Figure 1. The amino acids found in proteins have a common stereochemistry. In the structure illustrated in Figure , the amino group is always to the left side of the alpha carbon. In organic chemistry, this stereochemistry is referred to as L (for levo, meaning left). Thus, the amino acids found in proteins are Lâalpha amino acids. (Biochemists, being creatures of habit, usually do not refer to amino acid stereochemistry in the R and S nomenclature.) A few Dâamino acids are found in nature, although not in cellular proteins. (The D comes from dextro, meaning right.) For example, some peptide antibiotics, such as bacitracin, contain Dâamino acids. The carboxyll and amino groups of the amino acids can respectively donate a proton to and accept a proton from water.