Basic Principles of Luminescence

Some very common biological or biochemical lab methods are based on the existence of several “…escences” such as phosphorescence, chemiluminescence, bioluminescence and finally fluorescence. As an introduction to the topic of Fluorescent Proteins it may be useful to learn a bit more about “…escences”. The origin of these phenomena lies in the Latin word for see: -escentia, which already implies the connection to our visual system. All the “…escences” describe physical, chemical or biological processes that we can perceive with our eyes. We add the suffix “…escence” to words denoting change, action or process like the word convalescence.

So obviously, fluorescence is something we can see and something that concerns a change or a process. We will learn how fluorescence fulfills these conditions later on. First we will take a short look at the other “…escences” which are all luminescences themselves. Luminescence is the generic term for the emission of light which is not an effect of high temperature. So luminescence can be determined as an appearance of cold body radiation. This radiation can either be part of a chemical reaction or a cause of subatomic motions or stress on a crystal. Another way to generate emission is incandescence where light is emitted by a substance as the result of heat (e.g. hot metal).

Chemiluminescence is a light-emitting process based on a chemical reaction where the product has an excited intermediate. This intermediate emits light when falling into the ground state. Unlike fluorescence, electrons in chemiluminescent materials are excited by a chemical reaction and not by the absorption of photons. Chemiluminescence finds its technical application in light sticks for example. A well known chemiluminescent substance is luminol, which is used in criminalistics to find blood traces. Here Fe2+ ions which are present in hemoglobin function as a catalyzer to bring Luminol to its light emitting configuration.

If a living organism emits light we speak of bioluminescence, no matter how this light is produced. There are a lot of organisms that produce light, like glowworms (Lampyris noctiluca) or fireflies (Photinus pyralis). A very extraordinary organism in a row of several other fungi is the Jack O’Lantern mushroom (Omphalotus nidiformis), which glows in the dark. Numerous marine organisms like some corals, algae, crustacaea or even squids emit light, mostly in the blue or green spectrum. Another sea inhabitant is the bioluminating jellyfish Aequorea victoria, the source of the green fluorescent protein (GFP). Whereas the firefly, for example uses only a chemiluminescent process to produce light, A. victoria uses both a chemiluminescent and a fluorescent process. As scientists found out, the jellyfish generates blue light by a chemical reaction with the help of the protein Aequorin. This blue light is then used to excite the already mentioned GFP in a fluorescence reaction resulting in a green glow.

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