Amino acids contain a carboxylic acid (-COOH) group and an amino (-NH2) group. The amino groups are usually attached to the carbons which are alpha to the carboxyl carbons, so they are called alpha-amino acids. The naturally occurring amino acids are optically active, as they have four different groups attached to one carbon, (Glycine is an exception, having two hydrogen’s) and have the L-configuration. The R-groups of the amino acids provide a basis for classifying amino acids. There are many ways of classifying amino acids, but one very useful way is on the basis of how well or poorly the R-group interacts with water: 1. The hydrophobic R-groups which can be aliphatic (such as the methyl group of alanine) or aromatic (such as the phenyl group of phenylalanine). 2. The hydrophilic R-groups which can contain neutral polar (such as the -OH of serine) or ionizable (such as the -COOH of aspartate) functional groups.