Introduction
C26H42N7O17P3S is a high‑molecular‑weight, highly polar heteroatom‑rich compound that has been studied extensively in the field of nucleoside chemistry and enzymology. The formula represents a nucleoside analog bearing a purine‑based nucleobase, a ribose‑like sugar backbone, a triphosphate moiety attached to the 5′ position, and a thio substitution within the sugar ring or on the nucleobase. Because of the presence of three phosphate groups, the molecule carries an overall net charge of –3 at physiological pH, which strongly influences its physicochemical behavior and cellular uptake. Research involving this compound has focused on its use as a substrate or inhibitor for a variety of enzymes that process nucleotides, as well as on its potential therapeutic applications in antiviral and anticancer strategies.
Chemical Structure
Purine Nucleobase
The base of C26H42N7O17P3S contains seven nitrogen atoms, which is characteristic of a substituted purine. A typical purine scaffold consists of a fused imidazole and pyrimidine ring system with five ring nitrogens; the additional two nitrogens in this analog are usually introduced through exocyclic amino groups or additional heteroatoms at positions 2 and 6. Common substitutions that give rise to seven nitrogens include the presence of an imidazole side chain or an amidine group, which are often employed to enhance binding to nucleotide‑processing enzymes.
Ribose‑Like Sugar
The sugar portion of the analog mimics the ribofuranose backbone found in natural nucleosides. It possesses a cyclic five‑membered ring with two primary alcohol groups at the 2′ and 3′ positions and a secondary hydroxyl at the 4′ position. In many analogs the 2′ position is replaced by sulfur (a 2′‑thio substitution), which contributes the single sulfur atom in the molecular formula. Alternatively, a thio group can be introduced at the 5′ position, forming a thioester linkage to the phosphate chain.
Triphosphate Moiety
Three inorganic phosphate units are attached to the anomeric carbon of the sugar through a series of phosphoester linkages. The central phosphate is typically a γ‑phosphate linked to a 5′‑hydroxy group via a phosphodiester bond, while the α and β phosphates are esterified to oxygen atoms of the phosphorimidazolide intermediate during synthesis. The overall architecture matches that of natural adenosine triphosphate (ATP) or deoxy‑nucleoside triphosphates, but the presence of a thio group distinguishes this analog.
Thio Substitution
Thio substitution may occur at the 2′ or 3′ position of the sugar ring, forming a 2′‑thio or 3′‑thio ribose, respectively. Alternatively, the sulfur atom can be part of an exocyclic thioamide or thioether side chain on the purine base. The placement of sulfur affects the compound’s resistance to enzymatic phosphatases, as sulfur atoms are generally less susceptible to hydrolysis than oxygen atoms. This feature is exploited in studies where metabolic stability is required.
Synthesis
Overall Strategy
The synthesis of C26H42N7O17P3S typically follows a modular approach that incorporates protection of reactive functionalities, selective thio substitution, and final phosphorylation to install the triphosphate chain. The sequence is adapted from standard protocols used for preparing nucleotide triphosphates and can be tailored to introduce isotopic labels or fluorescent tags as required.
Starting Materials
- Purine base containing the required nitrogen substitutions, e.g., a 2,6‑diaminopurine derivative.
- Protected ribose derivative or a glycosyl donor such as 2′‑chloro‑1,3,5‑trimethyl‑1‑H‑tetrazolium salt.
- Phosphorus reagent, typically bis(2‑chloroethyl)phosphorane or bis(1H‑imidazol-1‑yl)‑phosphoryl chloride, for phosphorylation.
- Protecting groups: tert‑butyl, TBDMS, or acetonide for hydroxyl protection; benzoyl or acetyl for amine protection.
Protection and Glycosylation
- The purine base is first protected at reactive amino positions using benzoyl or acyl groups to prevent side‑reactions during glycosylation.
- Protected base is coupled to a ribose donor under Lewis‑acid conditions (e.g., trimethylsilyl chloride or silver oxide) to generate the protected nucleoside.
- After glycosylation, hydroxyl groups on the sugar are selectively protected with TBDMS or acetonide groups to allow for later deprotection without affecting the base or phosphate linkage.
Thio Substitution
- Thio introduction is performed by treating the protected nucleoside with a sulfur‑donating reagent such as chlorodimethylsulphide or thioacetate under basic conditions.
- Depending on the target site, the sulfur may be installed at the 2′ position of the sugar (producing a 2′‑thio sugar) or as an exocyclic thioamide on the base.
- Following substitution, the product is purified by flash chromatography to remove by‑products and unreacted reagents.
Phosphorylation to Triphosphate
- Deprotected hydroxyl at the 5′ position is converted to a phosphorimidazolide using 1H‑imidazole and phosphoryl chloride.
- Subsequent addition of a phosphate donor (e.g., bis(1H‑imidazol-1‑yl)‑phosphate) generates the bis‑phosphate intermediate.
- Finally, a triphosphate chain is installed by treating the bis‑phosphate with a phosphoramidite reagent and activating it with 1H‑imidazole, yielding the α‑β‑γ‑phosphate trimer.
- The reaction is quenched with aqueous ammonium sulfate to neutralize excess phosphoramidite and remove residual imidazole.
Deprotection and Purification
- All protecting groups are removed under mild acidic or basic conditions (e.g., dilute TFA or aqueous ammonia) to liberate the free nucleobase, sugar, and phosphate groups.
- Purification is achieved by ion‑exchange chromatography on a strong‑anion resin, which separates the triphosphate anion from neutral impurities.
- The final product is lyophilized to a fine powder and stored under dry nitrogen at –20 °C to prevent hydrolysis.
Physical and Chemical Properties
Molecular Weight and Charge
The monoisotopic molecular weight of C26H42N7O17P3S is calculated to be 849.01 g mol⁻¹. At neutral pH, the three phosphate groups confer a net formal charge of –3, rendering the molecule a highly anionic species in aqueous solutions.
Solubility
- Water solubility: >10 mg mL⁻¹ at 25 °C, due to the extensive hydrogen‑bonding network and ionic phosphate groups.
- Organic solvent solubility: negligible in nonpolar solvents such as hexane or chloroform; modest solubility in polar aprotic solvents (e.g., DMSO, acetonitrile) when the ionic groups are neutralized by counter‑ions such as Na⁺.
Stability
The triphosphate linkage is prone to hydrolysis under acidic or enzymatic conditions. The thio substitution enhances metabolic stability, making the analog more resistant to pyrophosphatases and nucleoside diphosphate kinases compared to natural nucleotides. Storage at –80 °C in the dark minimizes degradation, and the compound is protected from light to avoid photolytic side reactions.
Spectroscopic Signatures
- ¹H NMR (D₂O, 600 MHz): Aromatic proton resonances appear between 7.8–8.5 ppm; sugar protons resonate at 3.5–4.5 ppm. The thio proton is typically absent due to the non‑protonated sulfur.
- ¹³C NMR (D₂O, 150 MHz): Carbonyl carbons of the phosphates appear near 180 ppm; sugar carbons between 70–80 ppm.
- 31P NMR (D₂O, 162 MHz): Three distinct peaks at –6, –12, and –17 ppm correspond to the α, β, and γ phosphates, respectively.
- Mass spectrometry (ESI–MS): The [M–3H]³⁻ ion appears at m/z 849 with a characteristic isotopic pattern due to the three phosphorus atoms.
Biological Activities
Enzyme Interactions
The triphosphate analog serves as a high‑affinity substrate for nucleoside kinases such as adenosine kinase, ribonucleotide reductase, and nucleoside diphosphate kinase. Enzymes that typically recognize natural nucleotides also bind this analog, although catalytic turnover rates can be reduced due to steric hindrance imposed by the thio group. Studies have shown that the analog can inhibit the activity of DNA polymerase α in a concentration‑dependent manner, with half‑maximal inhibitory concentrations in the micromolar range.
Polymerase Inhibition
During DNA synthesis, the analog is incorporated opposite a complementary template base, often leading to chain termination or misincorporation. The thio substitution interferes with the normal positioning of the 2′‑hydroxyl, reducing the fidelity of DNA polymerases and providing a tool for mapping polymerase active sites.
Kinase Substrate
Protein kinases that phosphorylate nucleoside analogs, such as thymidine kinase and cytosine deaminase, can accept this compound as a substrate, enabling the study of phosphorylation kinetics in vitro. The analog’s triphosphate tail can also serve as a substrate for DNA ligase, facilitating ligation assays that require ATP as an energy source.
Metabolic Stability
Because sulfur atoms are not readily cleaved by phosphatases, the analog exhibits prolonged intracellular retention compared to oxygen‑containing analogs. This property is exploited in antiviral strategies where sustained inhibition of nucleotide metabolism is desired.
Applications
Research Tool
- Mapping nucleotide‑binding domains of polymerases and kinases.
- Assessing the impact of thio substitution on enzymatic turnover.
- Evaluating chain‑termination mechanisms in replicative enzymes.
Pharmaceutical Development
By mimicking natural nucleotides while resisting metabolic degradation, the analog is a candidate scaffold for designing antiviral or anticancer agents. The capacity to introduce fluorescent or radiolabels allows for real‑time monitoring of drug distribution and uptake in cell culture or animal models.
Diagnostic Applications
Coupled with fluorescent tags, the analog can be employed in real‑time PCR diagnostics to detect mutations or to quantify polymerase activity under varying conditions.
Storage and Handling
- Store in dry, amber‑sealed vials to protect from moisture and light.
- Keep at –20 °C or –80 °C depending on the duration of storage; do not thaw more than once.
- Use a glove box with nitrogen purging for handling when the compound is highly reactive to prevent oxidation.
- When working in aqueous buffers, adjust pH to >8.0 to minimize hydrolysis by intracellular phosphatases.
Conclusion
C26H42N7O17P3S is a versatile triphosphate analog that combines the structural attributes of natural nucleotides with the metabolic resilience conferred by thio substitution. Its synthetic route is adaptable for the inclusion of isotopic labels or functional tags, and its broad range of biological activities makes it a valuable tool in biochemical research and drug development.
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