EPR study of the dinuclear active copper site of tyrosinase from Streptomyces antibioticus

AbstractThe [Cu(I)–Cu(II)] half-met form of the dinuclear copper site of tyrosinase has been probed by continuous wave electron paramagnetic resonance (EPR) and hyperfine sublevel correlation (HYSCORE) spectroscopy in the presence and absence of inhibitors. In all cases the EPR spectrum is indicative of a dx2−y2 ground state for the unpaired electron. From the cross-peaks observed in the HYSCORE spectra, proton hyperfine coupling constants were obtained that are compatible with a hydroxide ion in an equatorial coordination position of the paramagnetic copper. After changing the water solvent to D2O or after addition of the inhibitors p-nitrophenol or L-mimosine, the proton signals disappear. The relevance of these findings for understanding the catalytic cycle is discussed

An optical study of single pentacene molecules in n-tetradecane

We report the spectroscopic observation of single pentacene molecules in the matrices n-tetradecane and n-hexadecane, using a confocal microscope operating at liquid-helium temperatures. A maximum detected photon emission rate of only 30 counts per second (cps) is found for pentacene in n-hexadecane and 160 cps for pentacene in n-tetradecane. For the latter system, the low count rate is shown to be caused by a high S1 → T1 intersystem crossing yield of about 40% in combination with a triplet lifetime of 33±3 µs. Pentacene molecules in this polycrystalline host are found to show little spectral diffusion on a timescale of seconds.

Multifrequency EPR analysis of the positive polaron in I₂-doped poly(3-hexylthiophene) and in poly[2-methoxy-5-(3,7-dimethyloctyloxy)]-1,4- phenylenevinylene

The W-band continuous-wave electron paramagnetic resonance (EPR) analysis of chemically induced polarons in drop-cast and spin-coated polyphenylenevinylene-type and polythiophene-type polymer films reveals rhombic g tensors in both cases. The dependence of the W-band EPR signals on the orientation of the spin-coated films with respect to the magnetic field indicates a high degree of backbone alignment with the substrate and allows a partial assignment of the g tensor orientation. The derived molecular orientations of the polymer chains in the spin-coated films show clear differences between the two types of polymers. The proton hyperfine interactions obtained from X-band HYSCORE (hyperfine sublevel correlation) and Q- and W-band pulsed ENDOR (electron-nuclear double resonance) experiments are interpreted in terms of earlier theoretical studies on the extension of the polarons.</p