bstract:The ability of clip shaped molecules based on the building block diphenylglycoluril to form complexes with dihydroxybenzene guest molecules has been studied in detail. The binding strength of these complexes can be varied over a wide range {KA % 0-1 (P M_l), by applying small modifications in the host or the guest molecule. It is found that the complexation is a combination of different effects, viz., hydrogen bonding, j tjt stacking interactions, and a cavity effect. Fersht, A. R.; Shi. J.-P.; Knill-Jones, J.; Lowe, D. M.; Wilkinson, A. J.; Blow. D. M.; Brick, P.; Carter, C.; Waye, M. M. Y.; Winter, G. Nature 1985. 314. 235. . (d) Schweitzer, B. A.; Kool, E. T. ./.
The electron paramagnetic resonance (EPR) spectrum from the [4Fe-4S](3+) cluster in several high-potential iron-sulfur proteins (HiPIPs) is complex: it is not the pattern of a single, isolated S=1/2 system. Multifrequency EPR from 9 to 130 GHz reveals that the apparent peak positions (g values) are frequency-independent: the spectrum is dominated by the Zeeman interaction plus g-strain broadening. The spectra taken at frequencies above the X-band are increasingly sensitive to rapid-passage effects; therefore, the X-band data, which are slightly additionally broadened by dipolar interaction, were used for quantitative spectral analysis. For a single geometrical [4Fe-4S](3+) structure the (Fe-Fe)(5+) mixed-valence dimer can be assigned in six different ways to a pair of iron ions, and this defines six valence isomers. Systematic multicomponent g-strain simulation shows that the [4Fe-4S](3+) paramagnets in seven HiPIPs from different bacteria each consist of three to four discernible species, and these are assigned to valence isomers of the clusters. This interpretation builds on previous EPR analyzes of [4Fe-4S](3+) model compounds, and it constitutes a high-resolution extension of the current literature model, proposed from paramagnetic NMR studies.
The Umbella high-frequency electron paramagnetic resonance (EPR) facility is a part of the Nijmegen High Field Magnet Laboratory with special emphasis on multifrequency EPR for chemical and biological applications. At present the facility has various solid-state sources available for the frequency range between 95 and 400 GHz, combined with magnetic fields up to 30 T. For frequencies above 400 GHz a far-infrared laser is used. With superheterodyne detection techniques a typical dynamical range of 130 dB is obtained, with single-mode resonators, oversized cylindrical and Fabry-Perot resonators. Some examples will be shown to demonstrate the power of multifrequency EPR in high-spin systems.
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