Analysis of one-dimensional pure-exchange NMR experiments for studying dynamics with broad distributions of correlation times

Intrinsic conformational plasticity of native EmrE provides a pathway for multidrug resistance

EmrE is a multidrug resistance efflux pump with specificity to a wide range of antibiotics and antiseptics. To obtain atomic-scale insight into the attributes of the native state that encodes the broad specificity, we used a hybrid of solution and solid-state NMR methods in lipid bilayers and bicelles. Our results indicate that the native EmrE dimer oscillates between inward and outward facing structural conformations at an exchange rate (k(ex)) of ~300 s(-1) at 37 °C (millisecond motions), which is ~50-fold faster relative to the tetraphenylphosphonium (TPP(+)) substrate-bound form of the protein. These observables provide quantitative evidence that the rate-limiting step in the TPP(+) transport cycle is not the outward-inward conformational change in the absence of drug. In addition, using differential scanning calorimetry, we found that the width of the gel-to-liquid crystalline phase transition was 2 °C broader in the absence of the TPP(+) substrate versus its presence, which suggested that changes in transporter dynamics can impact the phase properties of the membrane. Interestingly, experiments with cross-linked EmrE showed that the millisecond inward-open to outward-open dynamics was not the culprit of the broadening. Instead, the calorimetry and NMR data supported the conclusion that faster time scale structural dynamics (nanosecond-microsecond) were the source and therefore impart the conformationally plastic character of native EmrE capable of binding structurally diverse substrates. These findings provide a clear example how differences in membrane protein transporter structural dynamics between drug-free and bound states can have a direct impact on the physical properties of the lipid bilayer in an allosteric fashion.

Molecular Dynamics of Poly(Ethylene Glycol) Intercalated in Clay, Studied Using 13C Solid-State NMR

In this study, Cross-Polarization Magic-angle Spinning CP/MAS, 2D Exchange, Centerband-Only Detection of Exchange (CODEX), and Separated-Local-Field (SLF) NMR experiments were used to study the molecular dynamics of poly(ethylene glycol) (PEG) inside Hectorite/PEG intercalation compounds in both single- and double-layer configurations. The results revealed that the overall amplitude of the motions of the PEG chain in the single-layer configuration is considerably smaller than that observed for the double-layer intercalation compound. This result indicates that the effect of having the polymer chain interacting with both clay platelets is to produce a substantial decrease in the motional amplitudes of those chains. The presence of these dynamically restricted segments might be explained by the presence of anchoring points between the clay platelets and the PEG oxygen atoms, which was induced by the Na⁺ cations. By comparing the PEG motional amplitudes of the double-layered nanocomposites composed of polymers with different molecular weights, a decrease in the motional amplitude for the smaller PEG chain was observed, which might also be understood using the presence of anchoring points.

Nonexponential polymer segmental motion in the presence and absence of ions: H2 NMR multitime correlation functions for polymer electrolytes poly(propylene glycol)-LiClO4

The authors measure 2H NMR multitime correlation functions to investigate the segmental motion of poly(propylene glycol) containing various amounts of the salt LiClO4. 2H NMR two-time correlation functions indicate that addition of salt affects not only the time scale of the segmental motion, but also the degree of the nonexponential relaxation behavior. To quantify the origin of the nonexponential segmental motion, the authors analyze 2H NMR three-time correlation functions. In general, nonexponential relaxation can result from homogeneous dynamics, i.e., intrinsic nonexponentiality, and from heterogeneous dynamics, i.e., existence of a distribution of correlation times G(ln tau). For the studied high and low salt concentrations, including neat poly(propylene glycol), the analysis shows that both homogeneous and heterogeneous contributions are important. 2H NMR four-time correlation functions allow the authors to measure the lifetime of the dynamical heterogeneities. For the studied salt concentrations, the rate exchange occurs on the same time scale as the segmental motion, indicating short-lived dynamical heterogeneities. To arrive at these results, the authors reconsider the interpretation of (2)H NMR three-time correlation functions. Results of analytical calculations and computer simulations show that it is necessary to extend the previous way of analysis so as to include effects due to correlated back-and-forth jumps.