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 engineered for amino group labeling in biomolecules such as proteins, peptides, and oligonucleotides. It has an excitation maximum at 684 nm and emission maximum at 710 nm, supporting deep tissue imaging with low background autofluorescence (APExBIO product page). The dye is highly soluble in DMSO (≥35.82 mg/mL) but shows poor aqueous solubility, necessitating organic co-solvents during conjugation. In vivo studies demonstrate its capacity for sensitive tumor imaging, with peak tumor uptake observed 30 minutes post-injection and signal retention up to 24 hours (see extended benchmarks). Its high extinction coefficient (209,000 M⁻¹cm⁻¹) and moderate quantum yield (0.2) make it an optimal choice for fluorescence-based detection in advanced molecular workflows.

Biological Rationale

Fluorescent labeling of biomolecules enables precise tracking and quantification in molecular biology, biochemistry, and biomedical imaging. Near-infrared (NIR) dyes, such as Cy5.5 NHS ester (non-sulfonated), are favored for their ability to penetrate biological tissues with minimal autofluorescence interference (Li et al., 2025). The NHS ester functional group allows for covalent attachment to primary amines, which are abundant in lysine residues of proteins and the 5′- or 3′-ends of oligonucleotides. This chemistry ensures stable amide bond formation, reducing probe dissociation during downstream applications (reviewed here; this article updates with new tumor imaging data).

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

Cy5.5 NHS ester (non-sulfonated) labels biomolecules via nucleophilic substitution. The NHS ester reacts with primary amines in aqueous buffers (typically pH 7.5–8.5) to form stable amide bonds. The reaction is efficient in the presence of organic solvents such as DMSO or DMF, which dissolve the hydrophobic dye (APExBIO). Following conjugation, the dye’s near-infrared absorbance and emission enable detection even within highly scattering tissues. The non-sulfonated variant exhibits increased hydrophobicity, favoring certain in vivo applications where rapid clearance or membrane permeability is required (contrast: this article clarifies non-sulfonated vs. sulfonated variants).

Evidence & Benchmarks

• Excitation maximum at 684 nm and emission maximum at 710 nm under standard aqueous buffer conditions (pH 7.4) (source).
• High molar extinction coefficient: 209,000 M⁻¹cm⁻¹ measured in PBS, 25°C (APExBIO).
• Quantum yield of 0.2 (in PBS, 25°C) allows for robust signal-to-noise in imaging (benchmark).
• Solubility in DMSO: ≥35.82 mg/mL, but <0.1 mg/mL in water at room temperature (product spec).
• Stable as a solid for up to 24 months at -20°C in the dark; unstable in solution (degrades in hours at RT) (cf. advanced protocol guidance).
• In vivo tumor xenograft mouse imaging: peak signal in tumors at 30 minutes post-injection, with detectable fluorescence up to 24 hours (protocol data).
• Demonstrated utility for labeling plasmid DNA, proteins, and small peptides with high specificity for primary amines (atomic labeling evidence).
• Minimal background in NIR imaging of subcutaneous tumors in mouse models (Li et al., 2025).

Applications, Limits & Misconceptions

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

• Protein and peptide labeling for immunofluorescence and western blot detection.
• Oligonucleotide labeling for FISH, qPCR, and nucleic acid hybridization assays.
• In vivo tumor imaging, including xenograft and syngeneic mouse models.
• Optical imaging in neuroscience research, such as tracking nanomaterial delivery (Li et al., 2025).

However, certain boundaries must be respected:

Common Pitfalls or Misconceptions

• Does not efficiently label thiol or carboxyl groups—specific to primary amines only.
• Insufficient aqueous solubility for direct use in water-based buffers without organic co-solvent.
• Not stable in solution: should be prepared immediately before use to prevent hydrolysis.
• Non-sulfonated variant may show lower serum stability in circulation compared to sulfonated analogs.
• Not suitable for applications requiring reversible or non-covalent labeling.

This article provides updated atomic-level benchmarks and practical boundaries compared to prior reviews such as this guide, which focuses on advanced microbiome imaging protocols.

Workflow Integration & Parameters

For successful labeling, dissolve Cy5.5 NHS ester (non-sulfonated) in dry DMSO or DMF. Add to the target biomolecule in buffered aqueous solution (pH 7.5–8.5; e.g., 50 mM sodium bicarbonate or phosphate buffer). Typical reaction times are 30–60 minutes at room temperature, protected from light. Molar ratios of dye:biomolecule range from 2:1 to 10:1, depending on labeling density required. Remove unreacted dye by dialysis, gel filtration, or spin column purification. Store labeled conjugates at 4°C, protected from light. Use within 1–2 weeks for optimal signal integrity (APExBIO protocol).

For tumor imaging, inject labeled probe intravenously and image at 30 min, 2 h, and 24 h post-injection. Use NIR imaging systems with appropriate excitation/emission filters (typically 670–710 nm). Refer to this application benchmark for workflows integrating Cy5.5 NHS ester with in vivo optical imaging and neuromodulation research. This article clarifies labeling stability and workflow integration compared to previous internal content.

Conclusion & Outlook

Cy5.5 NHS ester (non-sulfonated), as provided by APExBIO, is a high-performance near-infrared dye for protein, peptide, and oligonucleotide labeling. Its robust NHS ester chemistry, deep tissue penetration, and strong photophysical attributes enable ultrasensitive detection in molecular biology and biomedical imaging. When used with appropriate protocols, it supports advanced applications such as in vivo tumor imaging and neuromodulation research. Future outlook includes integration with multifunctional nanoparticle platforms for simultaneous imaging and therapeutic delivery (Li et al., 2025).

For further information, see the Cy5.5 NHS ester (non-sulfonated) product page (A8103) or consult recent application notes and reviews for protocol specifics.