Cy5.5 NHS Ester (Non-Sulfonated): Precision Near-Infrared Dye for Protein and Tumor Imaging

Executive Summary: Cy5.5 NHS ester (non-sulfonated) is a near-infrared dye with excitation at 684 nm and emission at 710 nm, used for labeling amino groups in biomolecules (ApexBio). The NHS ester group reacts specifically with primary amines, forming stable amide bonds for high-efficiency conjugation (Kang et al., 2025). This dye is ideal for deep-tissue and in vivo imaging due to its reduced background autofluorescence (Kang et al., 2025). Cy5.5 NHS ester is stable as a solid at -20°C for at least 24 months, but labile in solution, requiring immediate use after dissolution (ApexBio). Its use has been validated in tumor imaging models, showing clear tumor margins and favorable pharmacokinetics (Kang et al., 2025).

Biological Rationale

Near-infrared (NIR) fluorescent dyes are essential for non-invasive imaging in biological research. The NIR window (650–900 nm) minimizes tissue autofluorescence and light scattering, enabling deeper tissue penetration and higher signal-to-noise ratios compared to visible light fluorophores (Kang et al., 2025). Cy5.5 NHS ester is engineered for effective labeling of peptides, proteins, and oligonucleotides that possess primary amine groups. This specificity is advantageous for tracking biomolecules in complex biological environments such as tumors or live animal models. Labeling with NIR dyes is especially valuable in oncology, where clear tumor delineation and low background are critical for diagnostic and therapeutic research. Recent findings highlight the importance of imaging tumor-associated bacteria and microenvironments to study metastasis and therapy responses (Kang et al., 2025).

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

Cy5.5 NHS ester (non-sulfonated) contains an N-hydroxysuccinimide (NHS) ester reactive group. NHS esters react efficiently with primary amines (commonly present on lysine residues in proteins or at the 5' end of oligonucleotides) in mild, aqueous buffers (typically pH 7.2–8.5), forming stable amide bonds (ApexBio). The Cy5.5 chromophore absorbs maximally at 684 nm and emits at 710 nm, positioning it in the NIR region. This spectral property allows for deep-tissue penetration and minimal background interference. The non-sulfonated form is highly soluble in organic solvents such as DMSO (≥35.82 mg/mL) and DMF but has limited solubility in aqueous solutions, necessitating dissolution in organic co-solvents before mixing with biomolecule buffers. Once conjugated, the resulting labeled biomolecules can be detected with high sensitivity using NIR imaging platforms (Kang et al., 2025).

Evidence & Benchmarks

• Cy5.5 NHS ester (non-sulfonated) demonstrates robust, covalent labeling of primary amines in proteins and oligonucleotides, producing stable amide linkages (ApexBio, product page).
• Labeled biomolecules retain NIR fluorescence with excitation at 684 nm, emission at 710 nm, and low background autofluorescence in tissue (Kang et al., 2025, DOI).
• In live animal models, Cy5.5-labeled probes have enabled high-contrast optical imaging of tumors, allowing precise tumor margin delineation (Kang et al., 2025, DOI).
• The product is stable as a solid at -20°C for 24 months in the dark but degrades rapidly in solution, requiring immediate use after reconstitution (ApexBio, product page).
• Cy5.5 conjugation has been applied to label and track both proteins and plasmid DNA for molecular biology and imaging studies (Kang et al., 2025, DOI).

Applications, Limits & Misconceptions

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

• Fluorescent labeling of proteins, peptides, and oligonucleotides for in vitro and in vivo studies.
• Deep-tissue and live animal imaging, particularly for tumor visualization and pharmacokinetics.
• Bio-conjugation in molecular biology workflows, including immunofluorescence and probe generation.

Compared to alternative NIR dyes, Cy5.5 NHS ester offers distinct excitation/emission maxima, favoring certain imaging systems (ApexBio). For a comparison with sulfonated Cy5.5 derivatives or other NIR dyes, see our article on Cy7 NHS ester, which discusses the trade-offs between emission wavelength, aqueous solubility, and labeling efficiency. This article extends the Cy7 NHS ester analysis by focusing on the non-sulfonated Cy5.5's performance in protein conjugation and tissue imaging.

Common Pitfalls or Misconceptions

• Cy5.5 NHS ester (non-sulfonated) is not suitable for direct use in aqueous buffers without organic co-solvent due to low water solubility.
• Labeled biomolecules should not be stored for extended periods; the dye is unstable in solution.
• This reagent cannot specifically label thiol or carboxyl groups—only primary amines.
• Overexposure to light can lead to photobleaching and reduced signal.
• It is not suitable for applications requiring ultra-high aqueous solubility; consider sulfonated variants instead.

Workflow Integration & Parameters

For optimal use, dissolve Cy5.5 NHS ester (non-sulfonated) in DMSO or DMF to at least 35.82 mg/mL. Add the dye solution to the biomolecule in an aqueous buffer (pH 7.2–8.5, typically PBS or carbonate), maintaining an organic solvent content below 10% v/v. React for 30–60 min at room temperature, protected from light. Remove unreacted dye by size-exclusion chromatography or ultrafiltration. Store labeled biomolecules at 4°C, protected from light, and use promptly. For more details, see the A8103 kit protocol.

Conclusion & Outlook

Cy5.5 NHS ester (non-sulfonated) provides high-efficiency, specific labeling of primary amines for NIR imaging. Its robust performance in tumor imaging and biomolecule tracking is well-validated. Upcoming research will likely expand its application to more complex in vivo models, including simultaneous imaging of multiple targets. For further reading, see the recent advances in Cy7 NHS ester labeling, which this article updates by focusing on Cy5.5's excitation/emission features and workflow compatibility.