Tryptophan fluorescence, which is seen in proteins containing the amino acid tryptophan, is a phenomenon that occurs when tryptophan is exposed to ultraviolet (UV) light. Electrons in the tryptophan are excited to a higher energy state by UV absorption and then drop again to ground state, emitting fluorescence in the process. Tryptophan is an intrinsically fluorescent component of proteins and has many applications in biochemistry, where it is used as a probe to study protein folding and behavior.
As one of several amino acids, tryptophan is an organic molecule that links in chains with other amino acids to form proteins. It is also one of the 10 essential amino acids in the diet of human beings. Chemically, tryptophan is aromatic, meaning that its molecular structure includes a ring of atoms connected in such a way as to allow electrons to cycle around the ring. The electrons in this ring structure absorb UV light, allowing tryptophan fluorescence to take place.
Electrons in an atom or molecule typically exist in the lowest possible energy state, the ground state. When an electron absorbs energy from a photon, or light particle, the energy from that photon causes the electron to jump to a higher energy state — an excited state. The excited electron returns to ground state by emitting energy, often in the form of light known as fluorescence. Tryptophan fluorescence is an example of this process.
Ultraviolet light, the same type of light found in sunlight, is not visible to the human eye. Its wavelengths are too short for humans to perceive visually, ranging from 10 nanometers (nm) to 400 nm in length. Tryptophan can absorb wavelengths of light up to 280 nm in length and emits slightly longer wavelengths over a spectrum, with most emissions at about 320 to 375 nm in length.
Organic compounds including tryptophan are analyzed in laboratories using a technique known as fluorescence spectroscopy. A beam of UV light is directed at the protein or molecular structure under study, causing its electrons to become excited. The wavelength of fluorescence that the electrons emit as they fall to the ground state is then measured. Fluorescence spectroscopy can be useful in studying the folded structure of proteins.
Tryptophan fluorescence also provides a tool for analyzing biochemical compounds. In medical research, for example, tryptophan fluorescence is sometimes used as a probe to determine how proteins in tissues or blood are behaving or bonding with other molecules. Variations in fluorescence wavelengths can indicate chemical changes in tryptophan’s immediate environment.