Cy5.5 NHS Ester (Non-Sulfonated): Atomic Facts, Mechanism, and In Vivo Imaging Benchmarks

Executive Summary: Cy5.5 NHS ester (non-sulfonated) is a near-infrared fluorescent dye optimized for covalent labeling of amino-containing biomolecules, such as proteins, peptides, and oligonucleotides (APExBIO). It demonstrates an excitation maximum at 684 nm and emission at 710 nm, supporting deep-tissue imaging with low background autofluorescence (Kang et al., 2025). The compound is highly soluble in DMSO (≥35.82 mg/mL), but exhibits low aqueous solubility, requiring organic co-solvents for efficient conjugation. In vivo, Cy5.5 NHS ester-labeled probes allow subcutaneous tumor visualization with optimal uptake at 30 minutes post-injection and signal detection for up to 24 hours. Its stability is best maintained as a solid at −20°C in the dark, with immediate use recommended upon dissolution (APExBIO).

Biological Rationale

Near-infrared (NIR) fluorescent dyes facilitate optical imaging in biological tissues due to low light scattering and minimal background fluorescence in the NIR window (650–900 nm) (Kang et al., 2025). Cy5.5 NHS ester (non-sulfonated) reacts with primary amines on biomolecules, enabling the covalent attachment of a fluorescent tag for high-sensitivity detection (APExBIO). This mechanism is critical for optical imaging in oncology, where visualizing tumors and their microenvironment in vivo aids in diagnosis and therapy monitoring. The dye's spectral properties allow for multiplexed imaging and are particularly suitable for deep tissue penetration, surpassing the performance of visible-range dyes. Its application in labeling plasmid DNA, proteins, and peptides is foundational in molecular biology and translational nanomedicine (see related analysis—this article updates with in vivo tumor imaging benchmarks).

Mechanism of Action of Cy5.5 NHS Ester (Non-Sulfonated)

Cy5.5 NHS ester contains an N-hydroxysuccinimide (NHS) activated ester functional group. This group selectively reacts with free primary amino groups (–NH₂) under mild aqueous or buffered conditions (pH 7–9), forming a stable amide linkage between the dye and the biomolecule (APExBIO). The resulting conjugates retain the dye's near-infrared fluorescence properties, with excitation at 684 nm and emission at 710 nm. The high molar extinction coefficient (209,000 M⁻¹cm⁻¹) and moderate quantum yield (~0.2) ensure robust signal output. Cy5.5 NHS ester (non-sulfonated) is soluble in organic solvents such as DMSO and DMF, but its low water solubility necessitates initial dissolution in organic solvent before conjugation. The product is supplied as a solid and is light-sensitive, requiring storage at −20°C in the dark for up to 24 months. Once dissolved, the NHS ester is hydrolytically unstable and should be used immediately. APExBIO supplies this product under SKU A8103 (Cy5.5 NHS ester (non-sulfonated)).

Evidence & Benchmarks

• Cy5.5 NHS ester-labeled probes enable visualization of subcutaneous tumors in xenograft mouse models, with tumor uptake peaking at 30 minutes post-injection (Kang et al., 2025, DOI).
• The near-infrared emission (710 nm) permits deep tissue imaging with significantly reduced background compared to visible-range dyes (Kang et al., 2025, DOI).
• Fluorescent signal from Cy5.5 NHS ester-labeled molecules is detectable in vivo for up to 24 hours post-injection (Kang et al., 2025, DOI).
• The dye is stable for 24 months as a solid at −20°C in the dark, but is unstable in solution and should be used immediately after dissolution (APExBIO).
• Solubility in DMSO is ≥35.82 mg/mL, but aqueous solubility is low, necessitating organic co-solvents for effective labeling (APExBIO).

Applications, Limits & Misconceptions

Cy5.5 NHS ester (non-sulfonated) is widely used in labeling proteins, peptides, and oligonucleotides for in vivo and in vitro fluorescence imaging applications. Its spectral properties are optimal for deep-tissue visualization, making it a preferred choice in oncology research and molecular diagnostics. In tumor imaging, Cy5.5 NHS ester enables real-time, non-invasive delineation of tumor margins and assessment of molecular processes (reliable cell assay guide—this article extends with new tumor xenograft benchmarks).

Despite its advantages, Cy5.5 NHS ester (non-sulfonated) is not compatible with all biomolecule types. It reacts only with primary amines; therefore, molecules lacking accessible amine groups are not directly labelable. The dye’s low aqueous solubility and light sensitivity can present workflow challenges. Additionally, NHS ester hydrolysis in aqueous buffers can reduce labeling efficiency if not used promptly. For advanced multiplexed and neuromodulation applications, see contrasting mechanistic and workflow insights in this review.

Common Pitfalls or Misconceptions

• Misconception: Cy5.5 NHS ester can label any biomolecule. Correction: Only primary amine-containing molecules (e.g., lysine residues on proteins) are suitable targets.
• Pitfall: Using water as the primary solvent. Correction: The compound requires organic solvents such as DMSO or DMF for dissolution due to low water solubility.
• Misconception: The dye is stable in solution for repeated use. Correction: NHS esters hydrolyze rapidly; solutions must be prepared freshly before labeling.
• Pitfall: Assuming visible-range filter sets suffice for detection. Correction: Detection requires filter sets matching excitation (684 nm) and emission (710 nm) maxima.
• Misconception: The dye is suitable for all in vivo imaging durations. Correction: Signal duration is typically up to 24 hours post-injection.

Workflow Integration & Parameters

Dissolution and Conjugation: Cy5.5 NHS ester (non-sulfonated) should be dissolved in anhydrous DMSO or DMF at a concentration up to ≥35.82 mg/mL. The dye is then added to a biomolecule solution in an appropriate aqueous buffer (commonly pH 7.4–8.5, e.g., PBS or sodium bicarbonate buffer) containing the target amine group. Typical molar ratios for efficient labeling range from 2:1 to 10:1 (dye:protein). Reaction times are generally 30–60 minutes at room temperature, protected from light. Excess unreacted dye should be removed by gel filtration or dialysis.

Storage: Store Cy5.5 NHS ester as a solid at −20°C, protected from light. Dissolved dye should be used immediately.

Detection: Use fluorescence imaging systems with excitation at 684 nm and emission detection at 710 nm. Alternative filter sets may reduce sensitivity.

Interlink: For advanced multiplexing and translational nanomedicine uses, see this deep-dive, which the present article updates by providing specific in vivo tumor imaging protocols and evidence.

Conclusion & Outlook

Cy5.5 NHS ester (non-sulfonated) is a validated, high-performance near-infrared dye for amino group labeling in proteins, peptides, and oligonucleotides. Its robust spectral properties and in vivo imaging benchmarks make it a key reagent for molecular and tumor imaging workflows. APExBIO supplies the product under SKU A8103, with well-characterized storage and conjugation guidelines (Cy5.5 NHS ester (non-sulfonated)). Emerging research continues to expand its applications in multiplexed imaging and translational nanomedicine. Practitioners should adhere to recommended protocols for solvent use, conjugation, and detection to maximize signal reliability and reproducibility. For further mechanistic and workflow innovations, see the referenced and interlinked articles above.