Protein Chemistry. Structure, Properties, Research Methods - Shendryk A.N. 2022

Methods for Experimental Investigation of Protein Structure
Optical Properties of Proteins
Protein Light Absorption in the UV, Visible, and IR Spectral Regions

In the visible region, only Chromoproteins—Proteins containing a pigment moiety—absorb light. Examples include heme-containing proteins (featuring a porphyrin ring with an iron ion) such as Myoglobin, Hemoglobin, Cytochromes, catalase, peroxidase, and others.

In the UV region, all proteins except protamines exhibit a broad absorption band around 275-285 nm (due to Tyrosine, Tryptophan, and phenylalanine residues). The optical density (extinction coefficient) decreases when an amorphous protein folds into an a-helix. The wavelength at the absorption maximum remains unchanged. See the figure below:

This phenomenon is known as the hypochromic effect. It was first discovered in Nucleic Acids and, much later, in Peptides and Proteins. The magnitude of the hypochromic effect serves as an indicator of the extent to which the peptide chain is incorporated into a helical Structure.

In the IR region of the spectrum, prominent absorption bands of protein molecules are attributed to the peptide bond (C=O and NH groups): 3300 cm-1 for NH stretching vibrations, 1660 for C=O stretching, and 1550 for NH bending vibrations. IR spectroscopy also provides valuable insights into the Introduction/11.html">Secondary structure of protein peptide chains.

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Last update: 06/08/2026

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