Spotlights Cite This: J. Phys. Chem. Lett. 2018, 9, 6668−6668
pubs.acs.org/JPCL
Spotlights: Volume 9, Issue 22
J. Phys. Chem. Lett. 2018.9:6668-6668. Downloaded from pubs.acs.org by 95.181.217.243 on 11/15/18. For personal use only.
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FACILITATED AND NON-GAUSSIAN DIFFUSION OF CHOLESTEROL IN LIQUID ORDERED PHASE BILAYERS DEPENDS ON THE FLIP-FLOP AND SPATIAL ARRANGEMENT OF CHOLESTEROL Cholesterol may have a bad reputation among the general public, but scientists know that it is truly indispensable. Cholesterol plays a critical role in maintaining the fluidity and structural integrity of cell membranes and is involved in important biological processes such as domain formation and signal transduction. For these reasons, the thermodynamics and transport of cholesterol are important and active areas of study. However, a detailed mechanism for cholesterol transport at a molecular level remains elusive. As they describe in their Letter, Oh and Sung (10.1021/acs.jpclett.8b02982) used molecular dynamics simulations up to 100 μs for lipid bilayers with various concentrations of cholesterol to understand the anomalous transport of cholesterol and elucidate the transport mechanism. The authors confirmed that cholesterol undergoes anomalous subdiffusion at certain time scales and that the cholesterol stays stable at the lipid bilayer center during the flip-flop, consistent with previous studies. They found that cholesterol diffusion is facilitated in the liquid ordered phase compared with the lipid diffusion and that cholesterol shows Fickian yet non-Gaussian diffusion. In addition, they show that cholesterol molecules at the lipid bilayer center diffuse much faster than other cholesterol molecules within the leaflet and that the dynamic heterogeneity exists for the cholesterol molecules in lipid bilayers. The authors’ findings illustrate that the facilitated and non-Gaussian transport of cholesterol can be explained even quantitatively by the presence of cholesterol at the lipid bilayer center and dynamic heterogeneity.
errors, the amount of exchange-mixing is often adjusted, but the authors provide examples showing that this is unnecessary.
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VIBRATIONAL ENERGY IN PROTEINS CORRELATES WITH TOPOLOGY The exchange of vibrational energy in proteins is crucial for their function in several important processes, including conformational rearrangements, chemical reactions, and allostery. Understanding how this vibrational energy exchange occurs and identifying the main characteristic features will be instrumental for a deep comprehension of protein function. Maggi et al. (10.1021/acs.jpclett.8b02380) performed molecular simulations with a neuropharmacologically relevant transmembrane receptor, the human muscarinic M2 receptor, and their findings show a clear connection between the ability of each residue to exchange vibrational energy and geometrybased properties such as the proteins’ residues coordination number. The authors’ methods allowed them to identify the region with fast and slow vibrational energy transfer and to connect them with the conformational changes related to receptor activation and signal transduction.
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QUANTIFYING DENSITY ERRORS IN DFT Although the Kohn−Sham approach to density functional theory (DFT) is used in thousands of scientific papers every year, its claim to being first-principles is diluted by the fact that there are hundreds of possible exchange-correlation approximations available in most codes. For decades researchers have studied the self-consistent densities of DFT calculations to gain insight into the quality of approximations. Interest in the area intensified after a recent study appeared to show that the selfconsistent densities of recent, empirically parametrized functionals are of poorer quality than those that are more systematically derived, by careful comparison with accurate densities of atoms and ions. But Sim et al. (10.1021/ acs.jpclett.8b02855) report different conclusions in their Letter, arguing that any general mathematical measure of density error, no matter how reasonable, is too arbitrary to be of universal use. Because the density is a function and not a number, the variety of possible error-measures that people can create is endless. The authors give alternative measures, some of which produce starkly different rankings. They show how to estimate the significance of the density-driven error even when exact densities are unavailable. In cases with large density © 2018 American Chemical Society
Published: November 15, 2018 6668
DOI: 10.1021/acs.jpclett.8b03386 J. Phys. Chem. Lett. 2018, 9, 6668−6668
