The usage of a tag to detect biomolecules, such as antibodies and proteins, is essential to biological research. Common tag molecules include small fluorescent dyes, fluorescent proteins and enzyme tags. In addition to these, AAT Bioquest offers specialized tagging reagents for affinity detection, protein purification, self-labeling protein systems, photoaffinity labeling, and hapten-based immunoassay development. These building blocks enable custom conjugation workflows for advanced research applications; an overview of such available tags is presented in the table below.

Digoxigenin is a steroid hapten derived from foxglove plants that is widely used for non-radioactive labeling of nucleic acid probes in Southern blots, in situ hybridization, and ELISA-based detection systems. These reactive digoxigenin derivatives enable covalent attachment to proteins, oligonucleotides, and other biomolecules for subsequent detection with anti-digoxigenin antibodies, providing highly specific signals with minimal background interference. For nucleic acid labeling applications, see our Digoxigenin-11-dUTP and ReadiLink™ DIG Labeling Kits.

DNP (2,4-dinitrophenyl) is a versatile hapten used for antibody-based detection and, in specific biophysical applications, can also function as a fluorescence quencher for tryptophan- or tyrosine-based FRET studies. These reactive DNP derivatives enable conjugation to carrier proteins, peptides, and other molecules for immunodetection applications, hapten competition assays, and proximity-based fluorescence quenching studies. The PEGylated DNP-PEG4 variants offer improved water solubility for aqueous conjugation reactions. For detection reagents, see our Anti-Tag Molecules catalog.

Polyhistidine tags are among the most widely used affinity tags for recombinant protein purification and detection. These reactive building blocks enable researchers to append 6xHis or 10xHis sequences to proteins, peptides, and other molecules for downstream Ni-NTA affinity purification or anti-His antibody detection. The reagents are available with NHS ester, maleimide, azide, and alkyne functional groups for flexible bioconjugation strategies. For His-tagged protein purification, see our ReadiPrep™ Nickel NTA Magnetic Beads and Ni Agarose Resins.

The 6xHis tag is the standard polyhistidine sequence used in most expression systems, providing a compact affinity handle that typically does not interfere with protein folding or function. These reactive 6xHis building blocks can be conjugated to target molecules via amine, thiol, or click chemistry reactions.

The 10xHis tag provides enhanced binding affinity to Ni-NTA matrices compared to the 6xHis tag, which can improve purification yields for proteins with partially buried or structurally constrained His-tags. These extended polyhistidine reagents are particularly useful for proteins that show weak 6xHis binding.

Tris-NTA reagents contain three nitrilotriacetic acid chelating groups that provide substantially higher binding affinity for polyhistidine-tagged proteins compared to single NTA groups, enabling stable noncovalent protein labeling and improved capture efficiency in pull-down experiments. For fluorescent His-tag detection, see our HIS Lite detection probes with Cy3, Cy5, and iFluor options.

Benzylguanine (BG) and benzylcytosine (BC) building blocks enable researchers to create custom substrates for SNAP-tag and CLIP-tag self-labeling fusion protein systems derived from human O6-alkylguanine-DNA-alkyltransferase. The reactive derivatives can be conjugated to fluorophores, affinity handles, or other functional molecules to generate application-specific labeling reagents for live-cell imaging, protein tracking, and pull-down experiments. For ready-to-use fluorescent substrates, see our BG-TMR and BG-SiR conjugates.

SNAP-tag substrates are based on O6-benzylguanine (BG), which reacts specifically and irreversibly with the SNAP-tag protein to form a stable covalent bond. These building blocks allow researchers to conjugate virtually any molecule to SNAP-tagged proteins by first reacting the BG reagent with their molecule of interest.

CLIP-tag is a modified version of SNAP-tag that recognizes O₂-benzylcytosine (BC) instead of benzylguanine, enabling orthogonal dual-labeling experiments where both SNAP and CLIP fusion proteins can be labeled simultaneously with different reporters in the same cell.

These diazirine-based photoaffinity labels enable UV-activated crosslinking to capture transient protein-protein and protein-ligand interactions in their native binding conformations. Trifluoromethyl phenyl diazirines are among the most effective photoaffinity groups, generating highly reactive carbene intermediates upon UV irradiation at approximately 350 nm that insert into nearby C-H bonds to create covalent crosslinks with interacting molecules. For additional photoreactive crosslinkers, see our NHS-Diazirine and NHS-LC-Diazirine reagents in our Crosslinkers catalog.

AAT Bioquest offers reactive derivatives of drugs, controlled substances, and hormones for custom conjugation to carrier proteins, detection molecules, or solid supports. Available reactive groups include NHS esters (amine-reactive), maleimides (thiol-reactive), alkynes and azides (click chemistry), free amines, and carboxylic acids—providing flexibility for diverse bioconjugation strategies. These building blocks support development of immunogens for antibody production, tracers for competitive ELISA and fluorescence polarization assays, and affinity ligands for forensic toxicology, therapeutic drug monitoring, and food safety testing applications.

AAT Bioquest also offers specialty tagging reagents for specific research applications, including amine-blocking reagents for protein surface modification, thiol-reactive carrier proteins optimized for hapten immunization, and dual-modality imaging probes for combined fluorescence and radiometal detection.

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