Technical articles
Coupling Activators, Nucleoside Building Blocks, and Modification Design in Oligonucleotide Chemical Synthesis: Their Effects on Synthetic Efficiency, Purity, and Product Properties
Coupling Activators, Nucleoside Building Blocks, and Modification Design in Oligonucleotide Chemical Synthesis: Their Effects on Synthetic Efficiency, Purity, and Product Properties
1. What problems does oligonucleotide chemical synthesis need to solve?
The task of oligonucleotide chemical synthesis is not merely to link monomers together in sequence. It also involves increasing the proportion of full-length product, reducing deletion sequences and other impurities, preserving structural integrity after post-synthetic processing, and ensuring that the final product exhibits the intended hybridization behavior, stability, or biological properties. From an experimental perspective, two categories of problems must be assessed: whether the synthesis process proceeds smoothly, and whether the properties of the product match its intended use. The former mainly relates to deprotection, coupling, capping, and backbone conversion, whereas the latter depends more on nucleoside building blocks and modification design.
The nucleoside phosphoramidite method has remained the dominant route for many years because it enables stepwise chain elongation on a solid support in a cyclic manner, while unreacted reagents can be removed after each step, making it easier to control reaction efficiency and the sources of impurities.
2. The basic cycle of solid-phase nucleoside phosphoramidite synthesis
Conventional solid-phase oligonucleotide synthesis usually proceeds in the sequence of 5′-DMTr deprotection, coupling, capping, and oxidation or sulfurization. This framework applies to most routine DNA syntheses and also serves as the basis for the subsequent discussion of activators and modified nucleosides.
Step | Function of the Step | Main Risks |
Deprotection | Removes the 5′-dimethoxytrityl protecting group to expose a new 5′-hydroxyl group | Incomplete deprotection will affect the next coupling cycle; excessively harsh acid treatment or overly long exposure increases the risk of depurination |
Coupling | Allows the activated nucleoside phosphoramidite to react with the terminal 5′-hydroxyl group, forming a new internucleotide linkage | Insufficient coupling efficiency lowers the proportion of full-length product |
Capping | Blocks unreacted 5′-hydroxyl groups and prevents failed chains from continuing to elongate | Inadequate capping increases deletion sequences and makes subsequent separation more difficult |
Oxidation or sulfurization | Converts the trivalent phosphorus intermediate into a more stable pentavalent phosphorus structure, or into a sulfur-containing backbone | Incomplete conversion leads to backbone heterogeneity and structural impurities |
The key judgment is as follows: coupling and capping mainly affect deletion sequences, deprotection mainly affects the risk of acid-induced damage, and oxidation or sulfurization mainly affects backbone integrity. If the target is a natural phosphodiester backbone, oxidation is usually used. If the target is a phosphorothioate backbone, this step must be replaced by sulfurization.
3. The role of coupling activators and key points for their selection
Nucleoside phosphoramidite monomers have limited intrinsic reactivity and usually require coupling activators to promote the formation of reactive intermediates that more readily react with the 5′-hydroxyl group. Tetrazole and its derivatives, as well as systems such as 4,5-dicyanoimidazole, were all developed for this purpose. Differences among activators are reflected not only in reaction rate, but also in coupling completion, side reactions, and suitability for different monomers.
There are three situations in which the effects of activators are especially pronounced.
1. The first involves monomers with greater steric hindrance or more complex structures. Such monomers are intrinsically slower to couple and are more sensitive to activation conditions.
2. The second involves monomers bearing sugar, base, or backbone modifications. Under standard conditions, these do not always maintain stable coupling efficiency.
3. The third involves the synthesis of longer sequences. Even a slight decrease in coupling efficiency in a single cycle can significantly reduce the proportion of full-length product after many cycles of accumulation.
Therefore, when the proportion of full-length product is low, when a particular monomer is difficult to introduce, or when the latter part of a long-chain synthesis shows a marked drop in yield, priority should not be given only to checking the monomer itself. The type of activator, the solvent and moisture content of the system, the coupling time, and the molar excess of the monomer should also be examined. For difficult-to-couple monomers, optimizing the activation conditions first is usually more targeted than replacing the entire monomer system at the outset.
4. Effects of nucleoside building blocks and modification types on product properties
Nucleoside building blocks and modification design mainly affect product stability, recognition behavior, protein interactions, and final application, rather than only whether chain elongation can be completed during synthesis. In oligonucleotides, common modifications can generally be divided into three categories: sugar modifications, base modifications, and backbone modifications. These three categories do not focus on the same issues.
Modification Type | Main Effects | Common Significance |
Sugar modification | Conformation, hybridization ability, nuclease resistance | Adjusts binding ability and stability |
Base modification | Pairing recognition, labeling, special interactions | Used for detection, imaging, or specific recognition |
Backbone modification | Chemical stability, protein binding, in vivo behavior | Adjusts resistance to degradation and biological performance |
Among these, the phosphorothioate backbone is the most common type of backbone modification. It is often used to improve resistance to nuclease degradation, but it also changes protein interactions, thereby affecting cellular uptake, tissue distribution, activity, and toxicity-related behavior. Sugar modifications are often used to further tune stability and hybridization properties. The significance of modified nucleosides and modified backbones lies in adjusting the chemical properties and application performance of the product to the required range.
5. How to decide experimentally whether to optimize activation conditions first or adjust monomer design first
During experimental troubleshooting, the important first step is to distinguish whether the problem belongs to the synthesis process or to molecular design.
Experimental Observation | Priority for Inspection | Key Point for Judgment |
Low proportion of full-length product and a high level of deletion sequences | Check coupling and capping first | Whether coupling is sufficient and whether capping effectively blocks failed chains from further elongation |
The early part of the same sequence is normal, but the later part shows a clear drop in yield | Check stepwise yield and activation conditions first | In long-sequence synthesis, small losses in each cycle accumulate and become amplified |
Significant damage in purine-rich sequences or acid-sensitive sequences | Check deprotection conditions first | Whether the acid type, concentration, and contact time are appropriate |
Difficulty introducing modified monomers | Check activator and coupling time first | Whether standard activation conditions are sufficient for that monomer |
Acceptable purity but insufficient resistance to degradation | Check backbone and sugar modifications first | Such problems are usually not caused by the activator |
Chemical analysis is acceptable, but functional performance is unsatisfactory | Check the match between modification design and intended use first | Evaluation must return to hybridization, recognition, or in vivo behavior |
Summary: for problems related to chemical chain-extension efficiency, process conditions should be checked first; for problems related to product properties and application, nucleoside building blocks and modification design should be checked first.
6. Key operational control points in synthesis
6.1 Moisture control
Nucleoside phosphoramidite monomers and activation systems are both sensitive to moisture. Once water enters the system, it consumes active components and lowers effective coupling, thereby reducing the proportion of full-length product. This problem is usually more pronounced for modified monomers and longer sequences.
6.2 Deprotection conditions
The purpose of 5′-DMTr deprotection is to expose the 5′-hydroxyl group, but stronger conditions are not necessarily better. Kinetic studies on depurination have shown that the acid type, concentration, solvent, and contact time all affect the balance between depurination and DMTr removal. Therefore, while ensuring sufficient deprotection, acid-induced damage should be minimized as much as possible, especially for purine-rich sequences and longer sequences.
6.3 Capping control
The role of capping is to promptly block the 5′-hydroxyl groups of unreacted chains, preventing these chains from continuing to elongate in the next cycle and forming deletion sequences that are difficult to separate. When deletion impurities are high, it is not enough to examine only whether coupling is sufficient; the effectiveness of capping must also be checked.
6.4 Backbone conversion
The natural phosphodiester backbone is usually obtained through oxidation to give a stable structure, whereas the phosphorothioate backbone relies on a sulfurization step to form the target linkage. If this step is incomplete, backbone heterogeneity will result, affecting purity and structural consistency.
6.5 Compatibility with post-synthetic processing
Some problems do not occur during the coupling step, but instead arise during cleavage, deprotection, and purification. Certain modified monomers may undergo coupling successfully, yet may not tolerate conventional post-synthetic processing conditions. Therefore, when selecting modified nucleosides, it is necessary not only to consider coupling efficiency, but also to evaluate their compatibility with post-synthetic processing conditions.
7. Key Reagents and Nucleoside Building Blocks in Oligonucleotide Chemical Synthesis: Classification, Features, and Applications of Representative Chemicals (Tables 1-4)
Table 1 | Nucleoside and Deoxynucleoside Starting Materials
Classification | CAS No. | Aladdin Cat. No. | Name | Specification or Purity | Product Features and Applications |
RNA nucleoside starting material (adenine) | 58-61-7 | Adenosine | PharmPure™, USP, endotoxin <1000 EU/g; microbial limit ≤100 cfu/g | Can serve as a precursor for ribonucleoside monomers or modified nucleosides, and is used in studies on RNA-related monomers, labeled nucleosides, or functionalized nucleoside precursors | |
RNA nucleoside starting material (cytosine) | 65-46-3 | Cytidine | Moligand™, ≥99% | Can be used for the preparation of cytidine-derived ribonucleoside monomers, and also in research on modified nucleosides, nucleoside analogs, and nucleic acid chemistry | |
RNA nucleoside starting material (guanine) | 118-00-3 | Guanosine | PharmPure™, USP, endotoxin <500 EU/g; microbial limit ≤100 cfu/g | Can serve as a guanosine precursor for RNA monomer preparation, modified nucleoside design, and nucleic acid structure studies | |
RNA nucleoside starting material (uracil) | 58-96-8 | Uridine | UltraBio™, ≥99% | Can serve as a uridine precursor for RNA monomer synthesis, preparation of 2′-modified nucleosides, and oligonucleotide chemistry research | |
DNA deoxynucleoside starting material (adenine) | 958-09-8 | 2'-Deoxyadenosine Anhydrous | Moligand™, ≥98% (HPLC) (T) | Used to prepare deoxyadenosine phosphoramidite monomers and is a common precursor for DNA solid-phase synthesis and modified monomer development | |
DNA deoxynucleoside starting material (cytosine) | 951-77-9 | 2′-Deoxycytidine | ≥99% | Used to prepare deoxycytidine phosphoramidite monomers, and also for studies on DNA modified building blocks and nucleoside analogs | |
DNA deoxynucleoside starting material (guanine) | 961-07-9 | 2′-Deoxyguanosine hydrate | ≥99% | Used to prepare deoxyguanosine phosphoramidite monomers and is suitable for guanine-site incorporation and related protecting-group route studies | |
DNA deoxynucleoside starting material (thymine) | 50-89-5 | Thymidine | PharmPure™, USP, endotoxin <50 EU/g; microbial limit ≤100 cfu/g | Can serve as a thymidine monomer precursor for DNA phosphoramidite monomer preparation and oligonucleotide chain extension studies |
Table 2 | Standard DNA Phosphoramidite Monomers
Classification | CAS No. | Aladdin Cat. No. | Name | Specification or Purity | Product Features and Applications |
Standard DNA phosphoramidite monomer (dA) | 98796-53-3 | DMT-dA(bz) Phosphoramidite | ≥99%, mixture of isomers | Used for adenine-site incorporation in solid-phase synthesis and is one of the standard building blocks for routine DNA sequence assembly | |
Standard DNA phosphoramidite monomer (dC) | 102212-98-6 | DMT-dC(bz) Phosphoramidite | ≥99% | Used for cytosine-site incorporation in solid-phase synthesis and commonly applied to chain extension of routine DNA sequences and modified sequences | |
Standard DNA phosphoramidite monomer (dG, isobutyryl-protected) | 93183-15-4 | DMT-dG(ib) Phosphoramidite | ≥99% | Used for guanine-site incorporation in solid-phase synthesis and is suitable for DNA synthesis routes in which deprotection conditions are selected according to the base protecting-group scheme | |
Standard DNA phosphoramidite monomer (dG, N2-dimethylformamidine-protected) | 330628-04-1 | DMT-dG(dmf) Amidite | ≥98% | Used for guanine-site incorporation and can be used to compare the effects of different guanine protecting strategies on synthetic efficiency and post-deprotection conditions | |
Standard DNA phosphoramidite monomer (dT) | 98796-51-1 | DMT-dT Phosphoramidite | ≥99% | Used for thymine-site incorporation in solid-phase synthesis and is a fundamental building block in standard DNA monomer systems |
Table 3 | Coupling Activators and Phosphoramidite Construction Reagents
Classification | CAS No. | Aladdin Cat. No. | Name | Specification or Purity | Product Features and Applications |
Tetrazole-type coupling activator | 288-94-8 | T109596 | Tetrazole | ≥98% | A classic coupling activator used to activate phosphoramidite monomers and promote coupling with the terminal 5′-hydroxyl group |
Tetrazole-type coupling activator | 89797-68-2 | 5-(Ethylthio)-1H-tetrazole (ETT) | ≥98% | Used for phosphoramidite coupling activation and commonly employed in condition screening for difficult-to-couple monomers, RNA monomers, or modified oligonucleotides | |
Tetrazole-type coupling activator | 21871-47-6 | 5-(Benzylthio)-1H-tetrazole | ≥98.5% | Used for phosphoramidite coupling activation and can improve coupling completion for certain sterically hindered monomers or modified monomers | |
Imidazole-type coupling activator | 1122-28-7 | 4,5-Dicyanoimidazole (DCI) | ≥99% | Used for phosphoramidite coupling activation and is suitable for development of coupling conditions for standard monomers and some modified monomers | |
Phosphitylation reagent | 89992-70-1 | 2-Cyanoethyl N,N-diisopropylchlorophosphoramidite | ≥95% | Used to convert nucleoside precursors into phosphoramidite monomers and is a key reagent for preparing DNA, RNA, and modified oligonucleotide building blocks |
Table 4 | Deprotection, Capping, Oxidation/Sulfurization, Post-Synthetic Processing, and Key Solvents
Classification | CAS No. | Aladdin Cat. No. | Name | Specification or Purity | Product Features and Applications |
Main synthesis solvent | 75-05-8 | anhydrous Acetonitrile (ACN) | Anhydrous grade, ≥99.8%, H2O ≤0.003% | Commonly used for preparing phosphoramidite monomers, activators, and automated synthesis systems, and is also a standard anhydrous medium in solid-phase synthesis | |
Basic component/reaction solvent for capping systems | 110-86-1 | Pyridine | Anhydrous grade, ≥99.8% | Commonly used as a basic medium in capping systems and monomer synthesis, and can help adjust the reaction environment and reagent solubility | |
Monomer synthesis/anhydrous reaction solvent | 109-99-9 | T1491789 | Tetrahydrofuran (THF) | Anhydrous grade, ≥99.9%, stabilizer-free, H2O ≤30 ppm | Commonly used in nucleoside phosphitylation, monomer preparation, and related anhydrous reaction systems |
5′-Dimethoxytrityl deprotection reagent | 79-43-6 | Dichloroacetic acid (DCA) | AR, ≥98% | Used to remove the 5′-dimethoxytrityl protecting group and expose the terminal hydroxyl group for the next coupling cycle | |
5′-Dimethoxytrityl deprotection reagent | 76-03-9 | Trichloroacetic acid (TCA) | Ph. Eur., suitable for analysis, ACS, extra pure | Used to remove the 5′-dimethoxytrityl protecting group and can be used for comparison of deprotection conditions and process screening | |
Acetylation reagent for capping | 108-24-7 | A1506320 | Acetic anhydride | Ph. Eur., puriss. p.a., ISO, ACS, ≥99% (GC) | Used to cap unreacted 5′-hydroxyl groups and reduce short-chain impurities formed by continued elongation of failure sequences |
Capping additive | 616-47-7 | 1-Methylimidazole | ≥99% | Commonly used as an additive in capping systems, in combination with acetylation reagents to complete blocking of unreacted sites | |
Oxidizing reagent | 7553-56-2 | I434868 | Iodine | High purity, reagent grade, ≥99.8% (T) | Used to oxidize the trivalent phosphorus intermediate formed after coupling into a pentavalent phosphorus backbone and is a key reagent in routine phosphodiester oligonucleotide synthesis |
Sulfurizing reagent | 66304-01-6 | 3H-1,2-Benzodithiol-3-one 1,1-Dioxide | Moligand™, ≥98% | Used to convert phosphite intermediates into phosphorothioate linkages and is suitable for preparation of sulfur-containing backbone oligonucleotides | |
Sulfurizing reagent | 15088-78-5 | Bis(phenylacetyl) Disulfide | ≥98% (HPLC) | Used for phosphorothioate backbone construction and can serve as a sulfur-transfer reagent in the synthesis of sulfur-containing oligonucleotides | |
Cleavage/deprotection reagent | 1336-21-6 | A112077 | Ammonia solution | Guaranteed reagent, 25-28% | Used to cleave oligonucleotides from the solid support and to remove base-protecting groups and phosphate-protecting groups |
Component of rapid cleavage/deprotection systems | 74-89-5 | M102684 | Methylamine | AR, 30-33 wt. % ethanol solution | Commonly used in the preparation of ammonia/methylamine rapid cleavage and deprotection systems, enabling shorter cleavage-from-support and base deprotection times for oligonucleotides; suitable for development of rapid post-synthetic processing conditions, but compatibility with base protecting groups and modified monomers should be evaluated |
References
[1] Beaucage S. L., Caruthers M. H. Deoxynucleoside phosphoramidites: A new class of key intermediates for deoxypolynucleotide synthesis. Tetrahedron Letters. 1981;22(20):1859-1862.
[2] Wei X. Coupling activators for the oligonucleotide synthesis via phosphoramidite approach. Tetrahedron. 2013;69(18):3615-3637.
[3] Septak M. Kinetic studies on depurination and detritylation of CPG-bound intermediates during oligonucleotide synthesis. Nucleic Acids Research. 1996;24(15):3053-3058.
[4] Khvorova A., Watts J. K. The chemical evolution of oligonucleotide therapies of clinical utility. Nature Biotechnology. 2017;35:238-248.
[5] Clavé G., Reverte M., Vasseur J.-J., Smietana M. Modified internucleoside linkages for nuclease-resistant oligonucleotides. RSC Chemical Biology. 2021;2:94-150.
[6] Crooke S. T., Vickers T. A., Liang X.-H. Phosphorothioate modified oligonucleotide-protein interactions. Nucleic Acids Research. 2020;48(10):5235-5253.
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