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Publisher Summary: Vibrational spectroscopy such as resonance Raman (RR) and infrared difference spectroscopy (IRD) has mostly been applied to retinal proteins having the retinal covalently bound to the protein via a protonated Schiff base. This chapter discusses the basic principles of RR and IRD methods and some of their important applications. Normal Raman scattering is a weak effect, and recording a complete spectrum usually requires many hours. Resonance Raman spectroscopy has the unique advantage of selectivity in that, in the spectrum of such complex systems as retinal proteins, only vibrations of the chromophore will be reflected. In the retinal proteins, photoreaction is evoked by the absorption of light. Resonance Raman spectroscopy has greatly contributed to the understanding of the photoreaction of bacteriorhodopsin. At first sight, infrared spectroscopy would seem to be less suitable for the investigation of complex biological systems, because the method is not selective. Infrared spectroscopy can, however, be rendered selective—that is, reflecting only functional groups—by forming the difference spectra between two different functional states. The main advantage of IRD is its capability to detect protein molecular changes.
