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

Executive Summary: Cy5.5 NHS ester (non-sulfonated) is a near-infrared (NIR) fluorescent dye designed for amine-reactive labeling of peptides, proteins, and nucleic acids (APExBIO). The dye exhibits an excitation maximum at 684 nm and emission at 710 nm, supporting deep tissue optical imaging and minimal background fluorescence. It is highly soluble in organic solvents such as DMSO (≥35.82 mg/mL) but poorly soluble in water, necessitating use of co-solvents for conjugation workflows. Cy5.5 NHS ester labeling enables quantitative assessment of biomolecule localization and tumor imaging, with in vivo studies reporting maximal tumor uptake at 30 minutes post-injection and persistence up to 24 hours (Li et al., 2025). The product is stable for 24 months at -20°C in the dark and is widely used in advanced molecular biology and translational oncology workflows.

Biological Rationale

Cy5.5 NHS ester (non-sulfonated) is used to label biomolecules containing primary amines, a functional group abundant in lysine residues and N-termini of proteins, as well as in modified oligonucleotides. NIR dyes like Cy5.5 minimize tissue autofluorescence and allow detection through several centimeters of tissue due to low absorption and scattering in the 650–900 nm window (APExBIO). This property is critical for non-invasive imaging in live animal models, including tumor xenografts, where high signal-to-noise ratios are required for accurate localization. The NHS (N-hydroxysuccinimide) ester moiety targets amine groups, forming stable amide bonds under mild aqueous buffer conditions (pH 7.2–8.5) with organic co-solvent. This chemistry is widely adopted for labeling antibodies, enzymes, and synthetic peptides for diagnostic and mechanistic studies (contrast: this article updates best-practice conjugation and in vivo imaging guidance).

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

The Cy5.5 NHS ester reacts with primary amines to form covalent amide bonds, irreversibly attaching the fluorescent dye to the biomolecule. Upon excitation at 684 nm, the dye emits at 710 nm, enabling detection with NIR imaging platforms. The non-sulfonated form is neutral in charge, which may affect its cell permeability and biodistribution compared to sulfonated analogs. The high extinction coefficient (209,000 M⁻¹cm⁻¹) ensures strong absorbance and brightness, while the moderate quantum yield (0.2) balances signal intensity with photostability. The resulting conjugates retain biological activity and can be tracked in living systems. In tumor imaging, Cy5.5-labeled probes accumulate at the target site, providing high-contrast images for up to 24 hours post-injection (Li et al., 2025).

Evidence & Benchmarks

• Cy5.5 NHS ester (non-sulfonated) exhibits excitation at 684 nm and emission at 710 nm in PBS, enabling NIR detection through deep tissues (APExBIO).
• The dye is highly soluble in DMSO (≥35.82 mg/mL), allowing preparation of concentrated stock solutions for labeling reactions (APExBIO).
• In vivo tumor imaging studies show maximal uptake at 30 minutes and persistent signal for 24 hours in xenograft models (Li et al., 2025, see Fig. 4A).
• Labeling efficiency is high under standard conditions (protein, 20–100 µg/mL, pH 8.3, 1–2 h, RT), with minimal loss of protein function (see: cell assay optimization details).
• The solid product is stable for 24 months at -20°C when protected from light, but unstable in solution and should be freshly prepared (APExBIO).
• In the context of neuromodulation studies, NIR dyes including Cy5.5 are compatible with piezoelectric nanoplatforms for real-time in vivo imaging (Li et al., 2025).

Applications, Limits & Misconceptions

Cy5.5 NHS ester (non-sulfonated) is broadly applied in:

• Fluorescent labeling of proteins, peptides, and oligonucleotides for tracking and quantification (see: advanced molecular imaging applications).
• In vivo tumor imaging and biodistribution studies, especially for xenograft and syngeneic mouse models (contrast: this article focuses on tumor microbiome imaging, this article focuses on dye properties and workflow).
• Multiplexed cell-based assays, viability, and cytotoxicity workflows (cell assay optimization).
• Real-time imaging in biomimetic neuromodulation platforms (Li et al., 2025).
• Labeling of plasmid DNA for tracking delivery and expression (APExBIO).

Common Pitfalls or Misconceptions

• Not water-soluble: Cy5.5 NHS ester (non-sulfonated) requires organic solvents (DMSO, DMF) for dissolution; direct aqueous use leads to precipitation and inefficient conjugation (APExBIO).
• Not stable in solution: Dye solutions hydrolyze rapidly; always prepare fresh stocks just before use (APExBIO).
• Non-selective amine reactivity: The NHS ester reacts with all accessible primary amines; site-specific labeling requires alternative chemistries (see: strategic deployment discussion).
• Non-sulfonated variant: The non-sulfonated form is less hydrophilic than sulfonated analogs and may display altered pharmacokinetics (APExBIO).
• Not suitable for live-cell imaging without conjugation: Free Cy5.5 NHS ester is membrane-impermeant and cytotoxic; only labeled biomolecules are generally used in cell-based imaging.

Workflow Integration & Parameters

For optimal labeling, dissolve Cy5.5 NHS ester (non-sulfonated) in anhydrous DMSO or DMF at concentrations up to 35.82 mg/mL. Add the dye to the target biomolecule in a pH 8.3 buffer (e.g., 0.1 M sodium bicarbonate) at a typical dye-to-protein molar ratio of 5:1. Incubate at room temperature for 1–2 hours, protected from light. Remove unreacted dye by gel filtration or dialysis. Confirm labeling efficiency by absorbance at 684 nm. Store labeled products at 4°C, protected from light. For in vivo imaging, inject labeled probes intravenously and image at 30 minutes to 24 hours post-injection. The A8103 kit from APExBIO provides quality-controlled solid dye, ensuring reproducibility (APExBIO).

Conclusion & Outlook

Cy5.5 NHS ester (non-sulfonated) is a validated, high-sensitivity tool for in vivo fluorescence imaging, amine-reactive labeling, and multiplexed molecular workflows. Its NIR properties enable deep-tissue optical imaging with low background, while robust conjugation protocols support reproducible results. Researchers can further explore translational imaging strategies and advanced neuromodulation platforms by integrating Cy5.5-based probes (Li et al., 2025). For detailed guidance on emerging applications and troubleshooting, see extended analyses on tumor microbiome imaging and cell assay optimization. APExBIO continues to provide rigorously validated reagents for next-generation biomedical research.