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The Structure of Amino Acids – Carboxyl Group, Amino Group, and R Group Properties of Amino Acids

The structure of amino acids consists of a carboxyl group, amino group, and R group. The properties of amino acids depends on the chemistry of the R group.

Amino acids are molecules that consist of several atoms. These atoms are linked to each other through chemical bonds. The type of atoms that make up an amino acid determine its properties. Although each of the 20 common amino acids have a distinct orientation of atoms, they all share a basic structure.

Alpha Amino Acids

All 20 common amino acids contain a carbon atom that is bound to two distinct chemical groups and a third chemical group that distinguishes each amino acid. This carbon atom is labeled the alpha carbon. The two groups that are common to all amino acids is the carboxyl group (COO-) and the amino group ( -NH3). The third group that is bound to the alpha carbon is the R group.

R Group

The R group is the distinguishing portion in the structure of amino acids. Some are polar and hydrophilic, while others are nonpolar and hydrophobic. Some contain atoms that give the amino acid a polarity somewhere in between these two extremes. Valine is a nonpolar amino acid that has a R group that consists of two methyl groups bound to a carbon atom. Serine is a polar amino acid that contains a hydroxymethyl R group. The properties of amino acids is largely dependent on the R group.

Net Charge

Amino acids are positively charged, negatively charged, or neutral at pH 7. Positively charged amino acids are basic, while negatively charged amino acids are acidic. The pH at which the amino acid has a net charge of zero is the amino acid’s isoelectric point.


Nelson, David L. Lehninger Principles of Biochemistry 4th ed. W.H. Freeman and Company. 2005

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