Showing papers by "Jean-Jacques Girerd published in 2006"

A spectroscopic and voltammetric study of the pH-dependent Cu(II) coordination to the peptide GGGTH: relevance to the fifth Cu(II) site in the prion protein.

Abstract: The GGGTH sequence has been proposed to be the minimal sequence involved in the binding of a fifth Cu(II) ion in addition to the octarepeat region of the prion protein (PrP) which binds four Cu(II) ions. Coordination of Cu(II) by the N- and C-protected Ac-GGGTH-NH2 pentapeptide (P5) was investigated by using potentiometric titration, electrospray ionization mass spectrometry, UV–vis spectroscopy, electron paramagnetic resonance (EPR) spectroscopy and cyclic voltammetry experiments. Four different Cu(II) complexes were identified and characterized as a function of pH. The Cu(II) binding mode switches from NO3 to N4 for pH values ranging from 6.0 to 10.0. Quasi-reversible reduction of the [CuII(P5)H−2] complex formed at pH 6.7 occurs at E 1/2=0.04 V versus Ag/AgCl, whereas reversible oxidation of the [CuII(P5)H−3]− complex formed at pH 10.0 occurs at E 1/2=0.66 V versus Ag/AgCl. Comparison of our EPR data with those of the rSHaPrP(90-231) (Burns et al. in Biochemistry 42:6794–6803, 2003) strongly suggests an N3O binding mode at physiological pH for the fifth Cu(II) site in the protein.

Photoexcitation and relaxation dynamics of catecholato-iron(III) spin-crossover complexes.

Abstract: The photophysical properties of the ferric catecholate spin-crossover compounds [(TPA)Fe(R-Cat)]X (TPA=tris(2-pyridylmethyl)amine; X=PF6-, BPh4-; R-Cat=catecholate dianion substituted by R=NO2, Cl, or H) are investigated in the solid state. The catecholate-to-iron(III) charge-transfer bands are sensitive both to the spin state of the metal ion and the charge-transfer interactions associated with the different catecholate substituents. Vibronic progressions are identified in the near-infrared (NIR) absorption of the low-spin species. Evidence for a low-temperature photoexcitation process is provided. The relaxation dynamics between 10 and 100 K indicate a pure tunneling process below 40 K, and a thermally activated region at higher temperatures. The relaxation rate constants in the tunneling regime at low temperature, kHL(T0), vary in the range from 0.58 to 8.84 s-1. These values are in qualitative agreement with the inverse energy-gap law and with structural parameters. A comparison with ferrous spin-crossover complexes shows that the high-spin to low-spin relaxation is generally faster for ferric complexes, owing to the smaller bond length changes for the latter. However, in the present case the corresponding rate constants are smaller than expected based on the single configurational coordinate model. This is attributed to the combined influence of the electronic configuration and the molecular geometry.