The term 'amino acid chain' refers to a sequence of amino acids that are linked together by peptide bonds. Amino acids are the building blocks of proteins, and when they are linked together in a chain, they form a peptide. Proteins are large molecules that are essential for many biological processes, such as metabolism, cell signaling, and gene expression. Peptides are smaller molecules that are made up of two or more amino acids, and they can be used for various purposes, such as hormones, antibiotics, and drugs.