The infra-red absorption bands associated with the peptide groups in globular proteins, folded synthetic polypeptides and $\alpha $-fibrous proteins are very similar. The spectra of denatured globular proteins, extended chain synthetic polypeptides and $\beta $-fibrous proteins are also very similar. Denatured fibrous insulin is shown by polarized-beam techniques to consist of extended $\beta $ chains lying perpendicular to the fibril axis, with a layer structure involving inter-chain hydrogen bonded grids. Rhombohedral insulin crystals show dichroism consistent with an arrangement of folded chains packed perpendicular to the trigonal axis. A mechanism to explain denaturation of proteins is suggested in which precipitation is caused by a change from intra-chain to inter-chain hydrogen bonds. A method for investigating chain configurations of proteins in aqueous solution is described.