Infrared Spectroscopy

Assignments of the amide I band to various secondary structures in H20 and D20 are feasible by infrared spectroscopy. Amide I—III bands are caused by a vibration in the plane of the amide group of the polypeptide backbone; C = O stretch, N-H bend, and C-N stretch and N-H bend are the main vibrations for amide I, II, and III, respectively. The individual secondary structure fractions yield distinct maxima. However, the resolution of these structures in a globular protein that contains several of these structures is difficult because of the large half-widths of these structures, thereby interfering with their resolution. Least-squares optimization and Fourier deconvolution procedures have been used for the analysis of overlapping bands. Derivative spectroscopy is an alternative method for assessing the number and peak frequencies of the component peaks. The second derivative infrared spectra of various proteins have shown peaks associated with a-helical, yS-sheet, and /?-turn to be resolved and information pertaining to some amino acid side chains to be obtained.

Infrared spectra of 13 globular proteins in water were obtained in 1800-1480 cm""1. High correlation coefficients of0.80-0.99 were obtained between the infrared andX-ray estimates of ordered helix, disordered helix, ordered fi-structure, disordered ^-structure, turns, and remainder (25).

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