Characterization of the CuA center in the cytochrome c oxidase from Thermus thermophilus for the spectral range 1800-500 cm-1 with a combined electrochemical and Fourier transform infrared spectroscopic setup

In this study we present the electrochemically induced Fourier transform infrared (FTIR) difference spectra of the CuA center derived from the ba3-type cytochrome c oxidase of Thermus thermophilus in the spectral range from 1800 to 500 cm-1. The mid infrared is dominated by the (nu) (C = O) vibrations of the amide I modes at 1688, 1660, and 1635 cm^-1, reflecting the redox-induced perturbation of the predominantly (beta)-sheet type structure. The corresponding amide II signal is found at 1528 cm^-1. In the lower frequency range below 800 cm^-1, modes from amino acids liganding the CuA center are expected. On the basis of the absorbance spectrum of the isolated amino acids, methionine is identified as an important residue, displaying C - S vibrations at these frequencies. This spectral range was previously disregarded by protein IR spectroscopists, mainly due to the strong absorbance of the solvent, H2O. With an optimized setup, however, IR is found suitable for structure/function studies on proteins.