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The progesterone receptor is able to bind to a large number and variety of ligands that elicit a broad range of transcriptional responses ranging from full agonism to full antagonism and numerous mixed profiles inbetween. We describe here two new progesterone receptor ligand binding domain x-ray structures bound to compounds from a structurally related but functionally divergent series, which show different binding modes corresponding to their agonistic or antagonistic nature. In addition, we present a third progesterone receptor ligand binding domain dimer bound to an agonist in monomer A and an antagonist in monomer B, which display binding modes in agreement with the earlier observation that agonists and antagonists from this series adopt different binding modes.

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Selectively 13 C-labeled ubiquinone anion radicals in protic and aprotic solvents are investigated by EPR and ENDOR spectroscopy, yielding information about the effect of hydrogen bonds on the electronic g-tensor and the carbonyl carbon 13 C-hf tensors. Formation of the hydrogen bonds alter the g-tensor significantly to lower values and increases the A. component of the 13 C-hf tensor. Both effects can be explained by electrostatic interactions between the positively charged hydrogen and the electrons at the carbonyl oxygen leading to a redistribution of charge and rr-spin density. Two different hydrogen bonds were obtained for UQo' which are in agreement with the results of DFT (density functional theory) calculations.

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