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

Executive Summary: Cy5.5 NHS ester (non-sulfonated) is a near-infrared fluorescent dye optimized for covalent labeling of primary amines in peptides, proteins, and oligonucleotides [APExBIO product page]. It exhibits excitation and emission maxima at 684 nm and 710 nm, respectively, facilitating low-background, deep-tissue fluorescence imaging (Li et al., 2025). The NHS ester group enables rapid, stable amide bond formation under standard labeling conditions. Cy5.5 NHS ester is highly soluble in DMSO/DMF (≥35.82 mg/mL in DMSO), but unstable in aqueous or prolonged light exposure, requiring immediate use after dissolution. The dye has enabled robust tumor delineation and pharmacokinetic profiling in live animal models, adding value to molecular imaging and translational research (internal).

Biological Rationale

Near-infrared (NIR) fluorescence imaging is a cornerstone of modern molecular biology and biomedical research. NIR dyes, like Cy5.5 NHS ester (non-sulfonated), offer minimal autofluorescence, low light scattering, and superior tissue penetration compared to visible-spectrum fluorophores (Li et al., 2025). Labeling of primary amines in proteins, peptides, and DNA allows precise tracking and quantification in complex biological systems. The use of NIR dyes is essential for in vivo studies, where deep tissue imaging and multiplexed analyses are required. APExBIO's Cy5.5 NHS ester (non-sulfonated) meets this need with a well-characterized photophysical profile and robust conjugation chemistry. The specificity of NHS-ester chemistry for primary amines ensures targeted labeling with minimal off-target effects.

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

Cy5.5 NHS ester (non-sulfonated) contains an active N-hydroxysuccinimide (NHS) ester group that selectively reacts with primary amino groups, such as those on lysine side chains or the N-terminus of proteins. The reaction occurs efficiently at pH 7.2–8.5 in aqueous buffers supplemented with organic solvents (e.g., DMSO, DMF) due to its low aqueous solubility [APExBIO]. The NHS ester is hydrolyzed in water, so the dye should be freshly dissolved and used immediately. Upon reaction, a stable amide bond forms, covalently linking the Cy5.5 fluorophore to the biomolecule. The dye's core structure confers excitation at 684 nm and emission at 710 nm, enabling detection with standard NIR imaging platforms. Its non-sulfonated form offers enhanced hydrophobicity, which may benefit certain labeling strategies and in vivo retention profiles (internal).

Evidence & Benchmarks

• Cy5.5 NHS ester (non-sulfonated) enables covalent labeling of lysine residues in proteins and amino-modified oligonucleotides under mild conditions (pH 7.2–8.5, RT, 30–60 min) (APExBIO).
• Excitation/emission maxima (684 nm/710 nm) enable deep tissue imaging with minimal background autofluorescence (Li et al., 2025).
• Solubility in DMSO ≥35.82 mg/mL enables high-concentration stock preparation; aqueous solubility is poor (APExBIO).
• Stability: Solid form is stable for 24 months at -20°C in the dark; dissolved dye is unstable and must be used immediately (APExBIO).
• Optical tumor imaging in live mice demonstrated clear tumor delineation and favorable pharmacokinetics after Cy5.5 NHS ester conjugation (Li et al., 2025).
• Comparable or superior sensitivity to other NIR dyes in multiplexed and 2D electrophoresis workflows (internal).

Applications, Limits & Misconceptions

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

• Antibody, protein, and peptide labeling for in vitro and in vivo tracking.
• Optical imaging of tumors and tissues in live animal models.
• Labeling of oligonucleotides and plasmid DNA for nucleic acid detection.
• Multiplexed fluorescence assays and 2D electrophoresis.
• Functionalizing nanoparticles or biomimetic platforms for neuromodulation or drug delivery studies (Li et al., 2025).

Compared to the overview in this internal article, which focuses on foundational and translational aspects, this dossier updates the workflow specifics and benchmarks with recent in vivo data.

Common Pitfalls or Misconceptions

• Cy5.5 NHS ester (non-sulfonated) is not water-soluble; direct dissolution in aqueous buffers leads to precipitation and poor labeling efficiency.
• The dye is not stable in solution; pre-dissolved stocks degrade rapidly due to hydrolysis of the NHS ester.
• Labeling is specific only for primary amines; secondary amines, thiols, or carboxyls are not targeted.
• The dye is not suitable for applications requiring real-time imaging in highly reducing or nucleophilic environments, which accelerate hydrolysis.
• Non-sulfonated Cy5.5 may have higher nonspecific binding in highly hydrophobic or lipid-rich samples compared to sulfonated analogs.

For assay optimization details, see this guide, which focuses on cell-based workflows; the present article clarifies chemical stability and in vivo imaging parameters.

Workflow Integration & Parameters

• Preparation: Dissolve Cy5.5 NHS ester (non-sulfonated) in dry DMSO or DMF to ≥10 mM concentration immediately before use.
• Labeling: Add dye to biomolecule in appropriate buffer (pH 7.2–8.5, typically PBS or bicarbonate) at a 5:1 to 20:1 molar ratio; react for 30–60 min at room temperature, protected from light.
• Purification: Remove unreacted dye via dialysis, spin columns, or gel filtration. Verify conjugation by absorbance (684 nm) or SDS-PAGE.
• Storage: Store lyophilized dye at -20°C, protected from light; labeled conjugates should be aliquoted and frozen.
• Instrument compatibility: Compatible with most NIR fluorescence scanners, confocal microscopes, and in vivo imaging systems using Cy5.5 or similar channels.

For advanced multiplexing and neuromodulation applications, this article extends the scope beyond the approaches in this internal resource by detailing new in vivo benchmarks and chemical caveats.

Conclusion & Outlook

Cy5.5 NHS ester (non-sulfonated) from APExBIO is a validated, high-performance near-infrared dye for amino group labeling in advanced fluorescence imaging and molecular biology. Its robust photostability, high reactivity, and NIR emission profile support sensitive deep-tissue studies and translational research. Ongoing innovation in molecular imaging, including multiplexed and neuromodulation platforms, will continue to benefit from the reliable properties and consistent performance of Cy5.5 NHS ester (non-sulfonated) (Li et al., 2025). For detailed product specifications and ordering, see the A8103 kit.