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Biomedical subjects

J Breton

Publications and source records attributed to J Breton.

At least 181 records · Page 10Linked to original sources
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Chlorophyll orientation and exciton migration in the photosynthetic membrane.

Measurements of the linear dichroism and of the polarization of the fluorescence with oriented chloroplasts have revealed a definite orientation of the pigment molecules with respect to the membrane plane. The Qy transition moments of the chlorophyll a molecules are more closely inclined with respect to this plane for the forms absorbing at longer wavelengths than for those absorbing at shorter wavelengths. The fluorescence depolarization by energy transfer, determined with magnetically-oriented chloroplasts, indicates that the degree of local order increases with wavelength for the different (absorption wavelength) forms of chlorophyll a in vivo. Laser pulses of either picosecond or microsecond duration have been used to probe the emission spectrum, lifetime and quantum yield of fluorescence of chloroplasts at various temperatures. With single picosecond pulses, singlet-singlet annihilations occur within the light-harvesting chlorophyll molecules. In the case of microsecond pulses, triplet excitions act as efficient quenchers of the singlets. By monitoring both the yield of carotenoid triplets and of the fluorescence during and after a laser flash, one can show that the carotenoid triplets account for part of but not all the fluorescence quenching.

Chlorophyll↗
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Iron-sulphur clusters with labile metal ions.

A study has been carried out of the redox-linked metal ion uptake processes of the iron-sulphur cluster [3Fe-4S] in the bacterial ferredoxin, Fd III from Desulphovibrio africanus using a combination of electron paramagnetic resonance (EPR) and low-temperature magnetic circular dichroism (MCD) spectroscopy and direct, unmediated electrochemistry of the Fd in a film deposited at a pyrolytic graphite electrode. Reduction of the three-iron cluster is required before a divalent metal ion becomes bound as in the reaction sequence [formula: see text] The redox potentials of these processes and the metal binding constants have been determined. The affinities of the [3Fe-4S]0 cluster for divalent ions lie in the sequence Cd greater than Zn much greater than Fe. In addition, specific binding of a monovalent ion, Thallium(I), is detected for [3Fe-4S]1+ as well as for [3Fe-4S]0. The results provide a clear and quantitative demonstration of the capability of the open triangular tri-mu 2-sulphido face of a [3Fe-4S] cluster to bind a variety of metal ions if the protein environment permits. In each case the entering metal ion is coordinated by at least one additional ligand which may be from solvent (H2O or OH-) or from a protein side chain (e.g., carboxylate from aspartic acid). Hence the [3Fe-4S] core can be a redox-linked sensor of divalent metal ions, Fe(II) or Zn(II), that may trigger conformational change.

Amino Acid Sequence↗