Chemical Structure and Properties

Thiol PEG Amine (HS-PEG-NH2) is a linear, heterobifunctional polyethylene glycol (PEG) derivative. Its backbone consists of repeating ethylene oxide units (–CH2CH2O–) that provide outstanding aqueous solubility, chain flexibility, and steric shielding. The molecule is uniquely terminated at one end with a free thiol group (–SH) and at the other end with a primary amine group (–NH2).

The two reactive termini are chemically orthogonal, meaning they can be selectively addressed by distinct conjugation chemistries under mild, aqueous conditions without mutual interference. The thiol group has a pKa around 8–10 and is nucleophilic at near-neutral pH, while the primary amine (pKa ~9–10) is most reactive above pH 8.0 with electrophilic reagents such as NHS esters.

The PEG spacer length can vary by molecular weight (e.g., MW 1,000–10,000 Da), which directly influences the hydrodynamic radius, flexibility, and steric properties of the resulting conjugate. Higher molecular weight PEG chains provide greater steric protection and prolonged circulation times in vivo.

Conjugation Chemistry and Reaction Mechanisms

Thiol (–SH) Reactivity: The thiol terminus of HS-PEG-NH2 undergoes several well-characterized reactions:

• Maleimide–thiol (Michael addition): At pH 6.5–7.5, the thiol group reacts rapidly and selectively with maleimide-functionalized molecules or surfaces, forming a stable thioether bond. This is among the most widely used bioconjugation reactions.
• Gold surface binding: Thiols form strong coordinate covalent bonds with gold (Au–S bond), making HS-PEG-NH2 ideal for self-assembled monolayer (SAM) formation on gold nanoparticles (AuNPs) and gold biosensor surfaces.
• Disulfide exchange: Reaction with pyridyl disulfide (OPSS) or other activated disulfides yields a reducible disulfide bond, useful in stimuli-responsive delivery systems.
• Vinyl sulfone and iodoacetyl groups also react efficiently with the thiol under mild conditions.

Amine (–NH2) Reactivity: The primary amine terminus participates in:

• NHS ester coupling: At pH 7–9, amine reacts with N-hydroxysuccinimide (NHS) ester-activated molecules to form stable amide bonds — the cornerstone of many protein and antibody conjugation protocols.
• Reductive amination: Reaction with aldehyde-presenting surfaces or molecules (e.g., oxidized glycoproteins) in the presence of a reducing agent (e.g., NaCNBH3) yields a secondary amine linkage.
• Isocyanate and epoxide reactions: Provide additional cross-linking options for polymer and material science applications.

Key Applications in Research

HS-PEG-NH2 is employed across a broad spectrum of research disciplines owing to its dual-functional, orthogonal reactivity and the beneficial properties of the PEG spacer:

1. Gold Nanoparticle (AuNP) Surface Functionalization: The thiol end anchors HS-PEG-NH2 onto AuNP surfaces via Au–S bonds, while the free amine terminus enables subsequent conjugation of targeting ligands (antibodies, peptides, aptamers), drugs, or fluorescent labels. PEGylation simultaneously imparts colloidal stability, reduces non-specific protein adsorption, and prolongs circulation half-life in biological environments.

2. Biosensor and Diagnostic Surface Engineering: On gold electrode or gold film biosensor platforms, SAMs of HS-PEG-NH2 serve as antifouling layers with amine handles for oriented immobilization of capture probes (antibodies, oligonucleotides), improving assay sensitivity and specificity.

3. Drug Delivery and Nanocarrier Engineering: HS-PEG-NH2 is used to PEGylate liposomes, polymeric nanoparticles, and dendrimers. The amine group allows attachment of targeting moieties or therapeutic payloads, enabling active-targeting drug delivery systems.

4. Protein and Antibody Bioconjugation: By selectively reacting the amine with NHS-ester-activated proteins or using the thiol for site-specific conjugation to engineered cysteine residues, researchers create defined, stable conjugates for ELISA, Western blot secondary reagents, and therapeutic antibody–drug conjugates (ADCs).

5. Hydrogel and Biomaterial Fabrication: HS-PEG-NH2 can serve as a crosslinker or tethering agent in hydrogel systems, enabling cell-adhesive peptide incorporation or post-fabrication surface functionalization via the amine or thiol terminus.

Handling, Storage, and Practical Tips

Storage: HS-PEG-NH2 should be stored in a cool, dry environment, typically at –20 °C, protected from light and moisture. Repeated freeze-thaw cycles should be avoided; aliquoting into single-use portions upon first opening is strongly recommended.

Oxidation Prevention: The free thiol group is susceptible to oxidation, forming disulfides (–S–S–) that reduce conjugation efficiency. Handle under inert atmosphere (nitrogen or argon) where possible, and use freshly prepared, deoxygenated buffers. Addition of reducing agents such as TCEP (tris(2-carboxyethyl)phosphine) at low concentration immediately before use can reduce disulfide dimers and regenerate free thiol, without interfering with the amine terminus.

Buffer Compatibility: Avoid buffers containing primary amines (e.g., Tris, glycine) when performing NHS ester coupling via the amine terminus, as these will compete with the reagent. Phosphate-buffered saline (PBS, pH 7.2–7.4) or HEPES buffer is preferred. For thiol–maleimide reactions, maintain pH 6.5–7.5 to maximize selectivity and minimize maleimide hydrolysis.

Solubility: HS-PEG-NH2 is freely soluble in water and most aqueous buffers, as well as common organic solvents such as DMSO, DMF, and DCM. Prepare stock solutions in the desired solvent and use promptly.

Quality Check: Ellman's reagent (DTNB) assay can be used to quantify free thiol content prior to conjugation experiments, serving as a purity and activity check.

Product Specifications and Ordering Information

Aladdin Scientific offers Thiol PEG Amine (HS-PEG-NH2) under catalog number T164363. Researchers should consult the product datasheet for precise specifications including available molecular weights, purity grade, and lot-specific analytical data such as NMR and GPC characterization confirming PEG chain length distribution and functional group integrity.

When selecting an appropriate molecular weight, consider the application context: lower MW PEG (e.g., 1,000–2,000 Da) is preferred for dense surface functionalization where short spacers are needed, while higher MW PEG (e.g., 5,000–10,000 Da) is favored for stealth nanoparticle coatings and in vivo applications requiring prolonged circulation.

For bulk quantities, custom molecular weights, or specialized purity requirements, contact Aladdin Scientific's technical support team. Proper documentation including SDS and certificate of analysis (CoA) is available upon request for regulatory and quality management purposes.