文摘
The active site of several oxygen binding proteins can be mimicked with the ferric iron protoporphyrin IXderivative hemin, coordinating two imidazole molecules and embedded in sodium dodecyl sulfate (SDS)micelles; the detergent simulates the hydrophobic cavity of heme proteins. We studied the low-frequencyvibrational modes of the porphyrin-iron-imidazole bonding in infrared absorbance spectra. Assignment ofthe metal-ligand vibrations to signals at 396, 387, and 378 cm-1 was performed by isotope labeling of theimidazole ligand. These modes were also found to be temperature-dependent and to display a linear increaseof signal intensity between 25 and 150 K and, with a different slope, between 150 and 300 K. The modes at396 and 399 cm-1 show for 25 K an upshift about 4 cm-1 and the signal at 378 cm-1 a small downshift,indicating the involvement of antisymmetric stretching modes and, in the latter, of bending motions. Anharmoniccouplings to doming modes are discussed, and the doming mode and hydrogen-bonding signature spectralrange between 300 and 100 cm-1 is presented.