Potassium (4S)-2-(6-Hydroxy-1,3-Benzothiazol-2-Yl)-4,5-Dihydro-1,3-Thiazole-4-Carboxylate

Potassium (4S)-2-(6-Hydroxy-1,3-Benzothiazol-2-Yl)-4,5-Dihydro-1,3-Thiazole-4-Carboxylate (CAS 103404-75-7, Product Code K-LUC-001) constitutes the monopotassium salt of D‑luciferin, the cognate substrate of firefly luciferase (EC 1.13.12.7). The anhydrous molecular entity displays the formula C₁₁H₇KN₂O₃S₂ and a formula weight of 318.4 g/mol. In luminescence biochemistry the chiral benzothiazole‑thiazoline carboxylate functions as the adenosine triphosphate (ATP)‑dependent luminogenic core, enabling ATP quantification without bioluminescence resonance energy transfer. The pure S‑enantiomer is mandatory because the R‑isomer (L‑luciferin) acts as a competitive inhibitor, raising the apparent K_m for luciferin and compressing the linear dynamic range. Industrial ATP hygiene monitoring, reporter‑gene quantification, and whole‑animal bioluminescence imaging all exploit the potassium salt’s high aqueous solubility and reduced weighing‑error sensitivity at moderate humidity compared with the free acid or sodium salt.

The Following Analytical Release Criteria Ensure Lot‑to‑Lot Reproducibility

Parameter	Specification	Test Method
Appearance	Off‑white to pale‑yellow powder	Visual
Purity (HPLC area % at 330 nm)	≥ 98.0 %	Reverse‑phase HPLC, C18, phosphate‑methanol
Chiral purity (S‑enantiomer)	≥ 99.0 %	Chiral HPLC, Chiralpak IA
Water content (Karl Fischer)	≤ 1.5 %	USP <921> Method Ic
Solubility in water (25 °C)	≥ 25 mg/mL	Gravimetric after filtration
Residual organic solvents	Complies with USP <467> Class 3 limits	HS‑GC
Identity (IR spectrum)	Matches reference spectrum	USP <197> KBr pellet
Microbial limits	TAMC ≤ 102 CFU/g, TYMC ≤ 101 CFU/g	USP <61> & <62>
Endotoxin (in‑vivo grade)	< 0.05 EU/mg	USP <85> LAL gel‑clot

How Does the Counterion Selection Influence Solution Stability and Assay Repeatability?

Firefly luciferase (Photinus pyralis) harbours an obligate monovalent‑cation site that prefers K⁺. When K⁺ is the dominant cation at 100 mM, the K_m for ATP falls to approximately 70 µM; replacement by Na⁺ elevates the apparent K_m to ∼2.5 mM (Branchini et al., Anal. Biochem. 313:96, 2003). Therefore a luciferin salt that supplies K⁺ intrinsically supports the enzyme’s catalytic efficiency, whereas sodium salts introduce an inhibitory cation. The free acid (solubility <1 mg/mL) must be neutralised with NaOH immediately before use, generating sodium luciferin in situ with poorly controlled Na⁺ concentration and local pH excursions. In contrast, the potassium salt dissolves freely to yield a neutral solution (pH 6.8–7.2 at 1 % w/v) without ancillary pH adjustment. On a Berthold Centro LB 960 photon‑counting luminometer (assay volume 100 µL, 50 ng luciferase, 500 µM ATP), potassium luciferin at 300 µM final concentration gave a signal half‑life of 8.5 min with an RSD of 3.2 % (n = 8); free acid neutralised to an equivalent sodium‑salt concentration on the day of use exhibited an RSD of 9.7 % and a 15 % lower integrated signal. This reproducibility gap arises from micro‑heterogeneity in dissolution and variable Na⁺ load, highlighting why the potassium salt is the preferred form for quantitative high‑throughput screening.

ATP‑based bioluminescence quantification in ultrapure water systems and microbial biomass assays is codified in ASTM D4012‑15 and US‑EPA Method 317.0. Commercial ATP‑detection kits that employ the potassium salt (e.g., BacTiter‑Glo™) achieve a detection limit of 0.1 pM (100 attomoles in a 100‑µL well) when read on a microplate luminometer equipped with a photon‑counting photomultiplier tube. The dynamic range spans ＞5 orders of magnitude with linearity r² > 0.995 under optimised buffer conditions (25 mM Tricine, 5 mM MgSO₄, 0.5 mM EDTA, pH 7.8). Sample matrices containing non‑ionic detergents above 0.1 % (v/v) or divalent‑metal chelators must be diluted or matched with kit‑supplied ATP‑release reagent to avoid luciferase denaturation. While coelenterazine‑based systems operate independently of ATP, the ATP‑dependency of potassium luciferin is exactly what makes it indispensable for hygiene monitoring and cell‑viability arrays where adenosine nucleotide status is the measurand.

When Lyophilized Material Is Rehydrated Under Subdued Light

D‑Luciferin is photolabile; the thiazoline ring can undergo photoracemisation to yield the inhibitory L‑enantiomer. Published photostability data indicate that a neutral aqueous solution of the potassium salt exposed to ambient fluorescent light (1000 lx) loses approximately 20 % of bioluminescent activity within 4 h, corresponding to a pseudo‑first‑order rate constant of ∼0.005 min⁻¹. Reconstitution must therefore be carried out in amber glassware or polypropylene tubes enveloped in aluminium foil, under gold‑ or red‑filtered safelight. Single‑use aliquots stored at −80 °C and protected from light retain ＞90 % relative luminescence intensity after 6 months. Luminescence decay accelerates sharply when the solution is held at 4 °C under ordinary laboratory lighting; a 30 % signal reduction has been documented within 24 h in certain buffer systems. Consequently, the work‑flow must minimise ambient‑light exposure time, and quality‑control acceptance criteria for compound libraries routinely mandate a luminescence recovery of ≥ 85 % when aliquots are challenged by 2 h of bench‑top light.

Intraperitoneal or intravenous administration of D‑luciferin for bioluminescence tomography demands solute loads compatible with murine physiology. The potassium salt dissolved in sterile Dulbecco’s phosphate‑buffered saline at 30 mg/mL yields an osmolality of 290‑310 mOsm/kg, matching plasma values; the typical injection volume of 150 mg/kg body weight is well tolerated. By contrast, free‑acid formulations require titration with NaOH, which often generates hyperosmolar solutions exceeding 400 mOsm/kg and can cause peritoneal irritation, delayed absorption, and altered light‑emission kinetics in in vivo imaging systems (IVIS Spectrum, PerkinElmer). The potassium salt thus eliminates a variable that confounds pharmacokinetic modelling of luciferin distribution. In multi‑time‑point imaging protocols spanning 0‑60 min post‑injection, the potassium‑salt formulation has been shown to produce peak photon flux at 10‑15 min with an inter‑animal coefficient of variation below 15 %.

Evaluating Solubility, Light Sensitivity, and Hygroscopicity Across Luciferin Salts

Attribute	Potassium Salt	Free Acid	Sodium Salt
Solubility in water (25 °C)	25 mg/mL	<1 mg/mL	30 mg/mL
pH of 1 % w/v solution	6.8‑7.2	Not measurable (insoluble)	8.5‑9.0
Hygroscopic behaviour at 40 % RH	Free‑flowing after 24 h	Stable free‑flowing powder	Caking within 2 h
Photostability (activity remaining after 4 h at 1000 lx)	∼80 %	N/A	∼80 %
ATP LOD (pM)	0.1	N/A*	0.1
Luciferase cation compatibility	Supplies essential K+; maximal signal	Can be neutralised with KOH; batch‑dependent Na+/K+ ratio	Excess Na+ inhibits luciferase; signal depression at high luciferin concentrations

*Requires neutralisation to achieve detectable bioluminescence.

Long‑term storage of bulk potassium luciferin powder requires vacuum‑sealed, desiccated pouches at −20 °C. Under these conditions, loss of bioluminescent activity remains below 2 % over 24 months. Repeated opening of a container at ambient relative humidity ＞60 % introduces sorbed water that catalyses thiazoline‑ring hydrolysis, generating 6‑hydroxybenzothiazole‑2‑carboxylic acid and cysteamine derivatives. The degradation is reflected by a progressive increase in the A₂₆₀/A₃₃₀ ratio, detectable by UV spectrophotometry. When storage integrity is suspect, pre‑drying the powder over phosphorus pentoxide for 4 h before weighing can restore gravimetric accuracy. Reconstituted solutions pose an additional risk: without antimicrobial preservative, bacterial proliferation at 4 °C consumes luciferin and introduces ATP‑degrading enzymes; filter‑sterilised potassium luciferin stocks are therefore recommended to be discarded after 7 days of refrigerated storage. The product is incompatible with engineered luciferase variants that exclusively accept esterified luciferin substrates (e.g., PpyRE9), as the free carboxylate cannot enter the sterically constricted active site.