Assessed by HPLC (RP/IE) or Capillary Electrophoresis (CE).
(ii) Molecular Weight: Confirmed by Mass Spectrometry (MALDI-TOF MS or ESI-MS).
(iii) Concentration Quantification: Measured via UV spectrophotometry. (iv) Functional Validation.

Summary and Outlook

Solid-phase oligonucleotide synthesis is the core process transforming aptamer sequences into functional molecules. Precise execution and strategic chemical modifications ensure high-purity, high-quality aptamer synthesis.

Professional service providers (Alpha Lifetech) leverage extensive SELEX expertise to deliver comprehensive Aptamer Services, including screening, synthesis, and validation of high-affinity, high-stability Nucleic Acid Aptamers.

FAQ

1. After finalizing aptamer sequence design, what key chemical factors should be considered before chemical synthesis to optimize synthesis success and efficiency?

Sequence design based on SELEX must prioritize binding activity and specificity, with critical factors including:

(i) Sequence length: Coupling efficiency decreases exponentially with longer chains (>30 nt). Length directly impacts synthesis difficulty and purification complexity.
(ii) Base composition and homopolymeric regions: High GC content increases steric hindrance, risking incomplete coupling or depurination. Consecutive identical bases may cause chain misfolding or error accumulation.
(iii) Secondary structure prediction: Strong intramolecular structures (e.g., hairpins, G-quadruplexes) can block access to the growing chain’s active terminus, reducing coupling efficiency. Software-based prediction is essential.
(iv) Special modifications: Non-natural nucleotides (e.g., LNA, 2'-F, 2'-OMe), fluorophore/biotin labels, or phosphorothioate (PS) backbones require evaluation of: Stability and coupling efficiency of modified phosphoramidites; Compatibility with protecting-group strategies; Post-synthesis stability under processing conditions.
(v) 5’/3’ end modifications: Functional groups (e.g., amino, thiol, cholesterol) or spacers need tailored synthesis and protecting-group approaches.
2. What are the core steps and critical control parameters in solid-phase phosphoramidite synthesis of aptamers?

Solid-phase synthesis is the standard automated method. Each nucleotide addition cycle comprises:

(j) Detritylation: Acid (e.g., trichloroacetic acid) removes the 5’-DMT group, exposing the 5’-OH for coupling.
(ii) Activation and coupling: Phosphoramidite monomers (in acetonitrile) react with an activator (e.g., tetrazole) to form a phosphite triester bond with the 5’-OH.
(iii) Capping: Acetic anhydride/N-methylimidazole acetylates unreacted 5’-OH ends to prevent deletion mutations.
(iv) Oxidation: Iodine/water/pyridine oxidizes phosphite triester to stable phosphate diester. For PS bonds, sulfurizing reagents (e.g., Beaucage reagent) are used.

Steps (i) to (iv) are repeated until the full sequence is assembled.
3. How are cleavage and deprotection efficiently performed post-synthesis? How do protecting-group strategies influence condition selection?

Post-synthesis, the aptamer remains attached to the solid support (CPG) with protecting groups on bases/ends:

(i) Cleavage/deprotection: Ammonia or methylamine solution simultaneously: Cleaves the succinate linker (releasing the oligo from CPG); Removes base protections (benzoyl/iso-butyryl) and cyanoethyl phosphate groups via β-elimination.
(ii) Additional deprotection: RNA 2’-TBDMS groups: Require extended fluoride treatment (e.g., TEA·3HF); 5’ DMT or amino modifiers: Need targeted deprotection post-cleavage/purification.
4. What are the primary methods for purifying synthetic aptamers? How to select the optimal strategy based on aptamer traits and application needs?

Crude products contain target full-length chains and impurities. Key purification methods:

(i) RP-HPLC: Ideal for DNA aptamers <80 nt, especially with DMT-on strategy or hydrophobic tags (e.g., cholesterol).
(ii) Denaturing PAGE: Preferred for Long aptamers (>60 nt) or RNA. And cases requiring ultra-high purity or involving complex modifications incompatible with HPLC.
5. How to systematically perform quality control (QC) and functional validation of synthetic aptamers? What key parameters must be tested?

Post-purification QC ensures chemical integrity and bioactivity:

Physicochemical analysis:
(i) Purity: Analytical HPLC (RP/IE), capillary electrophoresis (CE), or PAGE.
(ii) Identity: Mass spectrometry (MS) for molecular weight.
(iii) Structure: UV/CD for secondary conformation.

Functional validation:
(i) Binding affinity: Measure dissociation constant (K_d) via SPR, ITC, FP, or ELASA.
(ii) Specificity: Test against structurally similar off-target molecules.

Functional activity:
(i) Inhibitors: Determine half-maximal inhibitory concentration (IC50).
(ii) Detection probes: Assess limit of detection (LOD), sensitivity, and specificity.

reference

[1] Maryam R K, Javad M, Akram E, et al. Synthesis and in vitro study of surface-modified and anti-EGFR DNA aptamer -conjugated chitosan nanoparticles as a potential targeted drug delivery system[J]. Heliyon, Volume 10, Issue 19, 2024, e38904.

[2] Zahra J, Reza D, Mojgan N, et al. Dual-targeting CD44 and mucin by hyaluronic acid and 5TR1 aptamer for epirubicin delivery into cancer cells: Synthesis, characterization, in vitro and in vivo evaluation[J]. Heliyon, Volume 10, Issue 2, 2024, e24833.

[3] Gabriele C, Fabiola C, Giuseppe C, et al. Aptamer-based applications in delivering cancer gene therapies and beyond: state of the art and the missing links to clinical translation[J]. Advanced Drug Delivery Reviews, Volume 224, 2025, 115639.

From Sequence to Molecule: Aptamer Chemical Synthesis Protocol and FAQs