 Their work on gramicidin and tyrocidine led to the first commercial antibiotics, and with Fritz Lipmann they found that the antibiotics include D-amino acids.During the late 1930s, Hotchkiss was also strongly critical of the Bergann–Niemann hypothesis of protein structure, the proposal by fellow Rockefeller biochemists Max Bergmann and Carl Niemann that protein structures always consist of multiples of 288 amino acids.