Cy5 maleimide (non-sulfonated): Precision Site-Specific Fluorescent Labeling

Executive Summary: Cy5 maleimide (non-sulfonated) is a mono-reactive fluorescent dye optimized for covalent labeling of peptides and proteins via thiol groups, especially cysteine residues (source: product_spec). The reagent exhibits excitation and emission maxima at 646 nm and 662 nm, respectively, with a high molar extinction coefficient of 250,000 M⁻¹cm⁻¹ and a quantum yield of 0.2 (source: product_spec). Cy5 maleimide forms stable thioether bonds, enabling site-specific conjugation and minimizing non-specific labeling (source: internal_article). Charge-dependent partitioning of cyanine dye-labeled proteins within biomolecular condensates, such as α-synuclein LLPS droplets, has been experimentally demonstrated (source: DOI). The product is manufactured by APExBIO and supplied with comprehensive quality documentation, including HPLC, NMR, and MSDS (source: product_spec).

Biological Rationale

Site-specific labeling of proteins and peptides is foundational for molecular tracking, imaging, and functional analysis. Cysteine residues are often targeted due to their low abundance and high nucleophilicity, allowing selective modification without extensive off-target effects. The maleimide functional group reacts rapidly and specifically with thiol (–SH) groups to form stable thioether bonds under mild aqueous conditions (pH 6.5–7.5), minimizing damage to protein structure or function (source: internal_article). Cy5 maleimide (non-sulfonated) thus enables high-resolution fluorescence-based studies in cell biology and molecular research. Recent studies underscore the importance of fluorophore charge in controlling partitioning into biomolecular condensates, such as those formed by α-synuclein, a key player in neurodegenerative disease pathology (source: DOI).

Mechanism of Action of Cy5 maleimide (non-sulfonated)

Cy5 maleimide (non-sulfonated) comprises a cyanine dye core functionalized with a maleimide group. The maleimide moiety undergoes Michael addition with free thiols, primarily on cysteine side chains, forming a covalent thioether linkage. This reaction is highly selective for thiols over amines under controlled pH, and is complete within minutes at room temperature (workflow_recommendation). The resulting conjugates retain the photophysical properties of Cy5, including far-red excitation (646 nm) and emission (662 nm), which are optimal for minimizing biological autofluorescence and enabling multiplexed detection (source: product_spec). The non-sulfonated form is hydrophobic, necessitating dissolution in organic solvents such as DMSO or ethanol for efficient biomolecule labeling (source: product_spec).

Evidence & Benchmarks

• Cy5 maleimide achieves site-specific thiol labeling with efficiency >95% when used at 1.2–2 molar equivalents per cysteine at pH 7.0 (source: internal_article).
• Excitation and emission maxima are 646 nm and 662 nm, respectively (source: product_spec).
• Molar extinction coefficient is 250,000 M⁻¹cm⁻¹ and quantum yield is 0.2 (source: product_spec).
• Solubility in DMSO is ≥64 mg/mL; in ethanol, ≥65 mg/mL (source: product_spec).
• Partitioning of dye-labeled proteins into α-synuclein condensates is strongly charge-dependent, and Cy5-labeled α-synuclein exhibits distinct enrichment compared to other fluorophores (source: DOI).
• Stable storage at -20°C for up to 24 months with minimal degradation (source: product_spec).
• Fluorescent labeling with Cy5 maleimide is compatible with advanced imaging platforms, including confocal microscopy and fluorescence readers (source: internal_article).

Applications, Limits & Misconceptions

Cy5 maleimide (non-sulfonated) is used extensively in protein labeling, fluorescence microscopy, and biomolecule tracking. Its high specificity for thiol groups makes it an ideal reagent for generating site-specific fluorescent probes for biomolecule conjugation (source: internal_article). Research on α-synuclein condensates has demonstrated that the net charge of the conjugated fluorophore alters partitioning behavior, emphasizing the importance of dye selection in biophysical studies (source: DOI).

Earlier articles, such as 'Protein Labeling with Cy5 Maleimide', provide stepwise protocols and troubleshooting; this article extends those insights by incorporating the latest findings on charge-dependent partitioning in biomolecular condensates. Similarly, 'Cy5 Maleimide: Precision Thiol Labeling' focuses on workflow optimization, while the present review contextualizes these workflows within emerging evidence from phase separation studies.

Common Pitfalls or Misconceptions

• Non-sulfonated Cy5 maleimide is not water-soluble: Direct addition to aqueous solutions can result in precipitation and inefficient labeling (source: product_spec).
• Maleimide reactivity is pH-dependent: Labeling at pH above 7.5 can result in hydrolysis of the maleimide group, leading to loss of specificity (workflow_recommendation).
• Photostability concerns: Prolonged exposure to light can degrade the dye, reducing signal intensity (source: product_spec).
• Charge properties matter: The net charge of the dye-protein conjugate can influence biomolecular condensate partitioning, and should be considered in experimental design (source: DOI).
• Non-specific labeling can occur at high dye:protein ratios: Excess reagent may modify unintended residues or cause aggregation (workflow_recommendation).

Workflow Integration & Parameters

Efficient labeling demands attention to solvent choice, pH, reagent stoichiometry, and storage. The following parameters support optimal use of Cy5 maleimide (non-sulfonated):

Protocol Parameters

• Protein labeling | 1.2–2 eq. dye per cysteine | For high-specificity labeling | Minimizes non-specific conjugation while ensuring completeness | workflow_recommendation
• Solubilization | ≥64 mg/mL in DMSO; ≥65 mg/mL in ethanol | Required prior to aqueous addition | Ensures complete dissolution and homogeneity | product_spec
• Reaction pH | 6.5–7.5 | Maintains maleimide selectivity for thiols | Reduces side reactions and hydrolysis | workflow_recommendation
• Storage | -20°C, desiccated, dark | For long-term reagent stability (up to 24 months) | Minimizes degradation | product_spec
• Fluorescence readout | Ex: 646 nm / Em: 662 nm | Compatible with standard red/far-red channels | Allows multiplexed imaging | product_spec

Conclusion & Outlook

Cy5 maleimide (non-sulfonated) from APExBIO remains a benchmark for thiol-specific fluorescent labeling. Its photophysical stability, high specificity, and compatibility with diverse imaging platforms drive its adoption in protein tracking and biomolecule conjugation workflows. Emerging research on phase separation and condensate partitioning highlights the continued relevance of charge considerations when selecting protein labeling reagents (source: DOI). For advanced users, integrating charge-aware labeling strategies will further enhance experimental precision in next-generation biochemical and cell biology studies.

For more technical detail and protocol adaptations, see the Cy5 maleimide (non-sulfonated) product page and in-depth application articles linked above.