Structural changes accompanying the change in the redox state of microperoxidase-8 (MP8), the heme-octapeptide obtained from cytochrome c, and its complexes with (methyl)imidazole ligands were studied by electrochemically induced Fourier transform IR (FTIR) difference spectroscopy. To correlate with confidence IR modes with a specific electronic state of the iron, we used UV-vis and electron paramagnetic resonance spectroscopy to define precisely the heme spin state in the samples at the millimolar concentration of MP8 required for FTIR difference spectroscopy. We identified four intense redox-sensitive IR heme markers, nu38 at 1,569 cm(-1) (ox)/1,554 cm(-1) (red), nu42 at 1,264 cm(-1) (ox)/1,242 cm(-1) (red), nu43 at 1,146 cm(-1) (ox), and nu44 at 1,124-1,128 cm(-1) (ox). The intensity of nu42 and nu43 was clearly enhanced for low-spin imidazole-MP8 complexes, while that of nu44 increased for high-spin MP8. These modes can thus be used as IR markers of the iron spin state in MP8 and related c-type cytochromes. Moreover, one redox-sensitive band at 1,044 cm(-1) (red) is attributed to an IR marker specific of c-type hemes, possibly the delta(CbH3)(2,4) heme mode. Other redox-sensitive IR bands were assigned to the MP8 peptide backbone and to the fifth and sixth axial heme ligands. The distinct IR frequencies for imidazole (1,075 cm(-1)) and histidine (1,105 cm(-1)) side chains in the imidazole-MP8 complex allowed us to provide the first direct determination of their pKa at pH 9 and 12, respectively.