Cy5.5 NHS Ester (Non-Sulfonated): Atomic Evidence for Near-Infrared Amino Group Labeling

Executive Summary: Cy5.5 NHS ester (non-sulfonated) is a near-infrared (NIR) fluorescent dye engineered for covalent labeling of primary amines in biomolecules, enabling high-sensitivity optical imaging (APExBIO Product Page). The dye features an excitation maximum at 684 nm and emission at 710 nm, with an extinction coefficient of 209,000 M^-1cm^-1 and a quantum yield of 0.2 [Cy5.5 NHS Ester Benchmarks]. Cy5.5 NHS ester is highly soluble in DMSO (≥35.82 mg/mL), but poorly soluble in water, requiring organic co-solvents for use. The NHS ester reactive group enables robust, covalent conjugation to peptides, proteins, or oligonucleotides with exposed amines, forming stable amide bonds [Mechanism Deep Dive]. The reagent is extensively employed for sensitive in vivo tumor imaging, molecular labeling, and advanced flow cytometry, with proven stability for 24 months when stored at -20°C in the dark. These properties make Cy5.5 NHS ester (non-sulfonated) a gold standard for deep-tissue fluorescence labeling workflows (Kang et al., 2025, Sci. Adv.).

Biological Rationale

Near-infrared fluorescent dyes enable non-invasive imaging of biological tissues due to low tissue autofluorescence and deeper photon penetration at 684–710 nm wavelengths [In Vivo Imaging Review]. Cy5.5 NHS ester (non-sulfonated) allows for selective labeling of biomolecules containing primary amino groups, facilitating their optical detection in complex samples. In oncology research, such labeling is critical for visualizing tumor margins, tracking biomolecules, and monitoring therapeutic delivery in live animal models (Kang et al., 2025). The non-sulfonated form of Cy5.5 NHS ester maintains high hydrophobicity, ensuring strong interaction with hydrophobic domains in proteins and minimal background in aqueous systems.

Mechanism of Action of Cy5.5 NHS ester (non-sulfonated)

Cy5.5 NHS ester contains an N-hydroxysuccinimide (NHS) ester moiety that reacts specifically with primary amines (-NH₂) on lysine residues or N-termini of proteins, peptides, and oligonucleotides. The reaction typically proceeds in mildly basic aqueous buffers (pH 7.5–8.5) containing organic co-solvents (e.g., DMSO, DMF), forming a stable amide bond and covalently attaching the fluorescent Cy5.5 dye [Mechanism Deep Dive]. The resulting conjugate exhibits the dye’s characteristic excitation and emission properties, enabling sensitive detection. Unreacted NHS esters are hydrolyzed and inactivated in aqueous environments; therefore, freshly prepared dye solutions are recommended. The dye’s molecular formula is C₄₄H₄₆ClN₃O₄; molecular weight is 716.31 g/mol. The product is supplied as a stable solid and should be handled under low-light conditions to prevent photobleaching.

Evidence & Benchmarks

• Cy5.5 NHS ester (non-sulfonated) shows excitation at 684 nm and emission at 710 nm, supporting deep-tissue imaging with low biological background (APExBIO, product datasheet).
• The extinction coefficient is 209,000 M^-1cm^-1 and the quantum yield is 0.2, providing high signal intensity in optical imaging assays (Cy5.5 NHS Ester Benchmarks).
• Cy5.5 NHS ester-conjugated antibodies and proteins have enabled precise in vivo imaging of tumor xenografts and microbiome interactions in mouse models (Kang et al., 2025).
• The non-sulfonated form demonstrates superior labeling efficiency for proteins in organic-aqueous buffers due to enhanced solubility in DMSO (≥35.82 mg/mL) and DMF (Mechanism Deep Dive).
• Proper storage at -20°C in the dark maintains dye stability for at least 24 months as a solid; dye solutions lose activity rapidly and should be used immediately (Atomic Evidence Article).

Applications, Limits & Misconceptions

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

• Covalent labeling of proteins, peptides, and oligonucleotides for fluorescence detection in molecular biology workflows.
• Optical imaging of tumors and subcutaneous tissues in small animal models, enabling non-invasive visualization of tumor margins and metastasis (Kang et al., 2025).
• Fluorescent detection in western blotting, flow cytometry, and immunohistochemistry [Benchmarks].

This article clarifies and extends previous reports by highlighting the product’s validated spectral parameters, long-term solid-state stability, and its role in emerging microbiome-tumor imaging studies. For example, while "Cy5.5 NHS Ester: Near-Infrared Fluorescent Dye for Biomolecule Labeling" details its use in deep-tissue imaging, this article additionally documents its performance in microbiome-modulated tumor models. Similarly, "Cy5.5 NHS Ester (Non-Sulfonated): Transforming In Vivo Tumor Imaging" focuses on in vivo imaging, while this article emphasizes chemical handling and workflow pitfalls. For extended technical protocols, see "Cy5.5 NHS Ester (Non-Sulfonated): Benchmarks for Near-Infrared Fluorescence Imaging".

Common Pitfalls or Misconceptions

• Misconception: Cy5.5 NHS ester (non-sulfonated) is highly soluble in water. Correction: The dye is poorly soluble in aqueous buffers and must be dissolved in organic solvents such as DMSO or DMF before dilution.
• Pitfall: NHS ester solutions are stable for long-term storage. Correction: Stock solutions degrade rapidly; only freshly prepared solutions should be used for conjugation.
• Boundary: The dye efficiently labels only primary amines. Secondary amines and other nucleophiles do not react efficiently under standard conditions.
• Boundary: The dye is not recommended for live cell surface labeling without prior optimization, due to its hydrophobicity and potential for non-specific membrane interaction.
• Misconception: All Cy5.5 NHS esters are equivalent. Correction: Sulfonated and non-sulfonated forms differ in hydrophilicity, labeling efficiency, and background signal; selection must match experimental needs.

Workflow Integration & Parameters

For optimal labeling, Cy5.5 NHS ester (non-sulfonated) should be dissolved to ≥10 mM in anhydrous DMSO. Protein conjugation is typically performed at pH 7.5–8.5 in phosphate or bicarbonate buffer, with a 5–20-fold molar excess of dye over protein. The reaction proceeds for 0.5–2 h at room temperature, protected from light. Excess dye is removed by size-exclusion chromatography or dialysis. The final conjugate is quantified using absorbance at 684 nm (ε = 209,000 M^-1cm^-1). For in vivo imaging, the labeled probe is injected into animal models and imaged using NIR fluorescence systems.

Storage recommendations: the solid dye is stable for 24 months at -20°C, protected from light. Dye solutions must be used immediately after preparation and not stored for future use. For comprehensive workflow examples and troubleshooting guides, refer to the APExBIO Cy5.5 NHS ester (non-sulfonated) A8103 product page.

Conclusion & Outlook

Cy5.5 NHS ester (non-sulfonated) represents a rigorously validated, high-performance near-infrared fluorescent dye for covalent labeling of biomolecules with primary amines. Its proven efficacy in deep-tissue and in vivo imaging, including advanced tumor and microbiome research, positions it as a key reagent for next-generation molecular biology and translational research. Future directions include expanded use in multiplexed imaging, microbiome-modified tumor models, and integration with nanovaccine delivery systems (Kang et al., 2025).