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Although Cu2+is ubiquitous, the relativistic destabilization of the 5d orbitals makes the isoelectronic Au2+exceedingly rare, typically stabilized only through Au–Au bonding or by using redox non-innocent ligands. Here we report the perovskite Cs4AuIIAuIII2Cl12, an extended solid with mononuclear Au2+sites, which is stable to ambient conditions and characterized by single-crystal X-ray diffraction. The 2+ oxidation state of Au was assigned using 197Au Mössbauer spectroscopy, electron paramagnetic resonance, and magnetic susceptibility measurements, with comparison to paramagnetic and diamagnetic analogues with Cu2+and Pd2+, respectively, as well as to density functional theory calculations. This gold perovskite offers an opportunity to study the optical and electronic transport of the uncommon Au2+/3+mixed-valence state and the characteristics of the elusive Au2+ion coordinated to simple ligands. Compared with the perovskite Cs2AuIAuIIICl6, which has been studied since the 1920s, Cs4AuIIAuIII2Cl12exhibits a 0.7 eV reduction in optical absorption onset and a 103-fold increase in electronic conductivity.

The growth hormone secretagogue receptor (GHS-R) is involved in the regulation of pulsatile GH release. However, until recently, natural endogenous ligands for the receptor were unknown. We fractionated porcine hypothalamic extracts and assayed fractions for activity on HEK293 cells expressing GHS-R and aequorin. A partial agonist was isolated and identified using microspray tandem mass spectrometry as adenosine. GHS-R activation by adenosine and synthetic adenosine agonists is inhibited by the GHS-R selective antagonists L-765,867, D-Lys3-GHRP-6, and by theophylline and XAC. Cross desensitization of the GHS-R occurs with both MK-0677 and adenosine. Ligand binding and site directed mutagenesis studies show that adenosine binds to a binding site that is distinct from the previously characterized MK-0677 and GHRP-6 binding pocket. We propose, that adenosine is a physiologically important endogenous GHS-R ligand and speculate that GHS-R ligands modulate dopamine release from hypothalamic neurons.