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    • Cy5.5 NHS Ester (Non-Sulfonated): Benchmarking a Near-Inf...

    2026-02-09

    Cy5.5 NHS Ester (Non-Sulfonated): Benchmarking a Near-Infrared Dye for Biomolecule Labeling

    Executive Summary: Cy5.5 NHS ester (non-sulfonated) is a near-infrared fluorescent dye with an excitation maximum at 684 nm and emission at 710 nm, enabling deep-tissue biomolecule labeling and low background fluorescence (Kang et al., 2025). The NHS ester group reacts selectively with primary amines, facilitating efficient covalent coupling to proteins, peptides, and oligonucleotides (APExBIO, 2024). The dye is stable as a solid (up to 24 months at −20°C in the dark) and requires organic solvents for dissolution due to low aqueous solubility. In vivo studies show rapid tumor uptake and persistent imaging signals, substantiating its utility for optical imaging of tumor models (Kang et al., 2025). This reagent is supplied by APExBIO and supports workflows in molecular biology, translational research, and preclinical imaging (see related content).

    Biological Rationale

    Optical imaging using near-infrared (NIR) dyes addresses the need for sensitive, deep-tissue visualization with minimal autofluorescence. The Cy5.5 NHS ester (non-sulfonated) dye is specifically engineered for labeling biomolecules containing primary amino groups, such as lysine residues in proteins or the 5’ end of oligonucleotides (Cy5.5 NHS Ester: Near-Infrared Fluorescent Dye for Biomol…). Its NIR excitation/emission profile (684/710 nm) enables imaging beyond the visible spectrum, improving tissue penetration and reducing background from endogenous fluorophores. This makes it particularly suited for in vivo imaging of processes such as tumor localization, metastasis, and biodistribution in animal models (Kang et al., 2025). The dye’s chemical stability as a solid (when protected from light and stored at −20°C) ensures reliable long-term use, while its reactivity as an NHS ester provides efficient labeling under mild conditions (APExBIO).

    Mechanism of Action of Cy5.5 NHS Ester (Non-Sulfonated)

    Cy5.5 NHS ester (non-sulfonated) operates via amide bond formation between its N-hydroxysuccinimide (NHS) activated ester group and primary amines on biomolecules. This reaction occurs optimally in slightly basic aqueous buffers (pH 7.5–8.5) and is typically performed at room temperature. The dye is first dissolved in anhydrous organic solvent (such as DMSO or DMF) due to its low aqueous solubility (solubility ≥35.82 mg/mL in DMSO). The labeling reaction proceeds rapidly, frequently reaching completion within 30–60 minutes. After conjugation, the product is purified to remove unreacted dye, yielding a fluorescently labeled biomolecule suitable for downstream applications (Cy5.5 NHS Ester (Non-Sulfonated): Benchmarking a Near-Inf…). The final conjugate maintains the NIR fluorescence properties of the parent dye, enabling high-sensitivity detection with excitation at 684 nm and emission at 710 nm. The extinction coefficient (209,000 M⁻¹cm⁻¹) and quantum yield (0.2) provide robust signal intensity for imaging and quantification (APExBIO).

    Evidence & Benchmarks

    • Cy5.5 NHS ester (non-sulfonated) enables specific, covalent labeling of proteins, peptides, and oligonucleotides through reaction with primary amino groups (APExBIO).
    • In vivo optical imaging studies demonstrate rapid tumor uptake of Cy5.5-labeled probes, peaking at 30 minutes post-injection, with signals persisting up to 24 hours in xenograft mouse models (Kang et al., 2025).
    • The dye’s excitation maximum (684 nm) and emission maximum (710 nm) are optimal for deep tissue imaging with low background fluorescence (Cy5.5 NHS Ester: Near-Infrared Fluorescent Dye for Biomol…).
    • Cy5.5 NHS ester is stable as a lyophilized solid for up to 24 months at −20°C in the dark, but is unstable in solution and must be freshly prepared before use (APExBIO).
    • The use of organic co-solvents (DMSO or DMF) is essential due to the dye’s low aqueous solubility, ensuring efficient conjugation and minimizing precipitation (Cy5.5 NHS Ester (Non-Sulfonated): Benchmarking a Near-Inf…).
    • In studies of tumor-associated bacteria and cancer metastasis, NIR-labeled probes such as Cy5.5 NHS facilitate sensitive detection and monitoring of biological processes in preclinical models (Kang et al., 2025).

    Applications, Limits & Misconceptions

    Cy5.5 NHS ester (non-sulfonated) is widely used for:

    • Labeling proteins, peptides, and oligonucleotides for molecular biology and translational research (Cy5.5 NHS Ester: Near-Infrared Fluorescent Dye for Biomol…).
    • Optical imaging of tumors, including subcutaneous xenografts, to assess biodistribution and tumor microenvironment (Kang et al., 2025).
    • Tracking labeled plasmid DNA in cellular and animal models.
    • Deep-tissue fluorescence imaging in vivo, leveraging NIR wavelengths for superior penetration and minimal autofluorescence (see related article — this article provides updated benchmarks and clarifies the stability profile in vivo).

    However, several misconceptions persist regarding its use. See the subsection below for clarification.

    Common Pitfalls or Misconceptions

    • Cy5.5 NHS ester (non-sulfonated) is not water-soluble; attempts to dissolve directly in aqueous buffers result in poor labeling efficiency or precipitation (APExBIO).
    • The dye is not stable in solution; solutions must be prepared freshly prior to use to avoid hydrolysis and loss of reactivity.
    • Non-sulfonated Cy5.5 NHS ester lacks the additional negative charge of sulfonated analogs, which may affect solubility and conjugate properties in some systems.
    • It is not suitable for direct use in live animal injections without prior conjugation to a targeting biomolecule.
    • Misinterpretation of its emission spectrum may lead to overlap with other NIR dyes; spectral compatibility should be verified prior to multiplexing (Optimizing In Vivo Fluorescence… — this article extends prior discussion by detailing spectral overlap considerations).

    Workflow Integration & Parameters

    For optimal results with Cy5.5 NHS ester (non-sulfonated), follow these workflow parameters:

    • Dissolve the dye in anhydrous DMSO or DMF (recommended concentration: 5–10 mM).
    • Add the dye solution to the target biomolecule in a buffered solution (pH 7.5–8.5, e.g., phosphate or bicarbonate buffer).
    • Incubate the reaction at room temperature for 30–60 minutes, protected from light.
    • Purge unreacted dye via size-exclusion chromatography or dialysis.
    • Store the labeled conjugate as recommended (usually at 4°C in the dark, with/without preservatives based on biomolecule type).

    For detailed troubleshooting and advanced applications, refer to Illuminating Translational Breakthroughs…, which expands on mechanistic insights and translational strategy not covered here.

    Conclusion & Outlook

    Cy5.5 NHS ester (non-sulfonated) from APExBIO is a validated reagent for high-sensitivity, near-infrared fluorescent labeling of amino-containing biomolecules. Its robust optical properties, stability as a solid, and established in vivo imaging benchmarks make it an asset for preclinical research in molecular biology and oncology. Ongoing advances in nanovaccine and microbiome-targeted cancer therapy underscore the value of reliable NIR probes for tracking complex biological processes (Kang et al., 2025). For product details, protocols, and ordering, see the Cy5.5 NHS ester (non-sulfonated) product page. This article clarifies best practices and addresses misconceptions, extending the evidence base established in prior reviews. Researchers are encouraged to leverage this reagent for reproducible, multiplexed, and translational imaging workflows.

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