The interaction between two paramagnetic metal centers, a [3Fe-4S](+) cluster and a [NiFe] center, is investigated in the hydrogenase from Desulfovibrio vulgaris Miyazaki F by pulsed ELDOR (electron-electron double resonance). The distance between the metal centers is known from X-ray crystallography. The experimental dipolar spin-spin interaction deviates from the value expected for two point-dipoles located at the centers of the metal clusters. An extended spin-coupling model accounting for the spin coupling in the [3Fe-4S](+) cluster yields the observed interaction under the assumption of a particular magnetic coupling scheme for the three Fe ions. These results demonstrate that pulsed ELDOR can be used to gain insight into the inner structure of a multinuclear metal cluster.

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Pulsed electron-electron double resonance (ELDOR) has been used to obtain structural information from a FAD-dependent sulfhydryl oxidase, Augmenter of Liver Regeneration (ALR). ALR is a homodimer with each subunit containing a noncovalently bound FAD cofactor. Both FADs may be converted into the blue neutral radical form by aerobic treatment with DTT. From three-pulse and four-pulse ELDOR experiments, a distance of 26.1 +/- 0.8 A could be determined between the FAD cofactors in human ALR. Taking into account the electron spin density distribution in a neutral flavin radical obtained from density functional theory calculations, a distance of 26.9 A could be estimated for the separation of the spin centers in the X-ray structure of rat ALR. The good agreement confirms that rat ALR may be used as a model for mechanistic discussions of human ALR. The experiments also demonstrate that neutral flavin radicals have the appropriate properties to be used as intrinsic spin labels for distance determinations in proteins.

This paper reports on the switching characteristics of a spiropyran-cyclodextrin inclusion complex on a Au(111) surface. The switching behavior in acetone/water and ethanol/water solution is compared with that of the complex as thin film (∼100 layers) on gold and with the complex in a microcrystalline form. For the inclusion complex, we find a significantly increased switching time from the closed spiro to the open merocyanine conformation as compared to a solution of pure spiropyran. The thin film has a similar switching time as the solution of the inclusion complex. For the microcrystalline form, on the other hand, the switching times are considerably shorter. The back reaction exhibits a switching time for the solution of the inclusion complex which is similar to the pure spiropyran solution whereas the microcrystalline sample switches faster and the thin film of the inclusion complex slower than all the other ones. Furthermore, reversible switching of an adsorbed layer of the complex on gold could be observed. The switching characteristics can be explained in a model considering steric and compressive strain.

Thin films of spiropyran with thicknesses in the range of 5–40 nm were prepared on nonfunctionalized single crystalline MgO(100) substrates in situ in high vacuum. The films were studied in detail for wavelengths between 400 and 800 nm by means of optical absorption spectroscopy. The absorption maximum of the spiropyran state is found at 390 nm, comparable to microcrystalline powder. In order to investigate the switching kinetics between the closed-ring spiropyran and the ring-opened merocyanine form, time-resolved measurements were performed. The experimental data could be fit for both switching directions using a biexponential model indicating optically and thermally induced processes. A quantum efficiency for the switching toward the merocyanine state of about 2% was calculated. The reversibility of the switching process was found to be better than 90%.

A series of de novo synthesised peptides including the artificial rigid paramagnetic amino acid TOAC at two positions with different distances from two to seven in the primary structure have been investigated by 9- and 94-GHz EPR spectroscopy under solid-state conditions. From simulations of the spectra of such two-spin systems, the distance and relative orientation of the paramagnetic centres can be deduced. This yields structural information on the peptides. A quantitative analysis of the spectra of individual peptides in different solvents as well as a qualitative analysis of the spectra of the peptide series shows that the peptides do not assume conformations corresponding to any of the common helical structures in proteins.