Phenylalanine, tyrosine, and tryptophan have aromatic side chains. The nonpolar aliphatic and aromatic amino acids are normally buried in the protein core and are involved in hydrophobic interactions of the protein core. Tyrosine has a weakly acidic hydroxyl group and may be located on the surface of proteins. Reversible phosphorylation of the hydroxyl group of tyrosine in some enzymes is important in the regulation of metabolic pathways. The aromatic amino acids are responsible for the ultraviolet absorption of most proteins, which have absorption maxima ∼280 nm. Tryptophan has a greater absorption in this region than the other two aromatic amino acids. The molar absorption coefficient of a protein is useful in determining the concentration of a protein in solution, based on spectrophotometry. Typical absorption spectra of aromatic amino acids and a protein are shown in Figure 2.3.
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