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

Executive Summary: Cy5.5 NHS ester (non-sulfonated) is a near-infrared fluorescent dye designed for covalent labeling of biomolecules containing primary amino groups, using NHS ester chemistry to form stable amide bonds [APExBIO, product A8103]. The dye features an excitation maximum at 684 nm and emission at 710 nm, supporting deep tissue imaging with reduced autofluorescence [Cy5TSA.com]. It is soluble in DMF and DMSO (≥35.82 mg/mL in DMSO) but requires organic co-solvent dissolution prior to aqueous conjugation. Cy5.5 NHS ester is validated for labeling peptides, proteins, and oligonucleotides, and is used in optical imaging of tumors in live animal models [Li et al., 2025]. Proper storage at -20°C in the dark ensures 24-month stability in solid form, while solutions must be freshly prepared before use.

Biological Rationale

Fluorescent labeling is fundamental to molecular biology, enabling visualization and quantification of biological processes. Near-infrared (NIR) dyes like Cy5.5 NHS ester (non-sulfonated) are favored for deep tissue and in vivo imaging due to minimal tissue autofluorescence, low light scattering, and high signal-to-noise ratios [Cy5.5 NHS Ester Guide]. NHS ester chemistry allows for selective, covalent labeling of primary amines on proteins, peptides, and oligonucleotides, preserving biomolecule function and enabling stable long-term imaging. The non-sulfonated form offers improved hydrophobicity, which can enhance membrane permeability in certain applications.

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

Cy5.5 NHS ester (non-sulfonated) reacts with unprotonated primary amines (typically lysine residues or N-termini) at physiological pH (7.2–8.5). The NHS (N-hydroxysuccinimide) ester moiety undergoes nucleophilic substitution by the amine, yielding a stable amide bond and covalently attaching the Cy5.5 fluorophore to the target molecule [Optimizing Biomolecule Labeling]. The reaction is efficient in organic/aqueous mixtures, typically using DMF or DMSO to dissolve the dye, followed by dilution into buffered solutions for conjugation. The resulting conjugate exhibits an excitation maximum at 684 nm and emission maximum at 710 nm, placing it in the NIR spectral window optimal for in vivo fluorescence imaging [APExBIO].

Evidence & Benchmarks

• Cy5.5 NHS ester (non-sulfonated) enables covalent labeling of primary amines in proteins, peptides, and oligonucleotides with typical yields >90% under optimized conditions (APExBIO, product page).
• The dye exhibits excitation/emission maxima at 684/710 nm, ideal for minimizing background in deep tissue imaging (Cy5TSA.com, article).
• In animal models, Cy5.5 NHS ester-labeled probes enable clear tumor delineation with high tumor-to-background ratios in optical imaging (Li et al., 2025, DOI:10.1002/adfm.202518001).
• Solubility is ≥35.82 mg/mL in DMSO, but <1 mg/mL in aqueous solutions, necessitating use of organic co-solvents for successful labeling (APExBIO, product page).
• Proper storage at -20°C in the dark keeps the solid dye stable for up to 24 months; dissolved dye rapidly hydrolyzes and must be used immediately (Optimizing Biomolecule Labeling, article).

Applications, Limits & Misconceptions

Main applications:

• Fluorescent labeling of proteins, peptides, and oligonucleotides for molecular biology workflows.
• Optical imaging of tumors and tissues in vivo, leveraging NIR fluorescence for high-contrast, deep-tissue visualization.
• Bio-conjugation studies, especially where stable amide bonds and minimal background fluorescence are required.
• Cell-based assays (viability, proliferation, cytotoxicity) where deep imaging and high specificity are needed [Optimizing Cell Assays].

This article extends previous guides by presenting new evidence from live animal tumor imaging and highlighting workflow integration parameters, such as solvent compatibility and storage best practices, not detailed in Cy5.5 NHS Ester: Advanced Near-Infrared Fluorescent Dye or Enabling Next-Gen In Vivo Imaging.

Common Pitfalls or Misconceptions

• Low aqueous solubility: Attempting to dissolve Cy5.5 NHS ester (non-sulfonated) directly in water results in poor labeling efficiency; always use DMF or DMSO as a stock solvent.
• Pre-made solutions are unstable: The dye hydrolyzes in solution; prepare fresh aliquots immediately before use.
• Light sensitivity: Extended exposure to light degrades the dye; store and handle in the dark.
• Non-reactivity with non-primary amines: Only primary amines (not secondary or tertiary) react efficiently under standard conditions.
• Not suitable for live-cell surface labeling without further modification: The non-sulfonated form is less water-soluble and may require protocol adjustments for cell-surface applications.

Workflow Integration & Parameters

To use Cy5.5 NHS ester (non-sulfonated):

• Dissolve the dye in dry DMSO or DMF at up to 35.82 mg/mL just prior to use.
• Prepare biomolecule in a suitable buffer (e.g., 0.1 M sodium bicarbonate, pH 8.3) to ensure primary amines are unprotonated.
• Add the dye solution to the biomolecule under gentle mixing, typically at a dye:protein molar ratio of 1:1 to 20:1, depending on labeling density desired.
• Incubate at room temperature (20–25°C) for 30–120 minutes in the dark.
• Remove excess dye using desalting columns or dialysis.
• Analyze conjugation efficiency by UV-Vis or fluorescence spectroscopy (absorption at 684 nm, emission at 710 nm).
• Store labeled conjugates at 4°C in the dark for short-term use (days to weeks) or at -20°C for long-term storage.

For advanced protocol optimization, see Optimizing Biomolecule Labeling, which details troubleshooting and workflow fidelity. This article extends those recommendations by emphasizing solvent compatibility, labeling yields, and imaging benchmarks in translational research.

Conclusion & Outlook

Cy5.5 NHS ester (non-sulfonated) from APExBIO (SKU A8103) provides a robust, high-specificity option for covalent labeling of primary amine-containing biomolecules. Its NIR spectral properties enable deep tissue, low-background imaging in both basic and translational research. Recent preclinical benchmarks, including successful tumor delineation in animal models, validate its role in advanced optical imaging pipelines [Li et al., 2025]. Ongoing workflow improvements and best practice guidelines continue to expand its utility across in vivo, ex vivo, and in vitro applications. For more detailed cell assay optimization, see this guide which this article extends by integrating new imaging and conjugation data.