title: Spectroscopic and electrochemical studies of horse myoglobin in dimethyl sulfoxide.
abstract: This paper reports the first report of rapid, reversible direct electron transfer between a redox protein, specifically, horse myoglobin, and a solid electrode substrate in nonaqueous media and the spectroscopic (UV-vis, fluorescence, and resonance Raman) characterization of the relevant redox forms of myoglobin (Mb) in dimethyl sulfoxide (DMSO). In DMSO, the heme active site of metmyoglobin (metMb) appears to remain six-coordinate high-spin, binding water weakly. Changes in the UV-fluorescence spectra for metMb in DMSO indicate that the protein secondary structure has been perturbed and suggest that helix A has moved away from the heme. UV-vis and RR spectra for deoxyMb in DMSO suggest that the heme iron is six-coordinate low-spin, most likely coordinating DMSO. Addition of CO to deoxyMb in DMSO produces a single, photostable six-coordinate CO adduct. UV-vis and RR for Mb-CO in DMSO are consistent with a six-coordinate low-spin heme iron binding His93 weakly, if at all. The polarity of the distal heme pocket is comparable to that of the closed form of horse Mb-CO in aqueous solution, pH 7. Direct electron transfer between horse Mb and Au in DMSO solution was investigated by cyclic voltammetry. Mb exhibits stable and well-defined electrochemical responses that do not appear to be affected by the water content (1.3-7.5%). The electrochemical characteristics are consistent with a one-electron, quasi-reversible, diffusion-controlled charge transfer process at Au. E degrees for horse Mb in DMSO at Au is -0.241+/-0.005 V vs. NHE. The formal heterogeneous electron transfer rate constant, calculated from delta E(p) at 20 mV/s, is 1.7+/-0.5 x 10(-4) cm/s. The rate, which is unaffected by the presence of 1.3-7.5% water, is competitive with that previously reported for horse Mb in aqueous solution.
authors.affiliation: Department of Chemistry, Northeastern University, Boston, MA 02115, USA.
authors.forename: Qiu Ci
authors.initials: QC
authors.lastname: Li
authors.forename: Patricia Ann
authors.initials: PA
authors.lastname: Mabrouk
mesh_terms: D000818:Animals; D001665:Binding Sites; D004121:Dimethyl Sulfoxide / Q000737:chemistry*; D004563:Electrochemistry / Q000379:methods; D004579:Electron Transport; D006046:Gold / Q000737:chemistry; D006418:Heme / Q000737:chemistry; D006736:Horses; D008786:Metmyoglobin / Q000737:chemistry; D009211:Myoglobin / Q000737:chemistry*; D017433:Protein Structure, Secondary; D013057:Spectrum Analysis / Q000379:methods; D014867:Water / Q000737:chemistry
chemical_list: D009211:Myoglobin; D014867:Water; D008786:Metmyoglobin; D006418:Heme; D006046:Gold; D004121:Dimethyl Sulfoxide