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

Executive Summary: Cy5.5 NHS ester (non-sulfonated) is a near-infrared dye optimized for labeling primary amines in peptides, proteins, and oligonucleotides, using stable NHS ester chemistry (APExBIO, A8103). It provides an excitation maximum at 684 nm and emission at 710 nm, supporting deep-tissue and in vivo fluorescence imaging with low autofluorescence background (see internal review). The dye is highly soluble in DMSO (≥35.82 mg/mL), but must be dissolved in anhydrous organic solvent before conjugation. Cy5.5 NHS ester enables robust tumor delineation and pharmacokinetic studies in live animal models (Kang et al., 2025). Proper storage (-20°C, dark) is required to maintain reagent stability for up to 24 months.

Biological Rationale

Fluorescent labeling is essential for visualizing biomolecules in vitro and in vivo. Near-infrared (NIR) fluorophores such as Cy5.5 NHS ester (non-sulfonated) enable deep-tissue imaging due to reduced tissue autofluorescence and increased penetration depth (excitation 684 nm, emission 710 nm) (internal validation). Amine-reactive dyes are preferred for site-specific labeling of proteins, peptides, and oligonucleotides containing primary amino groups. Such labeling is critical for applications in molecular imaging, tumor detection, and microbiome research (Kang et al., 2025). The product from APExBIO (A8103) is designed to meet these requirements by leveraging NHS ester chemistry for stable conjugation (see mechanistic analysis).

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

Cy5.5 NHS ester (non-sulfonated) contains an N-hydroxysuccinimide (NHS) ester group, which reacts selectively with primary amines at physiological to mildly basic pH (7.2–8.5). The reaction forms a stable amide bond, covalently linking the dye to lysine residues or N-termini of biomolecules (APExBIO). The process typically occurs in buffered aqueous solution with excess dye and limited water to maximize yield. NHS esters hydrolyze rapidly in aqueous solution; thus, Cy5.5 NHS ester must be freshly dissolved in organic solvent (DMSO or DMF) and immediately introduced into the conjugation reaction. The optical properties (λ_ex 684 nm, λ_em 710 nm) are retained after conjugation, enabling sensitive NIR fluorescence detection (internal review).

Evidence & Benchmarks

• Cy5.5 NHS ester (non-sulfonated) enables efficient labeling of proteins and oligonucleotides via stable amide bond formation with primary amines (APExBIO).
• Conjugates retain near-infrared spectral properties (excitation 684 nm, emission 710 nm) suitable for deep-tissue imaging (internal data).
• High solubility in DMSO (≥35.82 mg/mL) enables preparation of concentrated stock solutions (APExBIO).
• Optical imaging of tumors using Cy5.5-labeled agents in live animal models shows clear tumor delineation and favorable pharmacokinetics (Kang et al., 2025, Fig. 5A-C).
• Labeling protocols using Cy5.5 NHS ester have been optimized for cell viability, proliferation, and cytotoxicity assays, supporting reproducible high-sensitivity results (protocol review).

Applications, Limits & Misconceptions

Cy5.5 NHS ester (non-sulfonated) is widely used for fluorescence labeling in molecular biology, protein conjugation, and optical imaging of tumors. It is especially valuable in in vivo fluorescence imaging due to its NIR properties (product review).

• Fluorescent labeling of proteins, peptides, and oligonucleotides: Labeling is efficient for biomolecules containing accessible primary amines (APExBIO).
• Optical imaging of tumors: Used in live animal models for tumor delineation and pharmacokinetic profiling (Kang et al., 2025).
• Cell-based assays: Enables robust viability, proliferation, and cytotoxicity analyses via labeled probes (see detailed guidance).
• Molecular imaging and microbiome research: Supports translational studies targeting tumor-associated bacteria and tumor microenvironment (contextual analysis).

Common Pitfalls or Misconceptions

• Cy5.5 NHS ester (non-sulfonated) is not water-soluble; it must be dissolved in anhydrous DMSO or DMF before use (APExBIO).
• The NHS ester is unstable in aqueous solution and should be freshly prepared; prolonged incubation reduces labeling efficiency (validation).
• Labeling is specific to primary amines; secondary amines or other functional groups are not efficiently labeled (internal review).
• Fluorescence detection requires NIR-capable instrumentation (excitation/emission 684/710 nm); standard FITC or Cy3 filter sets are not suitable.
• Storage in solution (even at -20°C) rapidly degrades the NHS ester; long-term storage must be as a dry solid (APExBIO).

Workflow Integration & Parameters

For optimal labeling, Cy5.5 NHS ester (non-sulfonated) is dissolved in dry DMSO or DMF, then added to biomolecules in pH 7.2–8.5 buffer (e.g., phosphate or bicarbonate). Typical dye-to-protein ratios range from 3:1 to 10:1 (mol:mol), and reactions proceed for 30–60 minutes at room temperature, protected from light (protocols). Excess dye is removed by gel filtration or dialysis. The product is compatible with standard protein/oligonucleotide quantification and imaging workflows. The dye is supplied as a dry solid, stable for up to 24 months at -20°C in the dark. For further details and validated protocols, see the APExBIO Cy5.5 NHS ester product page. This article provides an expanded, evidence-driven view compared to previous overviews, emphasizing clinical translation and tumor microbiome imaging.

Conclusion & Outlook

Cy5.5 NHS ester (non-sulfonated) is a robust, validated tool for NIR fluorescent labeling of biomolecules, enabling high-sensitivity in vivo imaging, tumor delineation, and translational research applications. Its chemistry ensures stable conjugation, while spectral properties maximize imaging depth and minimize background. Proper handling and workflow integration are essential for reproducible performance. As new studies expand the understanding of tumor-microbiome interactions and the need for precise molecular imaging grows, Cy5.5 NHS ester remains a preferred choice for advanced fluorescent labeling in life sciences (Kang et al., 2025).