The structure of the Nafion ionomer used in proton-exchange membranes of H(2)/O(2) fuel cells has long been contentious. Using a recently introduced algorithm, we have quantitatively simulated previously published small-angle scattering data of hydrated Nafion. The characteristic 'ionomer peak' arises from long parallel but otherwise randomly packed water channels surrounded by partially hydrophilic side branches, forming inverted-micelle cylinders. At 20 vol% water, the water channels have diameters of between 1.8 and 3.5 nm, with an average of 2.4 nm. Nafion crystallites (approximately 10 vol%), which form physical crosslinks that are crucial for the mechanical properties of Nafion films, are elongated and parallel to the water channels, with cross-sections of approximately (5 nm)(2). Simulations for various other models of Nafion, including Gierke's cluster and the polymer-bundle model, do not match the scattering data. The new model can explain important features of Nafion, including fast diffusion of water and protons through Nafion and its persistence at low temperatures.

The M2 protein of influenza A virus is a membrane-spanning tetrameric proton channel targeted by the antiviral drugs amantadine and rimantadine 1. Resistance to these drugs has compromised their effectiveness against many influenza strains, including pandemic H1N1. A recent crystal structure of M2(22-46) showed electron densities attributed to a single amantadine in the N-terminal half of the pore 2, suggesting a physical occlusion mechanism for inhibition. However, a solution NMR structure of M2(18-60) showed four rimantadines bound to the C-terminal lipid-facing surface of the helices 3, suggesting an allosteric mechanism. Here we show by solid-state NMR spectroscopy that two amantadine-binding sites exist in M2 in phospholipid bilayers. The high-affinity site, occupied by a single amantadine, is located in the N-terminal channel lumen, surrounded by residues mutated in amantadine-resistant viruses. Quantification of the protein – amantadine distances resulted in a 0.3 Å-resolution structure of the high-affinity binding site. The second, low-affinity, site was observed on the C-terminal protein surface, but only when the drug reaches high concentrations in the bilayer. The orientation and dynamics of the drug are distinct in the two sites, as shown by 2H NMR. These results indicate that amantadine physically occludes the M2 channel, thus paving the way for developing new antiviral drugs against influenza viruses. The study demonstrates the ability of solid-state NMR to elucidate small-molecule interactions with membrane proteins and determine high-resolution structures of their complexes.

Advanced solid-state NMR techniques exploiting 'H spin diffusion are described,which rely on selecting 'H magnetization of one of the components on the basis of 'H chemical shift or mobility differences under multiple-pulse irradiation. To monitor the progression of 'H spin diffusion, the higher resolution of I3C CP MAS spectra is exploited for NMR detection. For evaluation ofthe data,full quasi-analytical solutions ofthe diffusion equation as well as initial-rate approximations are derived, applied, and discussed. In order to obtain quantitative results, the 'H spin-diffusion constant has to be determined. To this end, the domain sizes in poly(styrene)-poly(methy1 methacrylate) systems including symmetrical diblock copolymers have been measured. The comparison of the NMR results with SAXS and TEM data yields the spin-diffusion coefficient for PS and PMMA.The resulting value ofD=0.8 nm2/ms is expected to be typical for rigid organic systems.The domain sizes investigated range from approximately 1 nm to ca. 100 nm. Bestimmung der DomanengroJen in heterogenen Polymeren mittels Festkorper-NMREs werden moderne Festkorper-NMR-Methoden beschrieben, die 'H-Spindiffusion ausnutzen und 'H-Magnetisierung einer der Komponenten des Systems auf Grund unterschiedlicher chemischer 'H-Verschiebung oder von Mobilitatsunterschieden bei Multipuls-Einstrahlung selektieren.Um die zeitliche Entwicklung der 'H-Spindiffusion zu ermitteln,wird fur den NMR-Nachweis die hohere Auflosung der "C-CP-MAS-Spektren ausgenutzt. Zur Auswertung der Ergebnisse werden vollstandige quasi-analytische Losungen der Diffusionsgleichung sowie Naherungen fur kurze Zeiten abgeleitet, angewendet und diskutiert. Um quantitative Ergebnisse zu erhalten, mu6 die 'H-Spindiffusionskonstante bestimmt werden. Zu diesem Zweck wurden die DomanengroRen in Poly(styren)-Poly(methylmethacry1at)-Systemen, einschliefllich symmetrischer Diblockcopolymere, gemessen. Ein Vergleich der NMR-Ergebnisse mit Rontgenkleinwinkelund elektronenmikroskopischen Daten ergibt als Wert des Spindiffusionskoeffizienten fur PS und PMMA D=0.8 nm2/ms, der als typisch fur starre organische Systeme angenommen werden darf. Die untersuchten DomanengroIJen liegen im Bereich von etwa 1 nm bis 100 nm.