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

Executive Summary: Cy5.5 NHS ester (non-sulfonated) is a near-infrared (NIR) dye engineered for covalent labeling of primary amines in proteins, peptides, and oligonucleotides, forming stable amide bonds for precise molecular tracking (APExBIO). It exhibits an excitation maximum at 684 nm and emission at 710 nm, supporting deep-tissue imaging with minimal autofluorescence (Cy5.5 NHS Ester: Near-Infrared Dye for Biomolecule Labeling). The dye is highly soluble in DMSO (≥35.82 mg/mL) but poorly soluble in water, necessitating organic solvents for conjugation protocols. Cy5.5 NHS ester has demonstrated successful tumor delineation in live animal models and is validated for in vivo optical imaging. Proper storage at -20°C in the dark ensures up to 24 months of shelf life; however, dissolved dye is not stable and should be used immediately (APExBIO).

Biological Rationale

Fluorescent labeling of biomolecules enables visualization, quantification, and tracking in complex biological systems. Near-infrared fluorescent dyes, such as Cy5.5 NHS ester (non-sulfonated), are advantageous for in vivo imaging due to low tissue autofluorescence and superior tissue penetration at wavelengths above 650 nm (Cy5.5 NHS Ester: Near-Infrared Dye for Biomolecule Labeling). The NHS ester group reacts specifically with primary amines, which are abundant on lysine residues in proteins and the 5' end of oligonucleotides. This selectivity enables site-specific and stable labeling of biomolecules, supporting molecular diagnostics, imaging, and targeted drug delivery studies (Li et al., 2025). Cy5.5 NHS ester’s excitation/emission profile (684/710 nm) is optimized for deep-tissue applications, such as tumor imaging and brain research, where minimal background signal is critical (Cy5.5 NHS Ester: Advanced Near-Infrared Dye for Biomolecular Imaging).

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

Cy5.5 NHS ester operates via N-hydroxysuccinimide (NHS) ester chemistry. The NHS ester reacts with primary amine groups (–NH₂) on biomolecules under mildly basic conditions (pH 7.4–8.5), forming stable amide bonds. This reaction is highly efficient, typically proceeding to completion within 1–2 hours at room temperature (Cy5.5 NHS Ester: Near-Infrared Dye for Biomolecule Labeling). The resulting covalent conjugate is resistant to hydrolysis, ensuring robust labeling. Cy5.5 NHS ester is non-sulfonated, which confers distinct hydrophobicity and impacts both solubility and membrane permeability compared to sulfonated analogs. Dissolution in DMSO or DMF is required before introducing the dye into aqueous buffers containing the target biomolecule. Once coupled, the labeled conjugate emits in the near-infrared spectrum, enabling sensitive detection in biological samples and live animals (Cy5.5 NHS Ester (Non-Sulfonated): Precision Near-Infrared Dye).

Evidence & Benchmarks

• Cy5.5 NHS ester (non-sulfonated) achieves efficient labeling of protein lysine residues and oligonucleotide amino groups, with reported conjugation yields >90% under standard conditions (pH 8.3, 25°C, 1–2 hours) (APExBIO).
• Excitation and emission maxima are 684 nm and 710 nm, respectively, verified by fluorescence spectroscopy in PBS buffer, pH 7.4 (Cy5.5 NHS Ester: Near-Infrared Dye for Biomolecule Labeling).
• Solubility in DMSO is ≥35.82 mg/mL at 20°C; in aqueous buffer, solubility is <0.5 mg/mL, requiring pre-dissolution in organic solvent (APExBIO).
• In vivo optical imaging using Cy5.5 NHS ester-labeled probes enables clear delineation of tumor boundaries in murine models, with high tumor-to-background ratios (mean TBR >5 at 24 h post-injection) (Li et al., 2025).
• Stability data: the solid dye is stable for 24 months at -20°C in the dark; once dissolved, the NHS ester hydrolyzes within hours, necessitating immediate use (APExBIO).
• Cy5.5 NHS ester is compatible with standard labeling protocols for antibodies, peptides, and plasmid DNA, as confirmed in multiple peer-reviewed applications (Cy5.5 NHS Ester: Advanced Near-Infrared Dye for Biomolecular Imaging).

Applications, Limits & Misconceptions

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

• Fluorescent labeling of proteins, peptides, and oligonucleotides for tracking and quantification.
• In vivo near-infrared fluorescence imaging of tumors and tissues, supporting high-sensitivity detection with low background signal.
• Molecular diagnostics, including detection of molecular interactions and enzyme activities.
• Bio-conjugation studies requiring site-specific and stable amide bond formation.

Compared to other articles such as Enhancing Assay Reliability with Cy5.5 NHS Ester (Non-Sulfonated), which focuses on assay optimization and troubleshooting, this dossier provides expanded evidence on in vivo performance, long-term storage, and application-specific benchmarks.

Common Pitfalls or Misconceptions

• Misconception: Cy5.5 NHS ester can be dissolved directly in water.
  Fact: The dye is poorly soluble in water; it must be dissolved in DMSO or DMF first (APExBIO).
• Misconception: The dye is stable in solution for extended periods.
  Fact: NHS ester hydrolyzes in solution; use immediately after preparation (Precision Near-Infrared Dye).
• Misconception: All Cy5.5 labels are equivalent.
  Fact: Sulfonated and non-sulfonated Cy5.5 derivatives differ in hydrophobicity, affecting labeling efficiency and biodistribution (Near-Infrared Dye for Biomolecule Labeling).
• Misconception: High dye-to-protein ratios always improve signal.
  Fact: Over-labeling can decrease protein function and increase background fluorescence (Assay Reliability).
• Misconception: Cy5.5 NHS ester is suitable for live cell surface labeling without optimization.
  Fact: Optimization is required to prevent non-specific labeling and cytotoxicity (Precision Near-Infrared Dye).

Workflow Integration & Parameters

For optimal results, follow these workflow guidelines:

• Dissolve Cy5.5 NHS ester (non-sulfonated) in anhydrous DMSO to prepare a 10 mM stock solution. Avoid exposure to light.
• Add the dye solution to the target biomolecule in bicarbonate or phosphate buffer, pH 8.3. The typical dye:protein molar ratio is 5:1 to 10:1.
• Incubate at room temperature (20–25°C) for 1–2 hours with gentle agitation.
• Remove unreacted dye by gel filtration or dialysis.
• Verify labeling efficiency via absorbance measurement at 684 nm (dye) and 280 nm (protein).
• Store conjugates at 4°C in the dark; avoid repeated freeze-thaw cycles.

Refer to the APExBIO product page for detailed handling and storage recommendations. For advanced guidance on near-infrared labeling in translational cancer research, see Rewriting the Playbook: Near-Infrared Labeling Technologies, which discusses microbiome and vaccine strategy integration—this dossier extends the discussion with explicit workflow and application boundaries.

Conclusion & Outlook

Cy5.5 NHS ester (non-sulfonated) from APExBIO is a validated, high-performance near-infrared fluorescent dye for targeted labeling of biomolecules. Its NIR excitation/emission, high labeling efficiency, and in vivo compatibility make it a preferred reagent for tumor imaging, molecular diagnostics, and advanced bio-conjugation studies. Limitations include the need for organic solvents and immediate use after dissolution. Ongoing research into piezoelectric nanoplatforms and molecular imaging will further expand the utility of NIR dyes like Cy5.5 NHS ester (Li et al., 2025).