Analysis of (ƒÓ,ƒÕ) using IR absorption spectroscopy
IR absorption spectroscopy has been applied to structural analysis of proteins. Peak position of the C=O stretching band (the Amide I band) gives the information of the secondary structure of the protein, as the AmI oscillator couples with each other differently in each secondary structure. By combining with the ¹³C isotope labeling technique, the IR band of the isotope-labeled ¹³C=O oscillator is distinguished from the remaining parts, and the site-specific analysis of the labeled residue is possible.

To make the quantitative discussion of the selected residue possible, we labeled successive two residues in the peptide with ¹³C and ¹⁸O, and analyzed the ¹³C=¹⁸O doublet band. The doublet band involves the two ¹³C=¹⁸O bands (set the peak position and the intensity of the higher and lower position to be ƒË_H, I_H, and ƒË_L, I_L, respectively). Difference in the peak position (ƒ¢ƒË=ƒË_H-ƒË_H) and the intensity ratio (R_int=I_H/(I_H+I_L) are the function of the force constants of each oscillator and the coupling strength between them.

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