title: Spin state and axial ligand bonding in the hydroxide complexes of metmyoglobin, methemoglobin, and horseradish peroxidase at room and low temperatures.
abstract: Absorption and resonance Raman spectra using Soret excitation of alkaline metmyoglobin (metMb), methemoglobin (metHb), and horseradish peroxidase (HRP) were obtained at room and low temperature. At 298 K both metMb and metHb exhibit two isotope-sensitive bands assigned to high- and low-spin nu(Fe-OH) stretching modes, respectively, which are correlated with the spin-state population. The low-spin stretch occurs 60 cm-1 to higher energy than the corresponding high-spin vibration. When the temperature is lowered, only the low-spin species is observed. HRP exhibits at both 298 and 20 K only the low-spin nu(Fe-OH) stretching mode, which occurs 50 cm-1 to lower energy than the corresponding modes observed in the globins. This is explained in the context of a strong hydrogen bond between the hydroxyl ligand and the distal His42 and/or Arg38. Lowering temperature causes in all of the examined proteins a strengthening of the Fe-OH bond and a contraction of the core of about 0.01 A, as determined by the upshifting of the low-spin nu(Fe-OH) stretching mode and the core size marker bands. Both effects are ascribed to an increase of the packing forces.
authors.affiliation: Università di Firenze, Dipartimento di Chimica, Italy.
authors.forename: A
authors.initials: A
authors.lastname: Feis
authors.forename: M P
authors.initials: MP
authors.lastname: Marzocchi
authors.forename: M
authors.initials: M
authors.lastname: Paoli
authors.forename: G
authors.initials: G
authors.lastname: Smulevich
mesh_terms: D000818:Animals; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D003903:Deuterium; D006735:Horseradish Peroxidase / Q000737:chemistry / Q000378:metabolism*; D006736:Horses; D006801:Humans; D006860:Hydrogen Bonding; D006863:Hydrogen-Ion Concentration; D006878:Hydroxides / Q000378:metabolism*; D008706:Methemoglobin / Q000737:chemistry / Q000378:metabolism*; D008786:Metmyoglobin / Q000737:chemistry / Q000378:metabolism*; D013053:Spectrophotometry; D013059:Spectrum Analysis, Raman; D013696:Temperature
chemical_list: D006878:Hydroxides; D008786:Metmyoglobin; D008706:Methemoglobin; D003903:Deuterium; D006735:Horseradish Peroxidase