Glossary
Fluorescent Label (FAM / FITC)
Both are fluorescein dyes, but FAM and FITC attach differently and differ in stability. Ex/Em ~495/520 nm, pH dependence and handling of labelled peptides.
Among the fluorescein-type tags coupled to synthetic peptides, two dominate: FAM, i.e. 5- or 6-carboxyfluorescein, and FITC, i.e. fluorescein isothiocyanate. Each has its absorption maximum near 495 nm and its emission near 520 nm, so both suit the usual FITC filters and the 488 nm laser line available on flow cytometers and confocal systems.
Why you cannot simply swap them
They differ in how they attach. The isothiocyanate of FITC couples to a free amine through a thiourea bond, which slowly hydrolyses and may break down over months once dissolved. FAM is linked through its carboxylic acid to give a robust amide bond, so FAM-tagged peptides are usually the better choice when storage stability or quantitative measurements are important. The dye is usually placed on the N-terminal amine or a lysine side chain, often with a PEG or aminohexanoic acid spacer in between so the bulky dye does not get in the way of target binding.
Handling points: fluorescein is strongly pH-dependent and fades below roughly pH 7, bleaches rapidly under illumination and must be protected from light from the first opening of the vial — amber glass, foil wrapping and dimmed lighting are routine. Both the molecular weight and the purity of a tagged peptide differ from those of the untagged sequence, so consult the COA for the conjugate, not for the parent.
Related terms and material
biotin label · I-125 radiolabel · HPLC. Reference material: labeled peptides. Further reading: labeled peptides guide.