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W D Phillips

Publications and source records attributed to W D Phillips.

At least 73 records · Page 4Linked to original sources
PubMed1972

Structure and properties of a synthetic analogue of bacterial iron--sulfur proteins.

The compound (Et(4)N)(2)[Fe(4)S(4)(SCH(2)Ph)(4)] has been prepared and its structure determined by x-ray diffraction. The Fe(4)S(4) core of the anion possesses a configuration of D(2d) symmetry that is closely related to the Fe(4)S(4) active-site structures of the high-potential iron protein from Chromatium and the ferredoxin from Micrococcus aerogenes. Electronic properties of the tetrameric anion have been partially characterized by measurement of proton magnetic resonance, Mössbauer, photoelectron, and electronic spectra, and magnetic susceptibility. Comparison of corresponding properties of [Fe(4)S(4)(SCH(2)Ph)(4)](2-) and the proteins implies that the oxidation levels of the synthetic tetramer, the reduced form of the high-potential protein, and the oxidized form of the 8-Fe ferredoxins are equivalent. The tetramer possesses the one-electron redox capacity associated with the 4-Fe centers of the ferredoxins. The structural and collective electronic features of [Fe(4)S(4)(SCH(2)Ph)(4)](2-) reveal it to be the first well-defined synthetic analogue of the active site of an iron-sulfur protein.

Bacterial Proteins↗
PubMed1971

Proton magnetic resonance studies of the ferredoxins from spinach and parsley.

Contact-shifted resonances have been detected in the pmr spectra of both oxidized and reduced forms of spinach and parsley ferredoxins. These resonances are assigned to the beta-CH(2) protons of four cysteine residues that are thought to bind the iron-sulfur redox center to the polypeptide chain. Temperature dependences of contact shifts reveal that the two iron atoms are antiferromagnetically coupled in both redox forms of each of these proteins. Thermal population of magnetic states gives rise to the contact shifts observed in the formally diamagnetic oxidized forms of these ferredoxins and accounts for the failure of contact shifts in the reduced forms exhibit to a Curie Law temperature dependence. It appears that the unpaired electron of reduced spinach and parsley ferredoxin is unequally distributed over the two iron centers. Valence states for the iron pairs of high-spin Fe(+3)-Fe(+3) and Fe(+2)-Fe(+3) for the oxidized and reduced forms, respectively, are compatible with the nmr results.

Electron Spin Resonance Spectroscopy↗
PubMed1970

Proton magnetic resonance study of ferredoxin from Clostridium pasteurianum.

Magnetic susceptibilities of both reduced and oxidized ferredoxin from Clostridium pasteurianum were obtained in solution. Whereas the reduced form exhibits a Curie law behavior, the magnetic susceptibility of oxidized ferredoxin in fact increases with temperature and suggests extensive antiferromagnetic exchange coupling between the component iron atoms. Contact-shifted resonances are observed for both forms of ferredoxin that are attributed to the beta-CH(2) protons of the eight cysteine residues. A model based on these results is presented.

Clostridium↗
PubMed1970

Proton magnetic resonance studies of Chromatium high-potential iron protein.

Contact-shifted nuclear magnetic resonances, arising from molecular paramagnetism, have been observed in both reduced and oxidized forms of the high-potential iron protein (HiPIP) isolated from Chromatium. Contact shifts of the reduced, formally diamagnetic form increase with temperature, indicating antiferromagnetic exchange coupling of the component iron atoms with thermal population of a magnetic state. In the oxidized form of HiPIP (formally S = 1/2), contact-shifted resonances attributed to the beta-CH(2) groups of two cysteine residues display approximate Curie law behavior, while contact-shifted resonances assigned to the two other cysteine residues continue to exhibit a temperature dependence characteristic of antiferromagnetic exchange coupling. A cluster model for the redox center of Chromatium HiPIP that appears compatible with the PMR and preliminary x-ray results(4, 11) is discussed.

Bacterial Proteins↗
PubMed1969

Sensitive criteria for the critical size for helix formation in oligopeptides.

We studied the conformation of a series of gamma-ethyl-L-glutamate oligopeptides by circular dichroism and 220 MHz nuclear magnetic resonance spectroscopy. By use of the first technique we noted enhancement of the n --> pi(*) and splitting of the pi --> pi(*) transitions commencing with the heptamer in trimethylphosphate and trifluoroethanol. With the second method we found changes in chemical shifts for the amide protons consistent with the onset of helicity at the heptamer in the solvents noted above. When DMSO-d(6) is used as a solvent, no such chemical shift changes occur because the oligopeptides do not assume helical conformations.

Magnetic Resonance Spectroscopy↗