Thymidine | CAS 50-89-5 | ≥98%

Thymidine (2′-deoxythymidine; dT) is a naturally occurring pyrimidine 2′-deoxyribonucleoside composed of thymine joined to 2-deoxy-D-ribofuranose through a β-N1-glycosidic bond. As one of the four canonical DNA deoxyribonucleosides, thymidine pairs with deoxyadenosine in the double helix and enters the thymidine salvage pathway: cytosolic thymidine kinase 1 (TK1) and mitochondrial thymidine kinase 2 (TK2) phosphorylate it to thymidine 5′-monophosphate (dTMP), which is sequentially phosphorylated by thymidylate kinase to dTDP and by nucleoside diphosphate kinase to thymidine 5′-triphosphate (dTTP) for nuclear and mitochondrial DNA synthesis. In research workflows, thymidine is used as a cell-cycle synchronisation reagent, as the basis of tritiated and analogue-based DNA-synthesis assays, as a probe substrate in thymidine kinase, thymidine phosphorylase, and pyrimidine-salvage studies, and as a chiral starting material in modified deoxynucleoside and oligonucleotide chemistry. Cellular uptake proceeds through equilibrative nucleoside transporters hENT1 and hENT2 (SLC29A1/A2) and concentrative nucleoside transporters including the pyrimidine-preferring hCNT1 and broad-selectivity hCNT3.

Cell Cycle Synchronisation and dNTP Pool Manipulation

Thymidine is one of the most widely used reagents for synchronising mammalian cell populations at the G1/S boundary and early S phase. Excess intracellular thymidine is rapidly phosphorylated to dTTP, which binds the specificity site (s-site) of ribonucleotide reductase (RNR) and allosterically shifts the α2β2 holoenzyme toward GDP reduction at the expense of CDP and UDP reduction; the resulting suppression of de novo dCDP/dCTP synthesis causes the dCTP pool to collapse while dTTP and other dNTP pools become imbalanced, stalling DNA replication. The double-thymidine block protocol exploits this reversible arrest with a typical schedule of 16–18 h in 2 mM thymidine, 8–9 h release, and a second 16–18 h block, releasing a synchronised S-phase population suitable for studies of replication timing, origin firing, mitotic entry, checkpoint activation, and dNTP-pool homeostasis. Release is achieved by removing thymidine or by adding exogenous deoxycytidine, which replenishes cytidine deoxynucleotide pools through the deoxycytidine-kinase (dCK) salvage route rather than through the inhibited RNR pathway.

DNA Synthesis and Cell Proliferation Assays

Tritiated thymidine ([³H]-thymidine) incorporation is the classical method for measuring DNA synthesis and cell proliferation in cultured cells, primary explants, and tissue specimens. After uptake and thymidine kinase-catalysed phosphorylation to [³H]-dTMP, [³H]-dTTP is incorporated into newly synthesised chromosomal DNA, and acid-precipitable radioactivity is read by liquid scintillation counting as a quantitative readout of replicative DNA synthesis. The same conceptual incorporation principle underlies non-radioactive thymidine analogues such as 5-bromo-2′-deoxyuridine (BrdU) and 5-ethynyl-2′-deoxyuridine (EdU), detected by antibody-based immunofluorescence/immunohistochemistry and by copper-catalysed azide–alkyne (CuAAC) click chemistry, respectively. These assays are widely used for proliferation profiling across cultured cell systems, lymphocyte-activation experiments in immunology, cell-cycle phase distribution studies by flow cytometry, and quantitative DNA-replication timing in cell biology research.

Thymidine Kinase Substrate and Pyrimidine Salvage Studies

Thymidine is the physiological substrate of thymidine kinase, the rate-limiting first step of the thymidine salvage pathway, and is used routinely as a probe substrate in TK kinetic and selectivity assays. Mammalian cells express two principal isoforms with distinct compartmentalisation and regulation: cytosolic TK1, an S-phase-regulated kinase with narrow natural-substrate specificity restricted to thymidine and deoxyuridine, whose catalytic efficiency increases roughly 200-fold during S phase via ATP- and enzyme-concentration-dependent transition from a low-activity dimer to a high-activity tetramer, with feedback inhibition by the end-product dTTP; and mitochondrial TK2, a constitutively expressed homodimer that phosphorylates thymidine, deoxycytidine, and deoxyuridine and supplies the dNTPs needed for mtDNA replication and maintenance in post-mitotic tissues. Thymidine is therefore a standard reagent for biochemical characterisation of TK1 and TK2 kinetics, isoform-discriminating substrate analogues, deoxyribonucleotidase substrate-cycling studies between TK1/dNT-1 and TK2/dNT-2, nuclear/mitochondrial dNTP-pool partitioning by [³H]-thymidine flux, thymidine phosphorylase and nucleoside-transporter assays (hENT1/2, hCNT1/3), and quantitative-PCR-based mtDNA copy-number readouts.

Starting Material for Modified Deoxynucleoside and Oligonucleotide Chemistry

Thymidine is one of the most-used chiral starting materials and reference 2′-deoxyribonucleosides in synthetic nucleoside chemistry. Its defined β-N1-glycosidic stereochemistry, free 3′- and 5′-hydroxyls, and chemically tractable thymine base provide established handles for selective 5′-O protection, 3′-O activation, and base-functionalisation chemistry. These features support the preparation of 5′-O-DMTr-thymidine 3′-O-(N,N-diisopropyl)-β-cyanoethyl phosphoramidite, the standard dT monomer of solid-phase oligonucleotide synthesis, and of a wide range of modified thymidine analogues including 5-substituted, sugar-modified, alkyne-, azide-, and thiol-functionalised variants used for click-chemistry conjugation, fluorescent base analogues, phosphorothioate/phosphoramidate backbone research, and structural probes for X-ray crystallography and NMR.

Other Applications

• Reference compound for HPLC, UHPLC, and LC-MS analysis of deoxyribonucleosides in nucleic-acid-hydrolysate profiling and nucleoside-pool measurement
• Substrate in thymidine phosphorylase kinetic assays and equilibrium studies of phosphorolysis to thymine + 2-deoxyribose-1-phosphate
• Reference standard in NMR and X-ray structural studies of pyrimidine 2′-deoxyribosides, including hydrogen-bonding, glycosyl torsion (anti/syn), and sugar-pucker (north/south) characterisation
• Metabolic precursor in DNA-replication and DNA-repair-pathway studies, including base-excision repair, mismatch repair, and replication-fork dynamics under nucleotide-pool perturbation