6-Carboxytetramethylrhodamine: Properties, Uses & Protocol

6-Carboxytetramethylrhodamine is a widely used orange-red fluorescent dye belonging to the rhodamine family, valued for its high photostability, strong molar absorptivity, and versatile amine-reactive carboxyl group that enables covalent conjugation to biomolecules in fluorescence-based research applications.

Common Abbreviation

6-TAMRA

Excitation Maximum

~542 nm

Emission Maximum

~568 nm

Fluorescence Color

Orange-red

Reactive Group

Carboxylic acid (—COOH), activatable as NHS ester

Primary Applications

Oligonucleotide labeling, FRET assays, flow cytometry, real-time PCR

SKU

C115506

Chemical Identity and Spectral Properties

6-Carboxytetramethylrhodamine (6-TAMRA) is a regioisomerically pure form of carboxytetramethylrhodamine in which the carboxylic acid group is positioned at the 6-position of the xanthene ring system. This positional purity distinguishes it from mixed isomer preparations and ensures reproducible spectral and conjugation behavior.

The dye absorbs maximally at approximately 542 nm and emits at approximately 568 nm, placing its signal in the orange-red channel that is well resolved from common green fluorophores such as fluorescein (FITC). Its molar extinction coefficient and quantum yield are favorable for single-molecule and ensemble fluorescence measurements. 6-TAMRA is compatible with standard argon-ion (514 nm or 532 nm) and green diode laser lines commonly found in flow cytometers, confocal microscopes, and qPCR instruments.

The dye exhibits good photostability relative to fluorescein, making it suitable for applications requiring prolonged imaging or multiple scan cycles. It is soluble in common polar organic solvents such as DMSO and DMF, and working solutions can be prepared in aqueous buffers at physiological pH.

Research Applications

Oligonucleotide and Nucleic Acid Labeling: 6-TAMRA is extensively used as a 3′ or 5′ fluorescent label on synthetic oligonucleotides, including TaqMan-style hydrolysis probes used in real-time quantitative PCR. Its orange-red emission is spectrally distinct from FAM (green) quenched pairs, enabling multiplex detection. When paired with a dark quencher such as BHQ-2, efficient fluorescence quenching occurs through FRET or contact quenching mechanisms.

FRET-Based Assays: As a FRET acceptor paired with fluorescein (donor) or as a donor paired with longer-wavelength acceptors, 6-TAMRA is used to study protein–protein interactions, nucleic acid hybridization dynamics, and conformational changes in macromolecules. Its defined excitation/emission profile facilitates accurate FRET efficiency calculations.

Flow Cytometry and Immunofluorescence: Antibodies, streptavidin, and other affinity reagents conjugated to 6-TAMRA via NHS ester chemistry can be used for cell surface marker detection. The orange-red channel is generally free of interference from autofluorescence in the green region, improving signal-to-noise ratios.

Peptide and Protein Labeling: The carboxylic acid can be activated with EDC/NHS or DIC/HOBt to form a reactive NHS ester, which reacts efficiently with primary amines (lysine residues or N-termini) at mildly alkaline pH (8.0–9.0), yielding stable amide bonds.

Labeling Protocol: NHS Ester Conjugation to Antibodies

The following general protocol describes the preparation of a 6-TAMRA NHS ester conjugate with a primary amine-containing biomolecule (e.g., IgG antibody). Adapt volumes and concentrations based on specific experimental needs.

1. Prepare dye stock solution: Dissolve 6-TAMRA (C115506) in anhydrous DMSO at 10 mM. Avoid aqueous solutions at this stage to prevent premature hydrolysis of the activated ester if using the NHS ester form.
2. Prepare protein solution: Exchange the protein into a conjugation buffer (0.1 M sodium bicarbonate, pH 8.3) using size-exclusion chromatography or dialysis. Target protein concentration of 2–10 mg/mL.
3. Activate the carboxylic acid (if using the free acid form): Mix 6-TAMRA free acid with EDC and NHS in a 1:2:5 molar ratio in MES buffer (pH 6.0) for 15 minutes at room temperature, then quench excess EDC with 2-mercaptoethanol (final 20 mM) before adding to protein.
4. Conjugation reaction: Add the dye solution slowly to the protein solution at a molar ratio of 5–20 dye molecules per protein molecule. Incubate at room temperature for 1–2 hours with gentle mixing, protected from light.
5. Purification: Remove unconjugated dye by gel filtration (e.g., Sephadex G-25) or dialysis against PBS. Collect the colored protein fraction.
6. Determine degree of labeling (DOL): Measure absorbance at 280 nm (protein) and 542 nm (6-TAMRA, ε ≈ 91,000 M⁻¹cm⁻¹). Apply correction factor for dye absorbance at 280 nm (CF₂₈₀ ≈ 0.34) to calculate protein concentration accurately. DOL = (A₅₄₂ / ε_dye) / [(A₂₈₀ − A₅₄₂ × CF₂₈₀) / ε_protein].
7. Storage: Store conjugate at 4 °C in PBS containing 0.1% BSA and 0.02% sodium azide, protected from light. For long-term storage, aliquot and freeze at −20 °C.

Note: Optimal DOL for antibody conjugates is typically 3–6 dye molecules per IgG to maintain antigen-binding activity and minimize self-quenching.

Handling, Stability, and Storage Recommendations

Light Sensitivity: 6-TAMRA and its conjugates are photosensitive. All handling, reactions, and storage should be performed under subdued or amber light. Use amber microcentrifuge tubes or wrap tubes in foil.

Solvent Compatibility: The free acid is best dissolved in DMSO or DMF before further dilution into aqueous buffers. Avoid prolonged exposure to strongly acidic or basic conditions, which can cause ring-opening and irreversible bleaching of the rhodamine chromophore.

Hygroscopicity: The dry powder should be equilibrated to room temperature before opening the vial to prevent condensation and hydrolysis. Desiccate the stock after each use.

Freeze-Thaw Cycles: Minimize freeze-thaw cycles for DMSO stock solutions. Prepare single-use aliquots for routine work.

Recommended Storage: Store the neat compound at −20 °C, desiccated, protected from light. Under these conditions, the compound is expected to be stable for at least 24 months. DMSO stock solutions should be stored at −20 °C and used within 6 months.

Frequently asked questions

What is the difference between 6-TAMRA and 5-TAMRA?

6-TAMRA and 5-TAMRA are positional isomers that differ in the location of the carboxylic acid group on the benzene ring of the xanthene scaffold (6-position vs. 5-position). The regioisomers have nearly identical spectral properties but differ in their chromatographic behavior and can yield conjugates with slightly different fluorescence quantum yields. Using a single regioisomer (e.g., 6-TAMRA) rather than a mixture improves lot-to-lot reproducibility in labeling experiments.

Which quencher is best paired with 6-TAMRA in qPCR probes?

BHQ-2 (Black Hole Quencher 2) is commonly recommended as a dark quencher paired with 6-TAMRA in TaqMan-type hydrolysis probes because its absorption spectrum overlaps well with the 6-TAMRA emission spectrum, enabling efficient FRET-based quenching. TAMRA itself can also serve as a quencher for FAM in dual-labeled probes, though this generates background fluorescence that dark quenchers avoid.

Can 6-TAMRA be used for live-cell imaging?

Yes, 6-TAMRA-labeled biomolecules can be used for live-cell imaging provided the conjugate is non-toxic at working concentrations and the labeling does not impair biological activity. The dye itself has limited cell permeability in its free acid form, so cell-surface labeling or microinjection approaches are typically employed. Its photostability and orange-red emission make it well suited for time-lapse fluorescence microscopy experiments.

How should I calculate the degree of labeling (DOL) for a 6-TAMRA conjugate?

Measure absorbance at both 542 nm and 280 nm using a UV-Vis spectrophotometer. Calculate the corrected protein absorbance at 280 nm by subtracting the dye contribution: A₂₈₀(corrected) = A₂₈₀ − (A₅₄₂ × CF₂₈₀), where CF₂₈₀ is the correction factor for 6-TAMRA at 280 nm (approximately 0.34). Then DOL = (A₅₄₂ / ε_dye) / (A₂₈₀(corrected) / ε_protein), where ε_dye ≈ 91,000 M⁻¹cm⁻¹ for 6-TAMRA.

References

1. Haugland, R.P. The Molecular Probes Handbook: A Guide to Fluorescent Probes and Labeling Technologies, 11th ed. Life Technologies, 2010.
2. Lakowicz, J.R. Principles of Fluorescence Spectroscopy, 3rd ed. Springer, 2006.
3. Berlier, J.E. et al. Quantitative comparison of long-wavelength Alexa Fluor dyes to Cy dyes: fluorescence of the dyes and their bioconjugates. Journal of Histochemistry & Cytochemistry, 51(12):1699-1712, 2003. doi:10.1177/002215540305101213

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