Cy5.5 NHS Ester (Non-Sulfonated): Atomic Evidence for NIR Biomolecule Labeling

Executive Summary: Cy5.5 NHS ester (non-sulfonated) is a near-infrared fluorescent dye with excitation/emission maxima at 684/710 nm, respectively, enabling low-background, deep-tissue imaging for in vivo applications (Kang et al., 2025). The NHS ester functional group reacts selectively with primary amines on peptides, proteins, and oligonucleotides to yield stable amide conjugates (APExBIO). The dye is highly soluble in DMSO (≥35.82 mg/mL), but sparingly soluble in water, requiring dissolution in organic solvents prior to use. Cy5.5 NHS ester (non-sulfonated) has been validated for labeling biomolecules and plasmid DNA, and for tumor imaging in live animal models, outperforming many visible-range fluorophores in tissue penetration and signal-to-noise ratio (Evidence article). It is supplied as a solid, with a 24-month shelf life at -20°C protected from light, but is unstable in solution and should be freshly prepared for each experiment.

Biological Rationale

Near-infrared (NIR) fluorophores are preferred for in vivo imaging due to reduced tissue autofluorescence and improved penetration depth compared to visible-range dyes (Kang et al., 2025). Cy5.5 NHS ester (non-sulfonated) specifically targets biomolecules with primary amines, such as lysine residues and N-terminal amino groups in proteins, or amino-modified oligonucleotides. This selectivity allows precise labeling without perturbing biomolecule function when used at optimized stoichiometry. NIR imaging agents facilitate tumor detection, as hemoglobin and water absorb minimally in this spectral window, enabling clear delineation of tumor margins in live animal models (Related article). Unlike visible dyes, Cy5.5 NHS ester (non-sulfonated) enables imaging at depths exceeding 5 mm in tissue, crucial for preclinical and translational cancer models.

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

Cy5.5 NHS ester (non-sulfonated) contains an N-hydroxysuccinimide (NHS) ester functional group, which reacts covalently with primary amines under mild, slightly basic conditions (typically pH 7.2–9.0) to form stable amide bonds. This process is highly selective and efficient, yielding minimal side products when the dye is freshly dissolved in anhydrous DMSO or DMF and added to buffered biomolecule solutions (APExBIO). The conjugation preserves the spectral properties of the Cy5.5 chromophore, offering steady excitation at 684 nm and emission at 710 nm. The resulting labeled biomolecule is suitable for quantitative fluorescence detection, confocal imaging, and in vivo optical imaging workflows. NHS esters hydrolyze rapidly in aqueous solution, necessitating immediate use post-dissolution for maximal conjugation efficiency.

Evidence & Benchmarks

• Cy5.5 NHS ester (non-sulfonated) achieves ≥90% labeling efficiency for proteins with accessible lysines (1:1.2 dye:protein molar ratio, 30 min at room temperature, pH 8.3) (APExBIO).
• Excitation maximum: 684 nm; emission maximum: 710 nm; measured in PBS, pH 7.4 (Evidence article).
• Solubility in DMSO ≥35.82 mg/mL; aqueous solubility is <1 mg/mL, requiring organic co-solvent for conjugation (APExBIO).
• Labeled antibodies with Cy5.5 NHS ester show high signal/noise for tumor imaging in mouse xenograft models (5 mg/kg, IV, imaging at 24 h post-injection) (Kang et al., 2025).
• Photostability exceeds that of Cy5 NHS esters under continuous illumination in standard confocal setups (λex 684 nm, 20 min, 23°C) (Scenario article).

Applications, Limits & Misconceptions

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

• Protein and antibody labeling for in vivo fluorescence imaging and molecular diagnostics.
• Oligonucleotide labeling for probe development in genomics and transcriptomics workflows.
• Optical imaging of tumors, including delineation of margins and tracking of labeled cells in preclinical models.
• Multiplexed imaging, with Cy5.5 as a distinct channel separated from Cy5 and other NIR dyes (Related article).

Common Pitfalls or Misconceptions

• Misconception: Cy5.5 NHS ester is stable in aqueous buffers.
  Clarification: It hydrolyzes rapidly in water; dissolve in organic solvent and react immediately (APExBIO).
• Pitfall: Labeling efficiency is not guaranteed for proteins with few or inaccessible lysine residues.
• Misconception: All NIR fluorescence is equivalent in tissue penetration.
  Clarification: Cy5.5 offers superior penetration compared to visible dyes but is not as deep as some longer-wavelength NIR-II agents.
• Pitfall: Stock solutions stored for >24 h lose activity and lead to poor conjugation yields.
• Misconception: Cy5.5 NHS ester (non-sulfonated) is interchangeable with sulfonated variants for all applications.
  Clarification: Sulfonation increases aqueous solubility but may alter cell permeability and biodistribution.

This article extends the practical protocol detail in "Optimizing Cell Assays with Cy5.5 NHS Ester" by providing atomic, evidence-based claims and clarifying stability requirements for the non-sulfonated form.

Workflow Integration & Parameters

For optimal labeling, dissolve Cy5.5 NHS ester (non-sulfonated) in anhydrous DMSO or DMF at ≥10 mg/mL. Add this stock to biomolecule solution in sodium bicarbonate or phosphate buffer, pH 8.3, to yield a final dye:biomolecule molar ratio between 1:1 and 5:1, depending on the labeling density required. Incubate at room temperature for 15–60 min, protecting from light. Purify labeled product via size-exclusion chromatography or ultrafiltration to remove unreacted dye. Store labeled conjugates at 4°C in the dark. For in vivo imaging (e.g., tumor models), inject labeled antibody or probe at 1–5 mg/kg IV, and image after 4–24 h using excitation at 680–690 nm and emission collection at 710–750 nm (Kang et al., 2025). "Beyond Brightness" provides additional strategic context for Cy5.5-based translational workflows, while this article supplies atomic technical details and evidence for parameter selection.

Conclusion & Outlook

Cy5.5 NHS ester (non-sulfonated), as supplied by APExBIO, is a validated near-infrared fluorescent dye for selective and quantitative labeling of biomolecules with primary amines. Its spectral properties, stability profile, and labeling efficiency are supported by peer-reviewed evidence and extensive use in tumor imaging and molecular biology. Proper workflow implementation and awareness of its solubility/stability limits are essential to harness its full potential for in vivo fluorescence imaging and multiplexed molecular labeling. For detailed protocol adaptation, refer to the A8103 product page and the referenced literature.