Spotlights Cite This: J. Am. Chem. Soc. 2018, 140, 1977−1977
pubs.acs.org/JACS
Spotlights on Recent JACS Publications
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CRYSTALLOGRAPHIC CHARACTERIZATION OF ETHANE-INDUCED “BREATHING” IN A METAL−ORGANIC FRAMEWORK Flexible metal−organic frameworks (MOFs) can expand or contract in response to stimuli, such as guest pressure, and exhibit steps in their adsorption isotherms that correspond to discrete structural changes. In situ characterization of such “breathing” behavior using single-crystal X-ray diffraction (SCD) is of fundamental interest, although it is typically precluded by loss of crystal integrity during a transition. Prem Lama and Leonard Barbour characterize ethane-induced phase changes in a flexible zinc MOF using SCD, following controlled exposure of a guestfree single crystal to increasing pressures of the gas at room temperature (DOI: 10.1021/jacs.7b10352). The authors find that the first ethane-expanded phase exhibits the same space group as the parent framework with negligible differences in geometry, coinciding with its small gas uptake at low pressure; the second phase is characterized by weak framework−guest interactions, although it demonstrates the largest ethane uptake. Characterization of all three phases indicates that ethane uptake is fully reversible. Importantly, additional diffraction characterization of methane-, propane-, and butane-expanded phases reveals that the nature of flexing is dependent on the identity of the n-alkane, offering valuable insights toward the design of new flexible frameworks. Katie Meihaus, Ph.D.
TEACHING QUANTUM SPINS NEW DANCE STEPS Quantum computing relies on quantum properties such as entanglement to perform calculations. The future of quantum computing will be advanced by the development of methods to control and manipulate electronic spin states and entanglement whithin complex systems. David Shultz, Martin Kirk, and colleagues describe a new platform for the precise generation of hyperpolarized spin states and the assessment of electronic structures (DOI: 10.1021/jacs.7b11397). The researchers synthesize a series of chromophoric molecules where each spin 1/2 entity is entangled with the remaining two spins. To start the spin polarization process, the researchers use optical excitation to generate a photoexcited two-spin system coupled to a ground-state radical. Typically, magnetic resonance methods are used to manipulate spins in this type of research, but Kirk and Shultz’s team instead turn to optical excitation, which allows the electronic structures of the molecules to be assessed by magnetic circular dichroism spectroscopy, an unconventional magneto-optical technique. Their findings have great implications for spin-based quantum computing. Erika Gebel Berg, Ph.D.
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GREATER INSIGHT INTO ION EXCHANGE WITH MULTISCALE KINETIC MODELING Chloride channel proteins are a large family of proteins including both passive channles and active chloride/proton ion anitporters. They are expressed in nearly every cell of every eukaryote, and disruption in their genetic encoding is linked to numerous diseases. Thus, they have great implications for both health and fundamental biology. To gain insight into their mechanism, Jessica Swanson, Gregory Voth, and colleagues have developed a multiscale kinetic model of a chloride channel (DOI: 10.1021/ jacs.7b11463). By combining the results of computer simulations with experimental data, the researchers show that the ratio of chloride/proton ion exchange across the membranes can be maintained by the collective regulation of different exchange pathways, rather than sequential events in a single pathway as previously hypothesized. The team has found that the exchange rate of chloride and proton ions is kinetically coupled via a key proton transporting residue, E148. Changes in the exchange rate of one ion in turn cause changes in the other, and all without the need for the substantial conformational changes typically employed byantiporters. Understanding the molecular mechanisms in play to maintain consistent ion exchange under physiological conditions will provide avenues for intervention when these mechanisms go awry, leading to diseases including Alzheimer’s disease and kidney disease. Sue Min Liu, Ph.D.
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EXPANDING A DYNAMICS SIMULATION FRAMEWORK TO NON-EXPERTS The study of large-scale conformational changes of complex biomolecules yields important insights into their purpose and function. Brooke Husic and Vijay Pande provide an overview of recent developments of a theoretical framework referred to as Markov state models (MSM) that are paired with advanced molecular dynamics simulations to provide a powerful new approach to analyze dynamical systems (DOI: 10.1021/ jacs.7b12191). The MSM approach is based on a “transition probability matrix” master equation where the dynamics are modeled using a set of states. It required significant experience and intuition to choose these states when the method gained widespread use in the early 2000's. After years of development, and specifically a breakthrough in 2013 with a method termed the variational approach to conformational dynamics, state selection can now be automated based on an objective criterion derived from the transition matrix. This important development has expanded the use of MSM to a wider community beyond the hands of a few experts. MSM methods continue to develop as experimental data are integrated into the MSM framework, making it an increasingly useful tool to explore rational drug design and to elucidate mechanisms of biological processes such as protein folding, ligand binding, and protein−protein association. Dalia Yablon, Ph.D. © 2018 American Chemical Society
Published: February 14, 2018 1977
DOI: 10.1021/jacs.8b01439 J. Am. Chem. Soc. 2018, 140, 1977−1977
