Example — the alpha carbon

This scaffold example introduces the common backbone of an alpha amino acid. Its central carbon connects to an amino group, a carboxyl group, a hydrogen atom, and a side chain. The side chain distinguishes one amino acid from another. In this diagram, R stands for that variable group; it is a placeholder rather than a particular atom.

Charge depends on the surrounding conditions. The drawing uses neutral group labels to emphasize connectivity, so it should not be read as a prediction of the predominant charge state in solution. Identifying the backbone and assigning its charge are separate tasks.

For a worked example, replace R with a methyl group, CH3. The resulting structure is alanine. The central carbon now has four different attached groups: hydrogen, methyl, amino, and carboxyl. That arrangement makes the alpha carbon a chiral center. Connectivity alone does not identify which spatial configuration is shown; a flat diagram needs additional stereochemical information for that purpose.

Now replace R with hydrogen instead. This produces glycine, whose alpha carbon carries two hydrogens. Because two attached groups are identical, that carbon is not a chiral center. Comparing these substitutions gives a quick way to connect side-chain identity with backbone symmetry.