[22] Infrared spectroscopy—Conformation

Publisher Summary This chapter describes infrared spectroscopy. The infrared spectra of high polymers are usually interpreted in terms of the vibrations of a structural repeat unit. The approach is analogous to the treatment of vibrational spectra of molecular crystals. It has the physical implication that only those fundamental vibrations can be observed in infrared (or Raman) spectra in which corresponding atoms in all repeat units move in phase. Three branches are infrared active (one of them very weak) and one is Raman active. Other polypeptide conformations—such as the α-helix and the parallel-chain pleated sheet—can be treated in an analogous manner. The conformational studies of polypeptides are to a large degree based on the fine structure of absorption bands that results from the coupling between chemical repeat units within the structural repeat unit. A band will split into as many components as there are chemical repeat units in the structural unit. Some branches might not be infrared active and others might overlap or be too weak to be observed.