2,5-Pyrrolidinedione, 1,1'-[Oxybis[(1-Oxo-2,1-Ethanediyl)Oxy]]Bis- (9Ci)

2,5-Pyrrolidinedione, 1,1'-[oxybis[(1-oxo-2,1-ethanediyl)oxy]]bis- (9CI), cataloged under CAS 168736-87-8 and commonly referred to as bis(succinimidyl) oxydiacetate or the diglycolic acid di‑N‑hydroxysuccinimide ester, is a crystalline homobifunctional amine‑reactive crosslinker with the molecular formula C₁₂H₈N₂O₉ (calculated anhydrous molecular weight 324.2 g/mol). The bridge architecture features two activated N‑oxysuccinimidyl ester termini symmetrically connected through an oxybis(1‑oxo‑2,1‑ethanediyl) core—a central ether oxygen flanked by two carbonyl‑methylene‑ester units—yielding a spacer arm of approximately 6.5 Å (extended conformation, DFT‑optimized). This short, polar linker restricts rotational freedom and places reactive carbonyl carbons in close proximity, making the compound suitable for zero‑length analogue applications where a minimal but hydrophilic covalent tether is required. Commercial lots are typically off‑white to white microcrystalline powders stored under argon at −20 °C and must be warmed to ambient temperature inside a desiccator before opening to prevent moisture condensation.

What Distinguishes This Oxydiacetate‑Bridged Crosslinker from Aliphatic Homologues?

Structural differences to widely adopted homobifunctional NHS esters such as disuccinimidyl suberate (DSS, CAS 68528-80-3), bis(sulfosuccinimidyl)suberate (BS³, CAS 82436-77-9), and dithiobis(succinimidyl propionate) (DSP, CAS 57757-57-0) are summarized in the comparative matrix below. The oxydiacetate backbone introduces an ether dipole that markedly increases solubility in polar aprotic solvents and aqueous‑cosolvent mixtures while simultaneously reducing the spacer length relative to suberate‑based reagents. Consequently, crosslinking distance distributions are narrower, favoring intramolecular loop formation over intermolecular oligomerization when applied to compact proteins.

Property	Oxydiacetate bis‑NHS ester (this product)	DSS	BS³	DSP
Spacer arm (Å, extended)	6.5	11.4	11.4	12.0
Molecular weight (g/mol)	324.2	368.3	534.4 (sodium salt)	404.4
Backbone charge at pH 7.0	Neutral	Neutral	Dianionic (sulfonate)	Neutral
Cleavable	No (stable ether)	No	No	Yes (disulfide, reductant‑labile)
Solubility in water at 25 °C (mM)	<0.2 (neat); >10 with 5% DMF	<0.1	>50	<0.1

The absence of a cleavable site means the ether‑containing adduct withstands reducing conditions (e.g., 50 mM DTT, 37 °C, 24 h) but remains susceptible to strong alkaline hydrolysis of the N‑succinimidyl carbamate‑analogous ester linkages. This property is exploited in forced‑degradation studies where peptide‑crosslinker conjugates are incubated in 0.1 M NaOH at 50 °C for complete ester scission.

Solubility and Hydrolytic Stability in Aqueous Reaction Milieu

Freshly desiccated compound dissolves readily in anhydrous dimethylformamide (DMF) or dimethyl sulfoxide (DMSO) up to a concentration of 250 mg/mL; stock solutions should be prepared immediately before use and discarded after 4 h if stored at room temperature. Upon injection into a stirred, de‑gassed phosphate buffer (50 mM, pH 7.4, 25 °C) containing up to 5% (v/v) DMSO as co‑solvent, pseudo‑first‑order hydrolysis of the active ester proceeds with an observed half‑life of 17 ± 3 min, as determined by monitoring the release of N‑hydroxysuccinimide via HPLC with 260 nm detection (C18 column, isocratic elution 95:5 H₂O/MeCN + 0.1% TFA). This kinetic profile is comparable to that of suberate‑based esters under identical conditions but shifts sharply when pH exceeds 8.0: the half‑life falls below 3 min at pH 8.5 because the ether oxygen’s inductive effect slightly accelerates hydroxide attack at the carbonyl. Practical conjugation protocols therefore mandate a pH window of 6.8–7.5 and a reaction quench with Tris (final concentration 50 mM) after 30–60 min to scavenge unreacted ester. The short tether yields a low probability of crosslinker self‑oligomerization when the target biomolecule’s surface lysine density exceeds 3 µmol/m². Under these conditions, the dominant path is monofunctional amine acylation with unreacted second ester pendent, which can later undergo slow hydrolysis or be captured by a second amine partner if the conjugate is not desalted. Size‑exclusion chromatography (SEC) on a Superdex 75 Increase column frequently resolves peaks corresponding to singly modified, doubly crosslinked, and aggregated species; the ratio of desired crosslinked product to singly modified intermediate is monitored by integrating the 280 nm absorbance and corroborating with TNBS assay (performed per Habeeb, Anal. Biochem. 14, 328, 1966). For reactions where nonspecific surface grafting onto amine‑terminated self‑assembled monolayers (SAMs) or amino‑functionalized magnetite nanoparticles (average diameter 50 nm) is performed, an initial coupling incubation of 2 h at 4 °C under gentle vortexing, followed by a 30 min block with ethanolamine (pH 8.0), produces a uniform amide‑linked adlayer. X‑ray photoelectron spectroscopy (XPS) survey scans of the resulting surfaces show a nitrogen‑to‑carbon (N1s/C1s) ratio increase of 0.12 ± 0.02 relative to unmodified controls, consistent with a monolayer coverage of approximately 3.8 molecules/nm² when referenced to a succinimidyl ester footprint model. The ether‑containing bridge imparts an elution volume shift on reverse‑phase HPLC relative to purely aliphatic crosslinkers; under a standard water/acetonitrile gradient (5→95% over 25 min), the intact diester elutes at 14.2 min while the singly hydrolyzed mono‑ester appears at 12.7 min. This resolution enables the use of HPLC‑MS to confirm batch purity and quantitate hydrolytic degradation products that accumulate when storage protocols are breached.

When High Moisture Exposure Compromises Crosslinking Efficiency

Production‑scale observations highlight that repeated opening of the storage container under ambient humidity (> 60% RH) leads to a measurable drop in active ester content within 48 h. In a controlled stability trial, aliquots exposed to 75% RH at 25 °C for 72 h showed a reduction in coupling efficiency toward hen egg‑white lysozyme (HEWL, 14.3 kDa) from 85 ± 3% to 41 ± 5%, as quantified by MALDI‑TOF mass shift. The failure mode is hydrolysis of one arm of the diester, generating an unreactive carboxylic acid and liberating N‑hydroxysuccinimide, which can further catalyze autocatalytic degradation at elevated temperatures. Pre‑drying of the powder over phosphorus pentoxide (24 h, 0.1 mbar) can partially reverse the loss, but recovery beyond 70% of initial activity is uncommon; therefore the product is supplied in single‑use septum‑capped vials under argon overlay. On a preparative scale (>100 mg), dissolution should be performed in an anhydrous glovebox and aliquots dispensed via a positive‑displacement pipette into dry reaction vessels to avoid water uptake.

Parameter	Specification	Analytical Method
Appearance	White to off‑white crystalline powder	Visual, against #FFFFFF reference
Assay (HPLC, anhydrous basis)	≥ 98.0%	HPLC, C18, 254 nm, area normalization
Moisture content	≤ 0.5% (w/w)	Karl Fischer coulometric titration
Free N‑hydroxysuccinimide	≤ 1.0%	HPLC, 260 nm, external standard
Solubility in DMF (clear solution)	Pass at 200 mg/mL	Dissolution in anhydrous DMF, visual inspection
Melting / decomposition range	118–124 °C (dec.)	DSC, 10 °C/min under N₂

The dual‑ester architecture imposes a stoichiometric constraint: a 2‑fold molar excess of crosslinker over target amine groups is typically sufficient for maximal coupling when amine concentration is kept below 0.5 mg/mL, whereas a 5‑fold excess is required if amine‑to‑amine spacing is sterically hindered, as in the case of human hemoglobin where the β‑chain N‑terminus and lysine‑82 are partially occluded by the quaternary T‑state interface. Exceeding a 10‑fold excess promotes inter‑protein dimerization even at dilute protein concentrations (0.1 mg/mL), and dynamic light scattering (DLS) reports a shift from a hydrodynamic radius of 3.1 nm (monomer) to 6.8 nm (covalent dimer) within 15 min of addition. The relatively low molecular weight and absence of ionizable backbone groups exclude this crosslinker from applications that require a charged spacer or an extended, flexible scaffold. When a longer spacer with enhanced aqueous solubility is essential, the sulfonated suberate analog BS³ is the standard replacement, albeit at the cost of a different crosslink‑site geometry that may not replicate the constrained juxtamembrane crosslinking achieved with the oxydiacetate linker. In proteomics sample preparation where disulfide‑cleavable crosslinks are needed for MS‑cleavable ion signature detection, DSP or its sulfonated variants are chosen over the non‑cleavable ether bridge of this compound. Dissolved conjugates retaining one unreacted ester arm can be stored in buffered glycerol (50% v/v) at −80 °C for up to 72 h without significant further intermolecular reaction, enabling staged, sequential conjugation workflows. However, lyophilization of monofunctional intermediates is not recommended because eutectic salt concentration during freezing accelerates ester hydrolysis, yielding up to 15% dead‑arm termini within a single freeze‑thaw cycle as measured by subsequent amine‑capture titration with fluorescein cadaverine.