Characterization of binding kinetics of A2AR to Gαs protein by surface plasmon resonance.

Because of their surface localization, G protein-coupled receptors (GPCRs) are often pharmaceutical targets as they respond to a variety of extracellular stimuli (e.g., light, hormones, small molecules) that may activate or inhibit a downstream signaling response. The adenosine A2A receptor (A2AR) is a well-characterized GPCR that is expressed widely throughout the human body, with over 10 crystal structures determined. Truncation of the A2AR C-terminus is necessary for crystallization as this portion of the receptor is long and unstructured; however, previous work suggests shortening of the A2AR C-terminus from 412 to 316 amino acids (A2AΔ316R) ablates downstream signaling, as measured by cAMP production, to below that of constitutive full-length A2AR levels. As cAMP production is downstream of the first activation event-coupling of G protein to its receptor-investigating that first step in activation is important in understanding how the truncation effects native GPCR function. Here, using purified receptor and Gαs proteins, we characterize the association of A2AR and A2AΔ316R to Gαs with and without GDP or GTPγs using surface plasmon resonance (SPR). Gαs affinity for A2AR was greatest for apo-Gαs, moderately affected in the presence of GDP and nearly completely ablated by the addition of GTPγs. Truncation of the A2AR C-terminus (A2AΔ316R) decreased the affinity of the unliganded receptor for Gαs by ∼20%, suggesting small changes to binding can greatly impact downstream signaling.

Mg Anode Passivation Caused by the Reaction of Dissolved Sulfur in Mg-S Batteries.

As Li-ion battery optimization approaches theoretical limits, interest has grown in designing next-generation batteries from low-cost earth-abundant materials. Mg-S batteries are promising candidates, exhibiting widespread abundance of elemental precursors and a relatively large theoretical energy density albeit at lower cell voltage. However, Mg-S batteries exhibit poor reversibility, in part due to interactions between dissolved polysulfides and the Mg anode. Herein, we employ electrochemical experiments using Ag2S quasi-reference electrodes to probe the interactions between Mg anodes and dissolved polysulfides. We show that Mg2+ reduction (charging) is impeded in the presence of polysulfides, while Mg metal oxidation (discharging) remains facile. Large reduction overpotentials arise due to the formation of a passivation layer on the anode surface, likely composed primarily of MgS. The passivation layer is removed under oxidative conditions but quickly reforms during reduction. We discover that dissolved S8 influences the rate of MgS formation by shifting the polysulfide disproportionation equilibria. Shorter-chain polysulfides react more readily than longer-chain polysulfides at the Mg electrode, and thus, film formation is mediated by the electrochemical generation of shorter-chain polysulfide species.