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    • Cy3 NHS Ester (Non-Sulfonated): Atomic Facts for Protein ...

    2026-02-23

    Cy3 NHS Ester (Non-Sulfonated): Atomic Facts for Protein and Oligonucleotide Labeling

    Executive Summary: Cy3 NHS ester (non-sulfonated) is a reactive fluorescent dye specifically engineered for covalent labeling of amino groups on biomolecules (APExBIO). The dye features excitation/emission maxima at 555/570 nm, an extinction coefficient of 150,000 M-1cm-1, and a quantum yield of 0.31, facilitating high-sensitivity detection in fluorescence microscopy (Li et al., 2025). It is soluble in DMSO (≥59 mg/mL) and ethanol (≥25.3 mg/mL with ultrasound), but not in water. APExBIO’s Cy3 NHS ester (SKU A8100) is validated for protein, peptide, and oligonucleotide labeling workflows, supporting imaging of organelle dynamics and targeted degradation strategies (related article). Water-soluble sulfo analogs are preferred for delicate proteins; this non-sulfonated form requires organic co-solvents for optimal conjugation.

    Biological Rationale

    Protein and nucleic acid labeling with fluorescent dyes enables sensitive detection in biochemical and imaging assays. Cy3 NHS ester (non-sulfonated) targets primary amines on lysine residues and N-termini of proteins, as well as amino-modified oligonucleotides (APExBIO). The resulting conjugates support quantification, visualization, and tracking of biomolecules in living and fixed systems. The orange fluorescence (excitation 555 nm, emission 570 nm) is compatible with standard TRITC filter sets. This spectral window enables multiplexing with dyes of different wavelengths, minimizing spectral overlap (see high-sensitivity workflows). Cy3 NHS ester is widely adopted in autophagy, targeted protein/organelle degradation, and cancer biology research, where tracking molecular cargo is essential (Li et al., 2025).

    Mechanism of Action of Cy3 NHS ester (non-sulfonated)

    Cy3 NHS ester (non-sulfonated) contains an N-hydroxysuccinimide (NHS) ester functional group. This moiety reacts specifically with primary amines in biomolecules under mildly basic conditions (pH 7.5–8.5), forming stable amide bonds (APExBIO). The dye’s polymethine (cyanine) core provides strong absorption at 555 nm and efficient fluorescence emission at 570 nm. The high extinction coefficient (150,000 M-1cm-1) and quantum yield (0.31) enable sensitive detection by standard fluorometers and microscopes. The lack of sulfonate groups renders it insoluble in water, necessitating DMF or DMSO as reaction media. After labeling, extensive washing removes unreacted dye, yielding highly specific conjugates for downstream use.

    Evidence & Benchmarks

    • Cy3 NHS ester (non-sulfonated) enables covalent labeling of proteins, peptides, and amino-modified oligonucleotides, facilitating robust detection in fluorescence-based assays (Li et al., 2025).
    • Excitation/emission maxima are 555 nm/570 nm in organic solvents, supporting use with TRITC filter sets for orange fluorescence (APExBIO).
    • The extinction coefficient is 150,000 M-1cm-1, and the quantum yield is 0.31 in DMSO, enabling high-sensitivity imaging and quantification (see workflow).
    • Solubility: ≥59 mg/mL in DMSO, ≥25.3 mg/mL in ethanol (ultrasonication), insoluble in water (APExBIO).
    • Validated in organelle labeling, autophagy, and targeted degradation studies, enabling visualization of mitochondrial clearance in cancer models (Li et al., 2025, Figure 1).

    Applications, Limits & Misconceptions

    Cy3 NHS ester (non-sulfonated) is used for:

    • Fluorescent labeling of proteins, peptides, and oligonucleotides for quantitative assays, western blotting, and single-molecule studies.
    • Imaging of organelle dynamics, protein localization, and tracking of biomolecular interactions.
    • Construction of labeled probes for flow cytometry, FRET, and multiplexed microscopy.
    • Supporting translational research in autophagy and targeted degradation, as demonstrated in p62-mimetic nanoparticle platforms for cancer therapy (Li et al., 2025).

    For further reading, this article dissects how Cy3 NHS ester (non-sulfonated) supports advanced imaging and organelle-targeting workflows, extending the focus here by providing strategic context for translational researchers.

    Common Pitfalls or Misconceptions

    • Not water-soluble: Cy3 NHS ester (non-sulfonated) requires organic co-solvents (DMSO or DMF); it will not dissolve in aqueous buffers.
    • Not suitable for delicate proteins in pure water: Use sulfo-Cy3 NHS esters for water-based labeling to avoid protein denaturation (internal Q&A extends this with practical troubleshooting).
    • Solutions are unstable long-term: Prepare fresh dye solutions before each use; storage as a solid at -20°C in the dark is recommended.
    • Excess light exposure degrades dye: Perform all procedures under low-light conditions.
    • Not compatible with all filter sets: Use TRITC or equivalent filters for optimal detection.

    Workflow Integration & Parameters

    For labeling, dissolve Cy3 NHS ester (non-sulfonated) in DMSO or DMF; typical working concentrations are 1–10 mM. Add to protein or oligonucleotide solutions buffered at pH 7.5–8.5. Incubate at 4–25°C for 30–120 minutes, protected from light. Remove unreacted dye by gel filtration, dialysis, or repeated washing. Measure degree of labeling (DOL) using absorbance at 555 nm. Store labeled conjugates at 4°C, protected from light; avoid repeated freeze-thaw cycles. This workflow comparison details integration strategies across sensitive imaging applications, which this article updates with recent benchmarks and mechanistic clarifications. For application notes in quantitative assays, see this protocol-driven piece.

    Conclusion & Outlook

    Cy3 NHS ester (non-sulfonated) from APExBIO (SKU A8100) is a validated, high-sensitivity dye for protein, peptide, and oligonucleotide labeling in fluorescence workflows. Its orange emission, high extinction coefficient, and robust linkage chemistry make it indispensable for imaging and detection in biomedical research. While water-solubility limitations require attention to solvent choice, the dye’s atomic properties underpin reproducible, quantitative labeling. Ongoing advances in autophagy-based targeted degradation and multiplexed imaging continue to expand Cy3 NHS ester’s relevance, as reflected in recent translational research (Li et al., 2025).