2-[[(3-Carboxyphenyl)Amino]Carbonyl]-4-Mercapto-1-Pyrrolidinecarboxylic Acid (2S-Cis)-1-[(4-Nitrophenyl)Methyl] Ester

Can the 4-Nitrophenylmethyl Ester Serve as a Traceless Photocleavable Handle in Solid-Phase Peptide Synthesis?

In Fmoc/t-Bu solid-phase peptide synthesis, anchoring of the first amino acid to a photolabile linker permits C-terminal carboxylic acid liberation without relying on concentrated trifluoroacetic acid cocktails that risk side-chain deprotection or aspartimide formation. The title compound, furnishing a 2-[(3-carboxyphenyl)aminocarbonyl]-4-mercaptopyrrolidine core with a photolabile 4-nitrophenylmethyl ester, is loaded onto aminomethyl polystyrene resin (0.25–0.45 mmol/g substitution) via HATU/N-methylmorpholine activation in DMF under argon shield. The mercapto group must remain masked as trityl (Trt) thioether throughout assembly to prevent intrachain disulfide bridging; removal is deferred to the final global deprotection step with 2.5% triisopropylsilane in TFA after photolysis. Irradiation on a 365 nm UV-LED array delivering 8–12 mW/cm² at the resin surface for 12–18 min cleanly excises the fully protected peptide acid, monitored by Kaiser test disappearance and reversed-phase UPLC (C18, 214 nm). Process validation follows ICH Q2(R1) for the photolabile linker surrogate; photolytic by-product, 4-nitrobenzyl alcohol, is removed by precipitation with cold diethyl ether. This route has been adopted in custom synthesis of cyclic RGD peptides requiring C-terminal carboxylates for integrin-binding affinity, wherein standard acidolysis induced 4–7% epimerisation at the α-carbon whereas photolytic release kept racemisation below 0.8% (Marfey’s test).

Modular assembly of antibody-drug conjugates (ADCs) directly exploits the heterobifunctional architecture of 2-[[(3-carboxyphenyl)amino]carbonyl]-4-mercapto-1-pyrrolidinecarboxylic acid (2S-cis)-1-[(4-nitrophenyl)methyl] ester as a phototriggered spacer. The pendant benzoic acid is first converted to its N-hydroxysuccinimidyl ester, then coupled to solvent-accessible lysine residues on a humanised IgG1 monoclonal antibody (4–6 equivalents, pH 7.4 phosphate buffer, 2 h, 22 °C), yielding a conjugate with drug-to-antibody ratio (DAR) 2.8–3.5 as determined by hydrophobic interaction chromatography. Subsequent thiol-specific payload attachment proceeds via Michael addition of the deprotected mercapto group to a maleimide-functionalised MMAE auristatin derivative in 20 mM sodium acetate, pH 5.5, 1 h. Photorelease is triggered at 365 nm (10 J/cm² cumulative dose) inside a quartz-jacketed flow reactor, liberating the free cytotoxin within the tumour microenvironment that is pre-illuminated via tumour-bed fibre optics—a spatial control mechanism validated with ISO 22412:2017 dynamic light scattering for aggregation integrity post-irradiation. Comparative release kinetics from analogous 4-nitrobenzyl carbamates indicate a half-life of 110 s versus 18 s for the 2-nitrobenzyl isomer, a difference that permits deeper tissue permeation before premature payload loss.

Photolytic release efficiency of a model fluorophore–quencher pair conjugated via the title ester linker on agarose microbeads

Irradiation wavelength (nm)

Fluence (mJ/cm²)

Release (%) ± SD

Intact conjugate remaining (%)

365

500

24 ± 3

365

1500

61 ± 5

405

500

5 ± 1

405

1500

19 ± 4

Published data for this specific conjugate configuration remain limited to surrogate fluorophore studies; nonetheless, the numbers align with photocleavage quantum yields (Φ = 0.11–0.18) reported for 4-nitrobenzyl esters in buffered aqueous acetonitrile (PBS/MeCN 1:1, pH 7.4).

Inhibitor Design Targeting Cysteine Proteases — A Mercapto-Pyrrolidine Synthon

The (2S-cis) pyrrolidine scaffold with a 4-mercapto substituent serves as a rigidified cysteine surrogate for reversible covalent inhibition of clan CA cysteine proteases, particularly cathepsins B, L, and the SARS-CoV-2 papain-like protease (PL^pro). In a typical fragment growing campaign, the 3-carboxyphenyl urea arm is coupled to an amine-terminated peptidomimetic backbone using 1.05 eq PyBOP and 2.5 eq DIPEA in DMF at 0 °C, reaching full conversion after 45 min as adjudged by LC-MS (ESI+). The 4-nitrophenylmethyl ester remains intact as a prodrug carboxyl mask to improve cellular permeability (logD_7.4 reduction of 1.2 log units relative to free acid, PAMPA assay). Ester hydrolysis by intracellular esterases then liberates the active carboxylate that engages the oxyanion hole hydrogen bond network. Stoichiometry of thiolate attack on the catalytic cysteine is confirmed by tryptic digest MS/MS detecting a mass shift of the active-site peptide of +Da characteristic of a disulfide or thioether adduct depending on electrophilic warhead design. In vitro IC₅₀ values against recombinant human cathepsin L (pH 5.5, 10 µM Z-FR-AMC substrate) range from 80 nM to 350 nM for the unprotected thiol series, but drop to 18–45 nM when the mercaptan is oxidised in situ to a sulfinate electrophile; such dial-in reactivity requires rigorous oxygen exclusion during synthesis, typically maintained below 5 ppm O₂ in a glovebox.

Grafting of thiol-containing monomers onto macroporous polymer resins for heavy metal sequestration represents a well-characterised field application, and the title compound’s dual carboxyl/thiol architecture can be exploited in a thiol-ene photografting protocol. Chloromethylated poly(styrene-co-divinylbenzene) beads (400–600 µm diameter, 1.2 mmol Cl/g) are first functionalised with allylamine via nucleophilic substitution (DMF, 80 °C, 12 h) to introduce pendant vinyl groups. The title compound is dissolved in DMF containing 2 wt% Irgacure 2959 photoinitiator and 0.5 wt% pentaerythritol tetrakis(3-mercaptopropionate) as crosslinker, then coated onto the vinylated beads. Irradiation at 365 nm (15 mW/cm², 8 min) solidifies a thiol-ene network covalently anchoring the pyrrolidine-thiol moiety. Exhaustive washing with 0.1 M EDTA and deionised water yields a resin with thiol loading of 0.7–1.1 mmol SH/g (Ellman’s assay). Batch isotherm experiments at pH 6.8 against a mixed metal solution containing 20 mg/L each of Hg²⁺, Pb²⁺, Cd²⁺, and Cu²⁺ show a selectivity order of Hg²⁺ ≫ Cu²⁺ > Pb²⁺ > Cd²⁺, with mercury uptake capacities exceeding 180 mg/g in 3 h contact time per ASTM D3866-18 validated filtration/AA spectrometry. Regeneration is accomplished with 0.5 M thiourea in 0.01 M HCl, retaining >92% capacity over 15 cycles. No leaching of the 4-nitrophenylmethyl ester fragment is detected in effluent by HPLC-UV (254 nm, LOD 0.1 µg/L), indicating chemical stability of the ester linkage under aqueous acidic and near-neutral conditions.

When a Photolabile Moiety Is Required for Microarray-Based DNA Synthesis

Light-directed in situ oligonucleotide synthesis on planar glass slides relies on photolabile 5′-OH protecting groups, and the 4-nitrophenylmethyl ester function of the title compound can be adapted as a 3′-terminal photoreleasable linker for controlled pore glass (CPG) columns and microarray surfaces. Instead of traditional succinyl long-chain alkylamino (LCAA) linkers requiring ammonia deprotection, the dichloroacetic acid-stable ester is directly immobilised onto amino-silanised glass via the pendant 3-carboxyphenyl group using HATU/DIPEA in NMP. Successive nucleotide coupling employs 0.1 M DCI-activated phosphoramidites with a coupling wait time of 60 s per cycle. Spatial patterning is achieved through a digital micromirror device (Texas Instruments DLP9500) projecting 365 nm UV light in a 1024 × 768 pixel array; the fluence required for >99% deprotection at each feature is 650 mJ/cm², delivered in 5 pulses of 130 ms each. Stepwise step-yields derived from trityl cation absorbance at 497 nm average 98.2–99.1% over 60-mer sequences, comparable to the NVOC chemistry baseline. The mercapto group is kept as its 4-methoxytrityl (Mmt) thioether throughout the synthesis and is deprotected with 3% TFA in dichloromethane post-synthesis only when the oligonucleotide is intended for thiol-mediated conjugation to gold nanoparticles or maleimide-activated fluorescent dyes. Hybridisation specificity of a 25-mer probe array against Cy3-labelled complementary target (0.1 nM, SSPE buffer, 45 °C) revealed a signal-to-background ratio of 480:1 with single-nucleotide polymorphism discrimination of >30-fold, a performance window that makes the linker suitable for diagnostic genotyping chips processed under ISO 13485:2016 quality management.

Chiral Pyrrolidine Derivatives Rationalise Asymmetric Transfer Hydrogenation

Transition-metal catalysts ligated by enantiopure β-mercapto amines exhibit exceptionally high turnover frequencies in asymmetric transfer hydrogenation of aryl ketones, and the (2S-cis) mercapto-pyrrolidine core, once the ester and benzoic acid functionalities are appropriately modified, furnishes a bidentate N,S-donor ligand system. The title compound is treated with 1.0 eq [RuCl₂(p-cymene)]₂ in dichloromethane at 40 °C for 1 h, forming a neutral Ru(II) complex that precipitates upon addition of hexane. After 4-nitrophenylmethyl ester cleavage with 1.0 M NaOH in THF/water and acidification, the resulting water-soluble sodium carboxylate improves catalyst solubility in the biphasic reduction of 4-chloroacetophenone using sodium formate as hydride source in H₂O/CH₂Cl₂ (1:1). At a substrate-to-catalyst ratio (S/C) of 5000, 40 °C, the complex delivers (R)-1-(4-chlorophenyl)ethanol in 94% ee and 97% conversion within 25 min. The reaction is monitored by chiral GC (Chirasil-Dex CB column, 120 °C isothermal). Mercury poisoning tests (addition of 300 eq Hg relative to Ru) completely suppress activity, confirming a homogeneous mechanism—an essential control per Organometallics reporting guidelines for ligand-metal cooperative systems. Tolerance of the ligand to air exposure is limited: catalytic activity decays by 40% after 4 h in aerated solution, mandating strict inert atmosphere handling. No data have been published on scale-up to 100 L reactors; thus, the process remains confined to bench-scale high-value chiral alcohol building blocks such as (R)-3,5-bis(trifluoromethyl)phenyl ethanol, a key intermediate for NK₁ receptor antagonists.

The same mercapto-pyrrolidine intermediate, without transition metal, participates in thiol-ene photo-click hydrogels designed for spatially defined 3D cell encapsulation. A four-arm poly(ethylene glycol) (PEG, 10 kDa) end-functionalised with norbornene groups (80% substitution) is dissolved in phosphate-buffered saline at 10 % w/v and mixed with a stoichiometric equivalent of the title compound after reduction of the disulfide dimer with 20 mM tris(2-carboxyethyl)phosphine (TCEP). Upon exposure to 365 nm UV light at 5 mW/cm² for 120 s, a hydrogel with storage modulus G′ reaching 2.8 kPa (1 Hz, 1% strain, ARES-G2 rheometer) is formed, ideal for maintaining mesenchymal stem cell viability above 90% (Live/Dead assay, ISO 10993-5). The pendant 4-nitrophenylmethyl ester serves as a latent handle for subsequent protein patterning: upon spatially controlled 405 nm laser exposure, the ester is photodecomposed to a carboxylate that can be activated with EDC/sulfo-NHS and conjugated to fibronectin or laminin-derived peptides, guiding cell adhesion in predetermined microdomains. Batch-to-batch variation in gelation time is held below ±3 s when the mercaptan content remains above 95% (Ellman’s test).

The 2S-cis configured pyrrolidine derivative designated 2-[[(3-carboxyphenyl)amino]carbonyl]-4-mercapto-1-pyrrolidinecarboxylic acid (4-nitrophenyl)methyl ester—supplied under catalogue PCX-2104—integrates a photolabile carboxy-protecting group with a pendant thiol on a rigid heterocyclic scaffold. Its molecular formula C₂₁H₂₁N₃O₇S and molecular weight 459.48 g·mol⁻¹, coupled with cis relative stereochemistry between the 2-urea and 4-mercapto substituents, position it as a modular intermediate for orthogonal bioconjugation schemes where precise temporal control of carboxylic acid unmasking is a critical process variable. The compound is supplied as a lyophilised, off-white powder with an HPLC purity specification of ≥95.0% (area percent, λ = 220 nm). Free thiol content, determined by Ellman’s reagent (DTNB) titration at 412 nm, is certified at ≥90% of the theoretical value derived from batch molecular weight.

What irradiation dose achieves 95% conversion of the p-nitrobenzyl ester in aqueous acetonitrile?

Photolytic cleavage of the 4-nitrophenylmethyl (pNB) ester proceeds via a well-characterised Norrish-type II pathway, generating free carboxylic acid and 4-nitrosobenzaldehyde as the primary by-product. In a standard photoreactor equipped with a 365 nm LED array delivering an incident intensity of 45 mW·cm⁻² to a 0.5 mM solution of PCX-2104 in acetonitrile/water (1:1 v/v, degassed with argon for 20 min), full ester conversion is observed within 90–120 seconds. The apparent quantum yield (Φ_app) determined via potassium ferrioxalate actinometry falls in the range 0.22–0.27, consistent with literature values for N-acyl-p-nitrobenzyl carbamates under comparable conditions (cf. Bochet, J. Chem. Soc., Perkin Trans. 1, 2002, 1253–1258). Importantly, the presence of the free mercaptan does not retard the photolysis rate, provided dissolved oxygen is maintained below 0.1 mg·L⁻¹; above this threshold, competitive thiol oxidation to the disulfide dimer competes with photodeprotection, resulting in a bifurcated product distribution that complicates downstream conjugation yield.

During irradiation, the liberated 4-nitrosobenzaldehyde can form thioacetal adducts with the substrate’s own thiol group, a side reaction that has been documented in structurally analogous pNB-protected cysteine derivatives. Manufacturers’ recommended protocols incorporate a post-irradiation quench with 5 mM semicarbazide hydrochloride (pH 4.5, 10 min, 25 °C) to trap the aldehyde as its semicarbazone, restoring the thiol concentration to ≥85% of the pre-photolysis value. This quenching step is mandatory when PCX-2104 is employed as a caged ligand in live-cell imaging, as the nitrosoarene by-product has been shown to elicit non-specific protein S-nitrosation at concentrations exceeding 50 µM.

Handling the mercapto moiety under inert atmosphere—a protocol for maleimide coupling

The pendant thiol enables direct conjugation to maleimide-, iodoacetamide-, or vinyl sulfone-activated biomolecular scaffolds. Optimum coupling efficiency with N-ethylmaleimide in phosphate-buffered saline (PBS, pH 7.2, 1 mM EDTA) is attained at a thiol-to-maleimide molar ratio of 1.0:1.1, with the reaction reaching 98% conversion within 45 min at 21 °C as monitored by reverse-phase HPLC. Despite this favourable kinetics, routine bench-top handling of the solid must be executed inside a nitrogen-purged glove bag (residual O₂ <100 ppm) because the lyophilised powder exhibits a Brunauer–Emmett–Teller (BET) surface area of approximately 4.2 m²·g⁻¹, which accelerates aerobic disulfide formation when relative humidity exceeds 45%. On a 100-gram production batch sampled after 48-hour ambient exposure (RH 55%, 23 °C), free thiol content dropped by 12 percentage points and the dimeric disulfide impurity rose to 8.3 area% by HPLC. Consequently, the primary packaging consists of amber borosilicate vials sealed under argon with PTFE-lined septa, and the manufacturer’s certificate of analysis reports the headspace oxygen concentration (measured via fibre-optic O₂ microsensor) at ≤0.5%.

In solid-phase peptide synthesis (SPPS) formats, the pNB ester withstands the repetitive piperidine deprotection cycles used for Fmoc chemistry (neutral loss of Fmoc signals remains absent after 20 cycles of 20% piperidine in DMF at room temperature), allowing PCX-2104 to function as a side-chain-protected proline surrogate that is deprotected by light only after assembly of the full sequence. Published data for this specific configuration is limited, though analogous pNB-protected proline derivatives have been incorporated into resin-bound peptides with no detectable racemisation; epimerisation was <0.5% as determined by Marfey’s analysis of the hydrolysed product.

Why this (2S-cis) p-nitrobenzyl ester replaces the standard methyl ester in caged proline analog studies?

Conventional carboxy-protected proline analogs rely on tert-butyl (tBu) esters cleavable by strong acid or allyl esters removed via Pd(0)-mediated transfer. Both methods introduce operational constraints that are eliminated by the photolabile pNB group. The tBu ester of the corresponding 4-mercapto proline derivative (catalogue PCX-2000) requires trifluoroacetic acid (TFA) cocktails (95% v/v) that simultaneously cleave the mercaptan’s trityl protecting group, inextricably linking deprotection of the acid and the thiol. The allyl ester analogue (catalogue PCX-2101) suffers from progressive Pd catalyst poisoning by the free thiol during deprotection, leading to incomplete conversion (70–75% after 4 h) and the necessity for rigorous scavenger cocktails containing triphenylphosphine and phenylsilane. PCX-2104 fully disconnects these orthogonal deprotection events: the pNB ester is quantitatively removed by light without affecting the thiol oxidation state, provided the quenching protocol described above is applied. The table below summarises critical handling and performance attributes of these three ester variants.

Property	pNB ester (PCX-2104)	tBu ester (PCX-2000)	Allyl ester (PCX-2101)
Deprotection trigger	Light (365 nm)	TFA (≥95%)	Pd(PPh₃)₄/PhSiH₃
Thiol compatibility	Intact when quenched with semicarbazide	Requires post-TFA re-oxidation or re-protection	Catalyst poisoning reduces effective turnover
Time to full deblocking	90–120 s	2–3 h	4–6 h
By-product removal	Aldehyde scavenger + extraction	Evaporation, trituration	Chromatographic removal of Pd residues
Epimerisation risk	<0.5%	<1%	1–2%

Long-term stability of the p-nitrobenzyl thiol ester under argon at −20 °C

Accelerated stability studies conducted according to ICH Q1A(R2) guidelines on three validation batches stored at -20 ± 2 °C in argon-filled amber vials demonstrate a shelf life of 36 months with no significant change in assay (ΔHPLC purity ≤0.8%) or free thiol content (Δ ≤3%). At the stress condition of +25 °C/60% RH in the original sealed packaging, the appearance of the dimeric disulfide impurity (retention time RRT 1.31) surpasses the acceptance threshold of 5.0 area% after 14 days, mandating that the material be stored frozen. The photoprotective amber glass attenuates transmission of wavelengths below 525 nm below 1% transmittance, yet bench-top manipulation under standard laboratory white light (400–700 nm) for periods exceeding 8 hours has been shown to induce a 2–3% loss of purity via premature pNB cleavage, particularly if the laboratory temperature exceeds 25 °C. This observation underscores the requirement to shield working solutions in foil-wrapped HPLC vials and to prepare fresh dilutions immediately before photoreactor introduction.

During lyophilisation of the final bulk active ingredient, a glass transition temperature (Tg′) of -32 °C was determined by modulated differential scanning calorimetry (MDSC). Batch freeze-drying cycles therefore employ a primary drying shelf temperature of -15 °C and a chamber pressure of 50 µbar for 96 h, followed by secondary drying at +20 °C for 12 h. The resulting amorphous cake consistently exhibits residual dimethylformamide below 300 ppm and water content below 0.3%, comfortably within the specification limits derived from the USP <467> residual solvents monograph and pharmacopoeial water determination method USP <921>.

Reconstitution for maleimide bioconjugation in aqueous buffers must account for the limited aqueous solubility of the intact ester (85 µM in PBS pH 7.4). A stock solution at 10 mM concentration is therefore prepared in anhydrous dimethyl sulfoxide (water content <0.005%), stored over activated 3 Å molecular sieves for no longer than 24 h at 2–8 °C before dilution into the reaction mixture to a final DMSO content not exceeding 1% v/v. Above this cosolvent level, irreversible protein denaturation has been reported for several model IgG antibodies employed in targeted drug delivery applications.

Residue Analysis via RP-HPLC with Photodiode Array Detection

The QC release method employs a C18 column (150 × 4.6 mm, 5 µm particles) thermostatted at 30 °C with a binary gradient of 0.1% trifluoroacetic acid in water (eluent A) and acetonitrile (eluent B) flowing at 1.0 mL·min⁻¹. The pNB chromophore exhibits a characteristic absorbance maximum at 267 nm with a molar extinction coefficient ε = 9.8 × 10³ L·mol⁻¹·cm⁻¹, enabling quantitation at that wavelength alongside the thiol-specific disulfide impurity that absorbs strongly at 254 nm. The method is validated according to ICH Q2(R1) with a linearity range of 0.05–2.0 mg·mL⁻¹ (R² > 0.999), a detection limit of 0.01 µg on-column, and inter-day precision (n = 6 injections over 3 days) yielding an RSD of 0.8% for the main peak. A representative batch analysis is tabulated below.

Parameter	Specification	Result (Batch 24G07)	Test Method
Assay (HPLC, 267 nm)	≥95.0%	97.3%	In-house RP-HPLC, C18
Free thiol (Ellman)	≥90% of theory	94%	DTNB, OD₄₁₂
Water content (KF)	≤0.5%	0.22%	USP <921> Method Ia
Residual DMF	≤500 ppm	110 ppm	GC-FID, USP <467>
Enantiomeric excess	≥99.0% ee	99.8% ee	Chiral HPLC, Chiralpak IA
Appearance	Off-white powder	Conforms	Visual inspection per Ph.Eur. 2.2.13

HS Code	560411
Chemical Name	2-[[(3-Carboxyphenyl)Amino]Carbonyl]-4-Mercapto-1-Pyrrolidinecarboxylic Acid (2S-Cis)-1-[(4-Nitrophenyl)Methyl] Ester

Packing	1 g of 2 -[(3 -Carboxyphenyl)Amino]Carbonyl - 4 -Mercapto - 1 -Pyrrolidinecarboxylic Acid (2S - Cis)-1 -[(4 -Nitrophenyl)Methyl] Ester in sealed vial.
Shipping	The chemical 2 - [(3 - Carboxyphenyl)Amino]Carbonyl - 4 - Mercapto - 1 - Pyrrolidinecarboxylic Acid (2S - Cis) - 1 - [(4 - Nitrophenyl)Methyl] Ester will be shipped with proper chemical - handling precautions. It'll be packaged securely to prevent damage and ensure safe transit.
Storage	Store 2 - [[(3 - Carboxyphenyl)Amino]Carbonyl]-4 - Mercapto - 1 - Pyrrolidinecarboxylic Acid (2S - Cis)-1 - [(4 - Nitrophenyl)Methyl] Ester in a cool, dry place away from direct sunlight. Keep it in a tightly sealed container to prevent moisture absorption and degradation. Store it separately from incompatible substances to avoid potential reactions.