Q.Glycine and Alanine are different with respect to one substituent on the α-carbon. What are the other common substituent groups?

Picture a single carbon atom standing at the centre of a molecule with four arms reaching out, each arm gripping something different. That central carbon, and what it holds, is the whole idea of an amino acid.

An amino acid is an organic compound that carries two special groups — an amino group and an acidic carboxyl group — attached to the same carbon. Because that shared carbon is the first, or alpha, carbon, these molecules are called alpha-amino acids. It helps to think of an amino acid as a substituted methane: the central carbon has four positions, and they are occupied by:

• a hydrogen atom,
• a carboxyl group (COOH), the acidic part,
• an amino group (NH2), the basic part, and
• a variable side chain called the R group.

The three fixed groups are the same in every amino acid; it is the R group that changes. That single variable is why so many different amino acids are possible, though only twenty of them occur in proteins. When the R group is just a hydrogen you get glycine, when it is a methyl group you get alanine, when it is a hydroxy-methyl group you get serine, and so on. Depending on how many amino and carboxyl groups they carry, amino acids can be acidic (such as glutamic acid), basic (such as lysine) or neutral (such as valine), and some, like tyrosine and phenylalanine, are aromatic.

A special trait follows from having both an NH2 and a COOH group: both can ionize. So the exact form an amino acid takes changes with the pH of its surroundings, and one such form carries a positive and a negative charge at the same time — the zwitterionic form. …