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The effect of hydrodynamic interactions (HI) on the long-time self-diffusion in quasi-two-dimensional fluids of paramagnetic colloidal particles is investigated using a combination of experiments and Brownian dynamics (BD) simulations. In the BD simulations, the direct interactions (DI) between the particles consist of a shortranged repulsive part and a long-ranged part that is proportional to 1/r 3 , with r the interparticle distance. By studying the equation of state, the simulations allow for the identification of the regime where the properties of the fluid are fully controlled by the long-ranged interactions, and the thermodynamic state solely depends on the dimensionless interaction strength. In this regime, the radial distribution functions from the simulations are in quantitative agreement with those from the experiments for different fluid area fractions. This agreement confirms that the DI in the experiments and simulations are identical, which thus allows us to isolate the role of HI, as these are not taken into account in the BD simulations. Experiment and simulation fall onto a master curve with respect to the dependence of D L = D L /(D 0 1/2), with D 0 the self-diffusion coefficient at infinite dilution and D L the long-time self-diffusion coefficient. Our results thus show that, although HI affect the short-time selfdiffusion, for a quasi-two-dimensional system with 1/r 3 long-ranged DI, the reduced quantity D L is effectively not affected by HI. Interestingly, this is in agreement with prior work on quasi-two-dimensional colloidal hard spheres.

G protein-coupled receptors (GPCRs) regulate diverse pathways in human physiology, and therefore represent major pharmaceutical drug targets. A growing number of high-resolution GPCR structures have provided detailed information about the mode of ligand binding, facilitating the development of better drugs. However, due to the dynamic nature and low abundance of these proteins, it remains challenging to obtain large quantities of stable GPCRs for structural studies. Therefore, we achieved thermostabilization of the human muscarinic M 2 receptor by replacing the serine residue at position 3.39 with arginine (S110R) to obtain the thermostabilized M 2 receptor in dramatically increased yields. We determined the structures of the thermostabilized M 2 receptor bound to the non-selective muscarinic antagonist NMS and the subtypeselective antagonist AF-DX 384 at resolutions of 2.3 Å and 3.0 Å , respectively. Crystals of the mutant receptor bound to NMS diffracted to a higher resolution than those of the wild type, presumably because the mutation rigidified the receptor to the antagonist-bound conformation. Comparison of the crystal structures and pharmacological analyses suggested that the side chain of the arginine at 3.39 in the S110R mutant mimics the role of the allosteric sodium ion. In addition to increasing thermostability, the S110R mutation also enhanced the affinity for AF-DX384, enabling co-crystallization of the protein with the ligand. MD simulations using this structure revealed that tightening of the AF-DX 384-residue contacts in M 2 receptor compared to M 3 receptor caused to the ligand selectivity of AF-DX 384.

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