Fluorescence involves the absorption of a photon followed by the emission of a less energetic photon (i.e. a photon with at least a slightly longer wavelength). The brightness of a fluorescent dye is determined by two factors:
Phycobiliproteins are among the best dyes available in both respects. They are extremely effective light absorbing antennae, the largest of which can contain 40 or more fluorophores (light-absorbing moieties) per molecule. In contrast, most synthetic dyes comprise only a single fluorophore.
In addition, the quantum efficiency of phycobiliproteins is extremely high. While techniques for measuring quantum efficiency are of limited precision and the subject of some controversy, phycobiliproteins are always reported to be among the most efficient of dyes. Phycoerythrins have frequently been reported to possess quantum efficiencies in excess of 90%, while values for phycocyanins have been somewhat lower.
Taken together, these properties of phycobiliproteins can make them, on a molar basis, up to two orders of magnitude brighter than many synthetic dyes.
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