Optical spectroscopy to characterize protein conformation and conformational changes

Abstract

Abstract Proteins absorb light and emit radiation in the UV range of the spectrum. The absorbance is caused by the peptide groups, by the aromatic amino acids, and to a small extent, by disulfide bonds. Fluorescence emission originates from the aromatic amino acids. Some proteins that carry covalently linked cofactors, such as the haem proteins, also show absorbance in the visible range. Proteins also absorb infrared light. The latter two properties will not be dealt with in this chapter. During absorption, light energy is used to promote electrons from the ground state to an excited state. Electrons that participate in delocalized aromatic systems frequently absorb in the near-UV or the visible region. Fluorescence emission is observed when excited electrons revert from the first excited state to the ground state. When a chromophore is part of an asymmetric structure, or when it is immobilized within an asymmetric environment, left-handed and right-handed circularly-polarized light is absorbed to different extents. This phenomenon is called circular dichroism (CD).