Which of the following amino acids is involved in the formation of salt bridges?

Salt bridges are non-covalent interactions that occur between positively and negatively charged side chains of amino acids. These interactions play a crucial role in stabilizing the three-dimensional structures of proteins. The key amino acid involved in the formation of salt bridges is one that contains a positively charged side chain.

Arginine is the correct choice because it has a side chain composed of a guanidinium group, which carries a positive charge at physiological pH. This positive charge allows arginine to interact with negatively charged side chains of other amino acids, such as aspartate and glutamate, effectively forming a salt bridge. This interaction contributes to the stability of protein structures by helping to hold different parts of the polypeptide backbone together.

Other amino acids listed, such as alanine, threonine, and leucine, either have neutral side chains or are hydrophobic, lacking the charged residues necessary for forming salt bridges. Consequently, they do not facilitate the same type of stabilizing interactions through ionic bonding that arginine does.