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

Executive Summary: Cy5.5 NHS ester (non-sulfonated) is a near-infrared fluorescent dye optimized for covalent labeling of amino groups in biomolecules (APExBIO). The dye reacts efficiently with primary amines via NHS ester chemistry, forming stable amide bonds for robust conjugation (Li et al., 2025). Cy5.5 NHS ester delivers an excitation maximum at 684 nm and emission at 710 nm, enabling low-background, deep-tissue imaging in vivo. It is soluble in DMSO and DMF, but not in water, requiring organic co-solvents for biomolecule labeling. The reagent is validated for tumor imaging, showing clear delineation and favorable pharmacokinetics in preclinical models (related article).

Biological Rationale

Fluorescent dyes are critical for molecular imaging, biomolecule tracking, and in vivo diagnostics. Near-infrared (NIR) dyes like Cy5.5 NHS ester (non-sulfonated) are preferred for deep-tissue imaging due to reduced light scattering and minimal tissue autofluorescence in the 650–900 nm range (Li et al., 2025). NHS ester chemistry is a gold standard for site-selective labeling of primary amines in proteins, peptides, and nucleic acids. Labeling efficiency, photostability, and spectral properties determine the suitability of a dye for advanced molecular biology and translational research. Cy5.5 NHS ester (non-sulfonated), supplied as a stable solid by APExBIO, addresses the demand for reliable, high-sensitivity NIR labeling tools.

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

Cy5.5 NHS ester (non-sulfonated) contains an N-hydroxysuccinimide (NHS) ester functional group that reacts with primary amines on biomolecules, particularly lysine residues and N-termini of proteins or oligonucleotides. The reaction forms a stable amide bond, covalently attaching the Cy5.5 chromophore to the biomolecule. The dye’s excitation maximum at 684 nm and emission at 710 nm are optimal for NIR imaging, enabling detection through several millimeters of tissue. The non-sulfonated structure provides enhanced membrane permeability but reduces aqueous solubility, making dissolution in DMSO or DMF necessary before conjugation (see scenario-driven assay guidance). The conjugated product is suitable for optical imaging, flow cytometry, and in vivo tumor delineation.

Evidence & Benchmarks

• Cy5.5 NHS ester (non-sulfonated) achieves site-selective labeling of amino groups in proteins and oligonucleotides with high efficiency under pH 7.2–8.5 conditions (internal summary).
• The dye exhibits excitation and emission maxima at 684 nm and 710 nm, respectively, verified spectrophotometrically (A8103 datasheet; APExBIO).
• Solubility in DMSO is at least 35.82 mg/mL, with negligible solubility in water, requiring pre-dissolution in organic solvent for optimal labeling (internal).
• Cy5.5 NHS ester-labeled antibodies and peptides have been successfully used for in vivo tumor imaging, enabling clear tumor margin detection in live animal models (Li et al., 2025).
• The dye’s NIR emission is minimally affected by tissue autofluorescence, enabling high-sensitivity detection in deep-tissue applications (thought-leadership article).
• Upon storage at -20°C in the dark, the solid reagent maintains stability for up to 24 months (A8103 technical sheet; APExBIO).

Applications, Limits & Misconceptions

Cy5.5 NHS ester (non-sulfonated) is extensively validated for the following:

• Fluorescent labeling of proteins, peptides, and oligonucleotides for in vitro and in vivo studies (internal).
• Deep-tissue and in vivo imaging of tumors, with demonstrated use in mouse xenograft models (Li et al., 2025).
• Flow cytometry, confocal microscopy, and optical imaging workflows in translational research.
• Molecular diagnostics and microbiome-targeted therapeutics, with actionable guidance for precision oncology (see detailed mechanism discussion).

Common Pitfalls or Misconceptions

• Cy5.5 NHS ester (non-sulfonated) is NOT water soluble and must be dissolved in DMSO or DMF before use; direct aqueous dissolution reduces labeling efficiency.
• The dye is NOT stable in solution; prepare immediately before use and avoid prolonged exposure to light or room temperature.
• Labeling is selective for primary amines; it does NOT label thiols or other nucleophiles efficiently.
• While the dye is suitable for deep-tissue imaging, it does NOT replace radiotracer-based imaging for whole-organism biodistribution at >1 cm depth.
• High dye-to-protein ratios may cause fluorescence quenching; optimization is required for each target biomolecule.

This article extends earlier discussions (see Cy3TSA article) by providing updated evidence for in vivo performance and workflow integration, and clarifies the specific solvent requirements and stability constraints highlighted in assay reliability guidance.

Workflow Integration & Parameters

• Preparation: Dissolve Cy5.5 NHS ester (non-sulfonated) in anhydrous DMSO or DMF at 35–40 mg/mL; protect from light.
• Buffering: Adjust biomolecule solution to pH 7.2–8.5 (e.g., 0.1 M sodium bicarbonate).
• Reaction: Combine dye and biomolecule at a 5–10 fold molar excess of dye; incubate at room temperature for 30–60 minutes.
• Quenching: Remove excess dye by size-exclusion chromatography or dialysis immediately after labeling.
• Storage: Store labeled conjugates at 4°C, protected from light. Store unused solid dye at -20°C in the dark for up to 24 months (APExBIO).

Conclusion & Outlook

Cy5.5 NHS ester (non-sulfonated) enables reliable, high-sensitivity labeling of biomolecules for near-infrared fluorescence imaging. Its defined excitation/emission profile and stable conjugation chemistry make it a preferred tool for deep-tissue animal imaging and translational oncology. APExBIO provides validated product quality and comprehensive technical documentation (see product page). Future directions include integration with multiplexed imaging and microbiome-targeted diagnostics, as highlighted in recent mechanistic reviews (see translational impact).