STABILITY AND CHARACTERIZATION OF IRON(III) AND IRON(II) HEME PEPTIDES ENCAPSULATED IN AQUEOUS DETERGENT MICELLES - H-1-NMR AND UV-VIS SPECTROSCOPIC STUDIES

Proton NMR and optical spectral studies on a heme undecapeptide in aqueous SDS micelles at different pHs are reported. In a micellar solution the monodispersed heme peptide is found to be encapsulated inside the hydrophobic micellar cavity. pH dependence of the ferric heme peptide inside the SDS micelle shows equilibrium conversion from the diaqua ferric heme at low pH (approximately 2) to the monoaqua monohistidine complex at pH approximately 6, which on further increase in pH undergoes deprotonation to give the monohistidine monohydroxo heme peptide complex. pH titration by optical spectroscopy shows a pK of 7.2, corresponding to the aqua (high-spin) half-arrow-right-over-half-arrow-left hydroxo (low-spin) transition. The cyano complex of the heme peptide in micellar solution shows a large spread of the heme methyl signals compared to the signals in the cyano protohemin complex, and its spectrum is similar to those of cyano heme proteins. Ferrous heme peptide complexes have also been stabilized inside the SDS micelles. Optical as well as NMR spectra of the ferrous complex suggest a five-coordination geometry. The line widths of the heme methyl resonances for these heme peptide complexes are much broader compared to those of the corresponding protoheme complexes but are similar to those found in the spectra of heme proteins.