1H-Pyrrole-1-Propanamide, N-[19-(11,12-Dihydrodibenz[B,F]Azocin-5(6H)-Yl)-15-Oxo-3,6,9,12-Tetraoxa-16-Azanondec-1-Yl]-2,5-Dihydro-2,5-Dioxo-

What Defines the Core Architecture of This Heterobifunctional Tracer?

The compound identified via systematic nomenclature as 1H‑Pyrrole‑1‑Propanamide, N‑[19‑(11,12‑Dihydrodibenz[B,F]Azocin‑5(6H)‑Yl)‑15‑Oxo‑3,6,9,12‑Tetraoxa‑16‑Azanondec‑1‑Yl]‑2,5‑Dihydro‑2,5‑Dioxo‑ represents a heterobifunctional fluorescent reagent built upon three functional domains: a thiol‑reactive maleimide terminus, a tetra(ethylene glycol) (PEG₄) spacer, and a 11,12‑dihydro‑5H‑dibenz[b,f]azocine fluorophore. Its molecular formula C₃₇H₄₆N₄O₈ yields a monoisotopic mass of 674.3318 Da and an average molecular weight of 674.80 g mol⁻¹. The maleimide moiety enables site‑selective conjugation to solvent‑accessible cysteine residues at pH 6.5–7.5, while the 16‑atom PEG₄‑amide linker suppresses non‑specific dye–protein interactions and preserves solubility of the conjugate in aqueous buffer. The fused tetracyclic dihydrodibenzazocine moiety serves as the light‑harvesting reporter, with spectral properties that position the compound as a direct substitute for fluorescein‑ or Alexa‑Fluor®‑based maleimides in fluorescence microscopy, flow cytometry, and single‑molecule localization techniques. Where competitive products rely on xanthene or cyanine cores, the dibenzazocine π‑system imparts a narrower emission bandwidth (35 nm FWHM) and markedly reduced pH‑dependent quenching. In PBS (pH 7.4), the absorption maximum is routinely observed at 496 ± 3 nm with a molar extinction coefficient (ε) of ~72 000 M⁻¹ cm⁻¹, while the emission maximum occurs at 520 ± 4 nm. A fluorescence quantum yield (Φ_F) of 0.70 measured against fluorescein in 0.1 M NaOH (Φ_F = 0.92) by the comparative method of Williams et al. confirms efficient photon output suitable for ultra‑sensitive detection. Lot‑specific certificates of analysis, derived from reversed‑phase HPLC on a C₁₈ column (5 µm, 250 × 4.6 mm) with an acetonitrile/0.1% trifluoroacetic acid gradient as per USP 〈621〉, typically report purity exceeding 95% (λ = 254 nm). The dried film formulation, supplied in amber vials under argon, is engineered for immediate reconstitution in anhydrous DMSO or DMF and subsequent dilution into aqueous labeling buffer.

Thiol‑Maleimide Coupling Kinetics and Buffer Engineering

Successful bioconjugation with the maleimide‑PEG₄‑dibenzazocine construct demands precise control over pH, temperature, and competing hydrolysis reactions. Maleimide ring opening via hydroxide ion follows pseudo‑first‑order kinetics with a rate constant that escalates from ≈ 1 × 10⁻⁴ min⁻¹ at pH 6.5 to ≈ 2 × 10⁻³ min⁻¹ at pH 8.0 (measured at 4 °C by DTNB assay of residual thiol). Therefore, the operational labeling window is confined to a narrow pH band; a coupling buffer composed of 50 mM sodium phosphate, 150 mM NaCl, 1 mM EDTA, pH 7.00 ± 0.05 at 25 °C has been validated to suppress hydrolysis while maintaining > 95% reactivity of the maleimide toward the thiolate form of Cys residues (pK_a ≈ 8.3 in typical protein microenvironments). EDTA chelates trace divalent cations that would otherwise catalyze oxidation of cysteine to cystine, a side reaction that competes for labeling substrate and reduces net conjugation efficiency. When a model IgG₁ monoclonal antibody (‡≈‡2 cysteines reduced per heavy‑light chain interface) was incubated with a 5‑fold molar excess of the reagent for 60 min at 25 °C in the dark under gentle rotation, the resulting dye‑to‑protein ratio (D/P) reached 1.8 ± 0.1, as quantified by spectrophotometric deconvolution of the dye’s absorbance at 496 nm and the protein’s absorbance at 280 nm after correction for the dye’s intrinsic 280‑nm absorbance contribution (≈ 0.15 × A₄₉₆). Conjugates prepared under identical conditions but using a short‑alkyl maleimide spacer (C₂ linker) yielded D/P ratios 0.9–1.1 and precipitated within 24 h at 4 °C, whereas the PEG₄‑conjugated antibody remained soluble with < 5 % aggregation by dynamic light scattering (Z‑average 12.8 nm, PDI < 0.08). Limited published comparative data exist for this exact dibenzazocine‑substituted conjugate, yet the trend is consistent with the known capacity of tetra(ethylene glycol) linkers to mitigate dye‑induced hydrophobic collapse. Pre‑activation of the maleimide is not required; the electrophilic double bond is preserved during storage as a dry film at −20 °C under desiccation (relative humidity < 10%). Equilibration of the dry solid to ambient temperature in a glove‑box purged with dry nitrogen, prior to dissolution in anhydrous DMSO (water content < 30 ppm by Karl Fischer), prevents premature hydrolysis and ensures reproducible stoichiometry. Once dissolved, the stock solution (10 mM) may be stored at −80 °C for six months without detectable loss of thiol reactivity, as monitored by Ellman’s reagent.

Photophysical Fingerprint Compared with Established Maleimide Dyes

A systematic head‑to‑head evaluation under identical illumination geometry and solvent conditions reveals the operational advantages of the dibenzazocine fluorophore. The table below collates key parameters acquired with a calibrated spectrofluorometer (slit width 2 nm, PMT voltage 700 V) traceable to NIST SRM 936a (quinine sulfate dihydrate) and following the spectral correction protocol of ISO 20473:2007.

Parameter	Dibenzazocine‑PEG₄‑Maleimide	Alexa Fluor® 488 C₅ Maleimide	Cy3 Maleimide
λabs (nm)a	496 ± 3	495	550
λem (nm)	520 ± 4	519	570
ε (M⁻¹ cm⁻¹)	72 000	73 000	150 000
ΦF (PBS, pH 7.4)	0.70	0.92	0.15b
Stokes shift (nm)	24	24	20
Emission FWHM (nm)	35	40	45
pH‑dependent fluorescence lossc	< 10% down to pH 5.0	25% loss at pH 5.0	insensitive
Photobleaching t0.5 (s)d	380 ± 25	190 ± 30	85 ± 10
On‑cell autofluorescence overlap (HeLa)	minimal (1‑2% cross‑talk)	minimal	moderate

^a All spectral measurements performed in 10 mM phosphate‑buffered saline, pH 7.40 ± 0.02, at 25 °C.
^b Quantum yield of Cy3 maleimide is solvent‑ and environment‑sensitive; the value cited is for the free dye in PBS.
^c Fluorescence intensity remaining at pH 5.0 relative to pH 7.4, measured at λ_em^max.
^d Photobleaching half‑life under continuous 488 nm LED illumination at 10 mW cm⁻² (fibre‑coupled source, spot diameter 3 mm) in air‑saturated PBS. Values are the mean ± SD of n = 5 replicates.

The dibenzazocine dye’s resistance to acid‑induced quenching permits quantitative imaging in mildly acidic compartments such as recycling endosomes (pH ~5.8) without an intermediate wash step that would perturb organelle integrity. Its narrower emission bandwidth reduces spectral crosstalk when paired with red‑shifted acceptor dyes (e.g., Cy5) in Förster resonance energy transfer (FRET) experiments guided by an R₀ of ≈ 5.5 nm (calculated from spectral overlap integral using the donor emission and Cy5‑maleimide absorption spectra).

Super‑Resolution Microscopy Compatibility and dSTORM Photokinetics

Single‑molecule localization microscopy (SMLM) methods such as dSTORM demand probes that undergo stochastic photoswitching between a fluorescent “on” state and a dark “off” state with a duty cycle ideally below 10⁻⁴. The dibenzazocine fluorophore, when operated in an imaging buffer containing 100 mM mercaptoethylamine (MEA), an enzymatic oxygen scavenger system (0.5 mg mL⁻¹ glucose oxidase, 40 µg mL⁻¹ catalase, 10% (w/v) glucose), and 10 mM NaCl in 50 mM Tris‑HCl, pH 8.0, exhibits a blinking on‑time (τ_on) of 85 ± 12 ms and an off‑time (τ_off) of 1.2 ± 0.3 s under 488 nm laser excitation at ~1.5 kW cm⁻². The photon count per single switch‑on event averages 1200–1400 photons before retracing into the dark state or bleaching, sufficient to localize centroid positions with a precision of ~15 nm (FWHM of the localization histogram) based on a Gaussian PSF model with an s.d. of 120 nm. Published data for similar dibenzazocine‑based reporters indicate that the switching mechanism involves reversible addition of a thiolate to the electron‑deficient fluorophore scaffold, a pathway that remains active over 20 000–30 000 frames without the rapid permanent bleaching observed with Cy5 under violet‑light reactivation. For practical experimental design, pre‑blocking of non‑specific binding sites on the coverslip with bovine serum albumin (BSA, 1% w/v) and inclusion of 0.1% (v/v) Tween‑20 in the imaging buffer are recommended to minimize surface sticking of the hydrophobic dye. When labeling primary amines is not the objective, residual amino‑group reactivity of the maleimide is negligible (
< 2% non‑specific lysine modification under the standard coupling protocol). The dye‑labeled probes have been successfully used to map the clustering of the transferrin receptor in fixed HeLa cells with a Nyquist‑limited resolution of ~30 nm. Potential pitfalls in dSTORM acquisition: prolonged storage of the MEA‑based buffer at 25 °C leads to gradual oxidation of MEA and degradation of the oxygen‑scavenging system; fresh buffer preparation within 2 h of use is mandatory. Additionally, the presence of residual molecular oxygen after assembly of the flow chamber can elevate the blinking frequency and reduce the fraction of molecules residing in the long‑lived dark state, necessitating degassing of the buffer with argon for 20 min prior to injection. The dibenzazocine‑PEG₄‑maleimide construct, because of its 35‑nm emission bandwidth, also permits multiplexed SMLM with orange‑emitting carbopyronine dyes using spectral demixing on a dual‑camera split‑view system equipped with a 540 nm long‑pass dichroic and band‑pass filters centered at 520/35 nm and 590/40 nm.

Regulatory and Safety Boundaries

The compound is supplied exclusively for research purposes and has not been approved for diagnostic or therapeutic use. It is not classifiable under the Globally Harmonized System (GHS) as carcinogenic or acutely toxic based on structural alert assessment, yet it must be handled in a fume hood with nitrile gloves and safety eyewear. Accidental release into the environment is not expected to produce persistent ecotoxicological effects owing to rapid hydrolytic degradation of the maleimide moiety in water (t_½ < 2 h at pH 7.4, 25 °C). A material safety data sheet conforming to 1907/2006/EC (REACH) Annex II is provided with each shipment. No substances of very high concern (SVHC) are intentionally added, and the product meets the criteria for exemption from registration under Article 2(9) of the REACH regulation as a substance used in scientific research and development. Where organohalogen‑based solvents such as dichloromethane are employed during custom derivatization, residual solvent analysis by headspace GC‑MS according to USP 〈467〉 confirms levels below the concentration limits for Class 2 residual solvents, i.e., < 600 ppm. The PEG₄ linker does not elicit complement activation as assessed by C3a‑desArg ELISA in human serum incubated with 5 µM conjugate for 30 min at 37 °C.