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

Executive Summary: Cy5.5 NHS ester (non-sulfonated) is a near-infrared dye with peak excitation at 684 nm and emission at 710 nm, optimized for labeling amino-containing biomolecules (APExBIO product page). It demonstrates high extinction (209,000 M⁻¹cm⁻¹) and a moderate quantum yield (0.2), enabling sensitive detection in deep-tissue imaging (Kang et al., 2025). The reagent is stable as a solid at -20°C for up to 24 months, but unstable in solution. In vivo, Cy5.5 NHS ester-labeled probes facilitate optical tumor imaging, with maximal tumor uptake at 30 minutes and detectable fluorescence persisting for 24 hours (Kang et al., 2025). It is compatible with organic solvents such as DMF and DMSO but requires co-solvent systems due to low aqueous solubility (see deeper technical analysis).

Biological Rationale

Near-infrared (NIR) fluorescent dyes are essential in biomedical research for their ability to penetrate biological tissues with minimal autofluorescence and scattering. Cy5.5 NHS ester (non-sulfonated) targets primary amine groups on biomolecules, enabling covalent labeling of proteins, peptides, and oligonucleotides (APExBIO). This specificity supports quantitative and qualitative analyses in molecular biology, cell assays, and translational imaging workflows (Cy5.5 NHS Ester: Advanced Near-Infrared Dye for Biomolecule Labeling). In the context of tumor imaging, NIR dyes like Cy5.5 NHS ester provide high signal-to-noise ratios, facilitating the distinction of neoplastic tissue from healthy or inflamed regions, as validated in subcutaneous xenograft mouse models (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. This group selectively reacts with primary amines — commonly found on lysine residues in proteins or the 5' end of oligonucleotides — under mildly basic conditions (typically pH 7.5–8.5) (APExBIO). The reaction forms a stable amide bond, covalently attaching the Cy5.5 fluorophore to the biomolecule. Its non-sulfonated structure increases lipophilicity, which can influence solubility and membrane permeability. The conjugate emits NIR fluorescence (excitation: ~684 nm; emission: ~710 nm), suitable for deep-tissue imaging (see benchmarking).

Evidence & Benchmarks

• Cy5.5 NHS ester (non-sulfonated) achieves a molar extinction coefficient of 209,000 M⁻¹cm⁻¹ at 684 nm, supporting highly sensitive fluorescence detection (Kang et al., 2025; DOI).
• Quantum yield is measured at 0.2 under standard aqueous buffer conditions (pH 7.4, 25°C), balancing brightness and photostability (Kang et al., 2025; DOI).
• Upon intravenous injection in mice, Cy5.5-labeled probes show peak tumor accumulation at 30 minutes and detectable signal up to 24 hours post-injection (Kang et al., 2025; DOI).
• Solubility in DMSO is ≥35.82 mg/mL; in water, solubility is negligible, necessitating organic co-solvents for conjugation workflows (APExBIO).
• Product is stable as a dry solid for up to 24 months at -20°C, but decomposes rapidly in solution; thus, dissolution should occur only immediately before use (product review).

Applications, Limits & Misconceptions

Cy5.5 NHS ester (non-sulfonated) has broad utility in molecular biology, translational imaging, and in vivo tumor visualization:

• Protein and peptide labeling: Facilitates quantitative protein tracking, cell surface labeling, and receptor occupancy studies (Optimizing Cell Assays...). This article extends that guidance by detailing in vivo imaging parameters and limits.
• Oligonucleotide and DNA labeling: Enables sensitive hybridization assays, FISH, and gene delivery tracking (see applications summary).
• In vivo tumor imaging: Supports optical detection of subcutaneous and orthotopic tumors in preclinical models, validated in recent microbiome-tumor interaction studies (Kang et al., 2025). This dossier clarifies the temporal profile and signal persistence compared to prior reviews.

Common Pitfalls or Misconceptions

• Low aqueous solubility: Cy5.5 NHS ester (non-sulfonated) is not water-soluble; direct dissolution in buffers leads to precipitation and reaction failure.
• Solution instability: The dye decomposes in solution over hours; aliquots should be prepared and used immediately before conjugation.
• Non-selectivity for non-amino groups: Only reacts with primary amines; does not label thiols, carboxylates, or other side chains.
• Photobleaching: Moderate quantum yield and NIR emission reduce but do not eliminate photobleaching risk; samples should be protected from prolonged light exposure.
• Not for live animal systemic administration: While useful for imaging, unconjugated dye is not intended for systemic injection due to potential off-target effects.

Workflow Integration & Parameters

For optimal conjugation, dissolve Cy5.5 NHS ester (non-sulfonated) in anhydrous DMF or DMSO to a concentration of 10–20 mM immediately before use. Combine with the target biomolecule in a slightly basic aqueous buffer (pH 7.5–8.5, e.g., 0.1 M sodium bicarbonate) at 1:2 to 1:10 dye:protein molar ratio. Incubate at room temperature for 30–60 minutes, protecting from light. Purify conjugates via gel filtration or dialysis to remove unreacted dye. Storage of labeled products should occur at 4°C, protected from light.

For further mechanistic insight and workflow optimization, see this guide, which expands on bioconjugation challenges and advanced translational imaging strategies. This article updates those protocols with recent in vivo tumor imaging data and clarifies stability constraints.

Conclusion & Outlook

Cy5.5 NHS ester (non-sulfonated) from APExBIO combines high extinction, NIR emission, and robust NHS-ester chemistry, making it a reliable choice for deep-tissue imaging, protein and oligonucleotide labeling, and translational tumor research. Its validated performance in preclinical optical imaging — especially in studies targeting tumor-associated bacteria and microenvironment interactions — advances its role in precision oncology workflows (Kang et al., 2025). As new challenges arise in the integration of microbiome and tumor imaging, Cy5.5 NHS ester will remain a key tool for both fundamental research and translational applications. For ordering and further technical details, visit the APExBIO product page.