Amino acids: Introduction

All amino acids except glycine are chiral molecules and, as such, the D-L and R-S notations apply to them. The 20 natural amino acids, those that make proteins, belong to the L series.

The different amino acids differ on the so-called "side chain" or "R group". This determines the singular properties of each amino acid: polarity, electric charge, hydrophobicity, volume... These properties are the key to the structure and function of a protein.

The following model depicts a generic amino acid, with its side chain represented by a large ball.

Please note that the doubly ionized form is displayed ( +H3N-CHR-COO), or zwitterion, which is the most abundant form in aqueous media at neutral pH. We will dwell on this question later: ionization of amino acids.

Let's start by identifying some amino acids, and then study some of their properties.