Amino-acids can link together by condensation reactions to form polymers called polypeptides. The peptide bond is an amide link. The -COOH group of one amino-acid reacts with the -NH2 group of another. A protein is a polypeptide or long chain of 50 or more amino-acids joined together by peptide linkages.Two different dipeptides can be formed from the same two amino acids. The carboxyl group of glycine could react with the -NH2 of alanine or the -NH2 group of glycine could react with the carboxyl group of alanine.The peptide bond can be broken by acid or alkaline hydrolysis. In acid hydrolysis polypeptide is heated under reflux with 6M HCl for 24hrs. In alkaline hydrolysis the polypeptide is heated with aqueous NaOH at just over 100°C.