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

Executive Summary: Cy5.5 NHS ester (non-sulfonated) is a near-infrared fluorescent dye designed for labeling biomolecules containing primary amines, including proteins, peptides, and oligonucleotides (APExBIO). The dye features an excitation maximum at 684 nm and emission maximum at 710 nm, supporting deep tissue and in vivo imaging with minimal background autofluorescence (Cy5.5 NHS Ester: Atomic Evidence). NHS ester chemistry allows for efficient, stable amide bond formation with amino groups, providing robust and reproducible labeling (Mechanism, Applications & Evidence). The reagent is soluble in DMF and DMSO (≥35.82 mg/mL in DMSO), but requires organic co-solvents for use in aqueous labeling reactions. Stability is ensured for up to 24 months at -20°C (protected from light), enabling reliable storage and workflow integration (Li et al., 2025).

Biological Rationale

Fluorescent labeling of biomolecules enables visualization, quantification, and tracking in complex biological systems. Near-infrared (NIR) dyes, such as Cy5.5 NHS ester (non-sulfonated), are optimized for reduced background interference, as biological tissues exhibit minimal autofluorescence above 650 nm (Transforming In Vivo Tumor Imaging). This spectral property is critical for in vivo imaging, including tumor localization and tracking of labeled cells or drugs (Advanced NIR Dye for Biomolecule Labeling). Amino group labeling allows for site-specific conjugation to lysine residues in proteins or amine-modified oligonucleotides, preserving biological function and enabling multiplexed detection schemes (APExBIO). The stable amide bond formed via NHS ester chemistry ensures that the fluorescent label remains covalently attached during downstream applications, including in vivo studies and harsh sample processing.

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

Cy5.5 NHS ester (non-sulfonated) employs N-hydroxysuccinimide (NHS) ester chemistry for selective reaction with primary amines. The NHS ester moiety undergoes nucleophilic substitution with free amino groups (e.g., lysine side chains, N-termini), forming a stable covalent amide bond and releasing N-hydroxysuccinimide as a byproduct (Mechanism, Applications & Evidence). This reaction is typically performed in mildly basic, aqueous buffers (pH 7.2–8.5) in the presence of a suitable organic co-solvent to enhance dye solubility. Cy5.5's spectral properties—excitation at 684 nm, emission at 710 nm—are retained upon conjugation, providing a robust, high-signal readout for imaging (Atomic Evidence for NIR Imaging).

Evidence & Benchmarks

• Cy5.5 NHS ester (non-sulfonated) enables covalent labeling of proteins, peptides, and oligonucleotides at primary amine positions, forming stable amide bonds (APExBIO, product page).
• The dye exhibits excitation and emission maxima at 684 nm and 710 nm, respectively, supporting deep tissue imaging with minimal autofluorescence in live animal models (Atomic Evidence).
• Solubility in DMSO is at least 35.82 mg/mL; aqueous solubility is low, necessitating use of organic co-solvents for labeling reactions (APExBIO).
• Stability of the solid dye is up to 24 months at -20°C, protected from light; solutions are unstable and should be prepared fresh before use (Mechanism & Workflow).
• In vivo tumor imaging with Cy5.5-labeled antibodies or nanoparticles yields high tumor-to-background ratios and clear tumor delineation (Li et al., 2025, Figure 3).
• Labeling efficiency and photostability benchmarks are consistent with other near-infrared cyanine dyes, supporting multiplexed imaging and long-term tracking (Advanced NIR Dye).

Applications, Limits & Misconceptions

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

• Fluorescent labeling of antibodies, proteins, and peptides for in vivo imaging and flow cytometry.
• Oligonucleotide and plasmid DNA labeling for molecular tracking studies.
• Optical imaging of tumors and live animal models, exploiting its NIR fluorescence for deep tissue penetration (Transforming Tumor Imaging).
• Multiplexed molecular imaging, given its spectral separation from visible fluorophores.

This article extends prior coverage by providing atomic, benchmarked evidence for labeling chemistry and workflow integration, whereas prior reviews have focused on protocol selection (Optimizing Cell-Based Assays), mechanism (Mechanism, Applications & Evidence), or translational imaging (Advanced NIR Dye for Biomolecule Labeling).

Common Pitfalls or Misconceptions

• Cy5.5 NHS ester (non-sulfonated) is not water-soluble; dissolving directly in aqueous buffer leads to poor labeling efficiency.
• The dye should not be stored in solution; it degrades rapidly and loses activity—always prepare fresh solutions immediately before use.
• NHS ester reactivity is limited to primary amines; secondary amines or other functional groups are not efficiently labeled.
• The dye is not suited for live-cell internal labeling without membrane-permeabilizing protocols, as the NHS ester is cell-impermeant.
• Photobleaching can occur under intense or prolonged illumination; minimize light exposure during storage and handling.

Workflow Integration & Parameters

For optimal labeling, Cy5.5 NHS ester (non-sulfonated) should be dissolved in DMSO or DMF to a concentration of 10–35 mg/mL immediately before use (A8103 kit). Add the dye solution to the biomolecule in a mildly basic buffer (pH 7.5–8.5, e.g., phosphate or bicarbonate) at a typical dye:protein molar ratio of 3–10:1. Incubate for 30–60 minutes at room temperature, protected from light. Remove excess dye by gel filtration, dialysis, or spin columns. Conjugates can be used directly for in vivo imaging, flow cytometry, or other analytical applications. The solid dye is stable for 24 months at -20°C in the dark; avoid repeated freeze-thaw cycles. Always reference the manufacturer's protocol for precise conditions.

Conclusion & Outlook

Cy5.5 NHS ester (non-sulfonated) is a validated near-infrared fluorescent labeling reagent for proteins, peptides, and oligonucleotides. Its robust amine selectivity and deep tissue imaging capacity make it a preferred choice for in vivo and translational research workflows. Emerging applications in multiplexed imaging and targeted drug delivery are supported by its unique spectral and chemical properties (Li et al., 2025). For further details and up-to-date protocols, consult APExBIO's Cy5.5 NHS ester (non-sulfonated) product page.