Amino acid
Amino acids are organic compounds containing amine (-NH) and carboxyl (-COOH) functional groups, along with a side chain (R group) specific to each amino acid. The key elements of an amino acid are carbon (C), hydrogen (H), oxygen (O), and nitrogen (N), although other elements are found in the side chains of certain amino acids. About 500 naturally occurring amino acids are known (though only 20 appear in the genetic code) and can be classified in many ways. They can be classified according to the core structural functional groups' locations as alpha- (α-), beta- (β-), gamma- (γ-) or delta- (δ-) amino acids; other categories relate to polarity, pH level, and side chain group type (aliphatic, acyclic, aromatic, containing hydroxyl or sulfur, etc.). In the form of proteins, amino acid residues form the second-largest component (water is the largest) of human muscles and other tissues. Beyond their role as residues in proteins, amino acids participate in a number of processes such as neurotransmitter transport and biosynthesis.
Aromatic amino acid
Amino acids do not absorb light in the visible region of the electromagnetic spectrum, however, all absorb light from the infrared region due to their conjugated double bonds and the aromatic amino acids also absorb ultraviolet radiation. The aromatic amino acids phenylalanine, tyrosine, and tryptophan have the ability to significantly absorb ultraviolet radiation above 250 nm.This characteristic of aromatic amino acids is used to quantify the concentration of proteins in an unknown sample. These amino acids are able to absorb light which excites its electron to the excited state. When the electron falls back to its ground state, it will either emit light or release energy. If the molecule is able to emit light it is known as a fluorescent molecule. Tryptophan is widely used as a fluorescent molecule.