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Transcript
Aromatic Amino Acids Aromatic amino acids are relatively nonpolar. To different degrees, all aromatic amino acids absorb ultraviolet light. Tyrosine and tryptophan absorb more than do phenylalanine; tryptophan is responsible for most of the absorbance of ultraviolet light (ca. 280 nm) by proteins. Tyrosine is the only one of the aromatic amino acids with an ionizable side chain. Tyrosine is one of three hydroxyl containing amino acids. Least hydrophobic Very hydrophobic Aliphatic Amino Acids Aliphatic R groups are nonpolar and hydrophobic. Hydrophobicity increases with increasing number of C atoms in the hydrocarbon chain. Although these amino acids prefer to remain inside protein molecules, alanine and glycine are ambivalent, meaning that they can be inside or outside the protein molecule. Glycine has such a small side chain that it does not have much effect on the hydrophobic interactions. The structures below are shown in the ionization state that predominates at pH 7. Less hydrophobic More hydrophobic