 Hence, alternative chemical models of proteins were considered, such as the diketopiperazine hypothesis of Emil Abderhalden.However, no alternative model had yet explained why proteins yield only amino acids and peptides upon hydrolysis and proteolysis.As clarified by Linderstrøm-Lang, these proteolysis data showed that denatured proteins were polypeptides, but no data had yet been obtained about the structure of folded proteins; thus, denaturation could involve a chemical change that converted folded proteins into polypeptides.