Some amino acids are nonpolar,
meaning that the overall electronic charge of the molecule is essentially
distributed among its atoms. Most nonpolar amino acids have side groups
composed of carbon and hydrogen atoms, which share their electrons equally
when bonded to one another. Proline is an unusual amino acid in that its side
group is linked to its amino group such that a five-membered nonpolar ring
structure is formed. Tryptophan contains two ring structures composed mainly
of nonpolar carbon–hydrogen bonds. One ring possesses a nitrogen atom, but
because the atoms to which it is bonded share their electrons equally, the
charge is evenly distributed and the molecule is nonpolar. The side chain of
phenylalanine is characterized by a nonpolar benzene ring composed entirely
of carbon-hydrogen bonds, while has a nonpolar sulfur-containing side chain.
Although sulfur usually tends to accumulate electrons, the charge on
methionine is evenly distributed because the sulfur atom shares its electrons
equally with the two carbon atoms to which it is bound. The amino acids
alanine, valine, leucine, leucine, and isoleucine all possess side groups
containing combinations of nonpolar carbon-hydrogen bonds, resulting in an
even charge dispersal throughout each molecule. Molecules which are nonpolar
tend to be hydrophobic, meaning that they avoid interactions with polar
molecules such as water. This is biologically significant because proteins
tend to adopt structures that position nonpolar amino acids away from the
aqueous external cellular environment.
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