Let There Be Light

Imagine a world without light and colour! In simple terms, it would be dark and black. The beauty of nature would disappear and artwork and photographs would all look the same. More importantly there would be no warmth and life on Earth (if it existed) would be very, very different. There are many phenomena and chemical reactions that produce light. Many organisms are also able to produce light using specific molecules and chemical reactions. Organisms produce light to attracted prey or a mate, or sometimes to escape predators. The production and emission of light by living organisms is known as bioluminescence. Our molecule this month is a brilliant example – the light glowing in fireflies caused by luciferin!

Light Bearing

Many living organisms contain a light-emitting molecule called luciferin. Luciferin is actually a general term for several different light-emitting molecules.

The name luciferin comes from “lucifer”, one of the many names given to the devil or Satan, and means “light-bearing”.

There are many organisms that use some form of luciferin to bioluminescence. We even suggested luciferin was responsible for Rudolph’s red nose!

The most well know member of the luciferin is firefly luciferin.

light dawn landscape nature luciferin firefly fireflies
Photo by Marek Piwnicki on Pexels.com

Lights In The Dark

Firefly-luciferin is, unsurprisingly, a type of luciferin molecule found in fireflies! The proper name of this molecule is (4S)-2-(6-hydroxy-1,3-benzothiazol-2-yl)-4,5-dihydrothiazole-4-carboxylic acid, but we’re going to stick to firefly luciferin!

The structure of luciferin

The structure of firefly luciferin is very interesting. There are three different rings in the structure, the acidic -COOH group, the alcoholic OH group and a couple of sulfur atoms thrown in as well! It is actually this -COOH group that is responsible for the bioluminescence.

In its pure, isolated form, it is a pale yellow solid.

Shining a Light on Luciferin

In 1885, Raphael Dubois showed how the reaction of luciferin produced light.

Firefly luciferin was isolated at John Hopkins University, USA, in 1949. However, it took several years to make enough of the molecule to allow serious study.

When You See The Yellow Light

In most species of firefly, the bioluminescence is a bright yellow colour. It believed that light was originally a warning to predators that the larva was not very tasty! However, over time the light is not used during the mating season to help the females choose a suitable male.

a firefly glowing with luciferin

Lights, Camera, Action!

In order for luciferin to produce light there is one important ingredient – oxygen! All luciferins react with oxygen, along with an enzyme called luciferase, to produce light. The actual reaction is quite complex, but let’s simplify.

So, remember that -COOH group we said was important? Well in firefly luciferin this group reacts with oxygen gas, with a little helping hand from the luciferase enzyme, to form a square ring. This ring is not very stable and breaks apart, eventually losing carbon dioxide and forming a molecule that is very energetic. Strictly speaking, we say the molecule is in an excited state.

One way the molecule can get rid of this extra energy is to emit it as light. This emission of light, is the bioluminescence we see!

A Molecule of Many Colours

While firefly luciferin produces a yellow light this is not always the case. Different species of fireflies can produce green-yellow and even orange light. This is because of slight differences to the structure and the surrounding conditions.

Luciferins are also found in different organisms. Snails, fish, bacteria and even fungi can have a type of luciferin that causes it to fluoresce all kinds of colours: blue, green and red.

Lights Out

Light is amazing, and without light, life would be very different, if it existed at all. What’s even more amazing is that some living organisms are able to produce their own light, a process known as bioluminescence. One of the most famous molecules responsible for bioluminescence is luciferin.

Luciferin comes in many forms, but the most widely studied is found in fireflies! With a little help from oxygen and a special enzyme, these small insects are able to emit a bright yellow light! Perfect to help find a mate, at least for fireflies. Shining a torch at people in the night is not likely to have the same results!


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