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Taking advantage of the improved spectral resolution of high-field electron paramagnetic resonance (EPR) at 95 GHz/3.4 Tas compared to conventional X-band EPR (9.5 GHz/0.34 T), detailed information on the polarity profile in a protein-protein interface is obtained. Nitroxide spin label side chains are introduced at positions 88 to 94 in the AS-1 sequence of the membrane adjacent HAMP domain of the transducer protein, NpHtrII, which is reconstituted in complex with sensory rhodopsin, NpSRII from Natronobacterium pharaonis. Position-dependent variations of the values of the nitroxide magnetic tensor components g.~, and A.: suggest that the spin label side chains at positions 88 to 93 of AS-1 are located between a hydrophobic anda hydrophilic microenvironment. The observed periodicity of the polarity properties of the respective spin label microenvironment agrees with an ~t-helical secondary structure of this part of AS-I and validates a recently published molecular model which locates residues 88 and 91 in the interface between helices F and G of NpSRII and AS-1 of NpHtrII close to the cytoplasmic lipid-water interface.

The paramagnetic resonance of dilute Ce 3+ ions in the bulk paramagnetic host Tb 3 Ga 5 O 12 is separated from the host response by means of optically detected magnetic resonance (ODMR) using the magnetic circular dichroism (MCD) of the absorption. This result shows that it is possible in principle to extend by means of the ODMR of the absorption the regime of paramagnetic resonance detection of impurity ions to magnetic hosts, which strongly interact with the microwave field and, hence, make the impurity signals unobservable by means of conventional EPR. The observed ODMR signals are attributed to Tb 3+ and Ce 3+ ions by correlation with the optical bands of these species. By means of ODMR via the Faraday effect it is shown that this effect is due to transitions between Tb levels. The identification of the observed magnetic resonance structures is confirmed by calculations based on known information on Tb 3+ and Ce 3+ in diamagnetic hosts. The role of molecular fields is discussed.