Brian R. Cherry

Proton-detected heteronuclear single quantum correlation NMR spectroscopy in rigid solids with ultra-fast MAS.

In this article, we show the potential for utilizing proton-detected heteronuclear single quantum correlation (HSQC) NMR in rigid solids under ultra-fast magic angle spinning (MAS) conditions. The indirect detection of carbon-13 from coupled neighboring hydrogen nuclei provides a sensitivity enhancement of 3- to 4-fold in crystalline amino acids over direct-detected versions. Furthermore, the sensitivity enhancement is shown to be significantly larger for disordered solids that display inhomogeneously broadened carbon-13 spectra. Latrodectus hesperus (Black Widow) dragline silk is given as an example where the sample is mass-limited and the sensitivity enhancement for the proton-detected experiment is 8- to 13-fold. The ultra-fast MAS proton-detected HSQC solid-state NMR technique has the added advantage that no proton homonuclear decoupling is applied during the experiment. Further, well-resolved, indirectly observed carbon-13 spectra can be obtained in some cases without heteronuclear proton decoupling.

Oxidative coupling of porphyrins using copper(II) salts.

A method for radical coupling of porphyrins using copper(II) salts as one-electron oxidants was developed. A Zn(II)-porphyrin bearing an aminophenyl group yielded porphyrin oligomers, and two tri-arylporphyrins were oxidized to form doubly and triply linked dimers. Bromination of doubly linked dimers gave macrocycles with twisted skeletons.

Concerted One-Electron Two-Proton Transfer Processes in Models Inspired by the Tyr-His Couple of Photosystem II

Nature employs a TyrZ-His pair as a redox relay that couples proton transfer to the redox process between P680 and the water oxidizing catalyst in photosystem II. Artificial redox relays composed of different benzimidazole–phenol dyads (benzimidazole models His and phenol models Tyr) with substituents designed to simulate the hydrogen bond network surrounding the TyrZ-His pair have been prepared. When the benzimidazole substituents are strong proton acceptors such as primary or tertiary amines, theory predicts that a concerted two proton transfer process associated with the electrochemical oxidation of the phenol will take place. Also, theory predicts a decrease in the redox potential of the phenol by ∼300 mV and a small kinetic isotope effect (KIE). Indeed, electrochemical, spectroelectrochemical, and KIE experimental data are consistent with these predictions. Notably, these results were obtained by using theory to guide the rational design of artificial systems and have implications for managing proton activity to optimize efficiency at energy conversion sites involving water oxidation and reduction.

Multiporphyrin arrays with π-π interchromophore interactions.

A recently reported synthetic method has been employed to prepare several arrays of free base and zinc porphyrins. In the arrays, the porphyrins are arranged around a central benzene ring. The lack of aryl rings in the linkages to the central benzene ring, coupled with the presence of only one meso-aryl substituent on each porphyrin, allows strong electronic interactions between the porphyrin macrocycles. In arrays containing two or six porphyrins, a variety of evidence indicates that the porphyrins exist as twist-stacked dimers reminiscent of the special pairs of bacteriochlorophylls found in some photosynthetic bacteria. These dimers feature van der Waals contact between the macrocycles, and demonstrate excitonic splitting due to π-π interactions. The excitonic effects split and blue-shift the Soret absorptions, and slightly broaden the Q-band absorptions and shift them to longer wavelengths. The interactions also lower the first oxidation potentials by ca. 100 mV, and the arrays show evidence for delocalization of the radical cation over both porphyrins in the dimer. The arrays demonstrate singlet-singlet energy transfer among the chromophores. Arrays of this type will be good models for some aspects of the interactions of photosynthetic pigments, including those of reaction center special pairs and possibly quantum coherence effects. They can also be useful in artificial photosynthetic constructs.

Microscale Mechanism of Age Dependent Wetting Properties of Prickly Pear Cacti (Opuntia)

Cacti thrive in xeric environments through specialized water storage and collection tactics such as a shallow, widespread root system that maximizes rainwater absorption and spines adapted for fog droplet collection. However, in many cacti, the epidermis, not the spines, dominates the exterior surface area. Yet, little attention has been dedicated to studying interactions of the cactus epidermis with water drops. Surprisingly, the epidermis of plants in the genus Opuntia, also known as prickly pear cacti, has water-repelling characteristics. In this work, we report that surface properties of cladodes of 25 taxa of Opuntia grown in an arid Sonoran climate switch from water-repelling to superwetting under water impact over the span of a single season. We show that the old cladode surfaces are not superhydrophilic, but have nearly vanishing receding contact angle. We study water drop interactions with, as well as nano/microscale topology and chemistry of, the new and old cladodes of two Opuntia species and use this information to uncover the microscopic mechanism underlying this phenomenon. We demonstrate that composition of extracted wax and its contact angle do not change significantly with time. Instead, we show that the reported age dependent wetting behavior primarily stems from pinning of the receding contact line along multilayer surface microcracks in the epicuticular wax that expose the underlying highly hydrophilic layers.

Secondary Structure Adopted by the Gly-Gly-X Repetitive Regions of Dragline Spider Silk.

Solid-state NMR and molecular dynamics (MD) simulations are presented to help elucidate the molecular secondary structure of poly(Gly-Gly-X), which is one of the most common structural repetitive motifs found in orb-weaving dragline spider silk proteins. The combination of NMR and computational experiments provides insight into the molecular secondary structure of poly(Gly-Gly-X) segments and provides further support that these regions are disordered and primarily non-β-sheet. Furthermore, the combination of NMR and MD simulations illustrate the possibility for several secondary structural elements in the poly(Gly-Gly-X) regions of dragline silks, including β-turns, 310-helicies, and coil structures with a negligible population of α-helix observed.

Isolation of a bis(imino)pyridine molybdenum(I) iodide complex through controlled reduction and interconversion of its reaction products

Analysis of previously reported [((Ph2PPr)PDI)MoI][I] by cyclic voltammetry revealed a reversible wave at -1.20 V vs. Fc(+/0), corresponding to the Mo(ii)/Mo(i) redox couple. Reduction of [((Ph2PPr)PDI)MoI][I] using stoichiometric K/naphthalene resulted in ligand deprotonation rather than reduction to yield a Mo(ii) monoiodide complex featuring a Mo-C bond to the α-position of one imine substituent, (κ(6)-P,N,N,N,C,P-(Ph2PPr)PDI)MoI. Successful isolation of the inner-sphere Mo(i) monoiodide complex, ((Ph2PPr)PDI)MoI, was achieved via reduction of [((Ph2PPr)PDI)MoI][I] with equimolar Na/naphthalene. This complex was found to have a near octahedral coordination geometry by single crystal X-ray diffraction and electron paramagnetic resonance (EPR) spectroscopy revealed an unpaired Mo-based electron which is highly delocalized onto the PDI chelate core. Attempts to prepare a Mo(i) monohydride complex upon adding NaEt3BH to ((Ph2PPr)PDI)MoI resulted in disproportionation to yield an equimolar quantity of (κ(6)-P,N,N,N,C,P-(Ph2PPr)PDI)MoH and newly identified ((Ph2PPr)PDI)MoH2. Independent preparation of ((Ph2PPr)PDI)MoH2 was achieved by adding 2 equiv. NaEt3BH to [((Ph2PPr)PDI)MoI][I] and a minimum hydride resonance T1 of 176 ms suggests that the Mo-bound H atoms are best described as classical hydrides. Interestingly, ((Ph2PPr)PDI)MoH2 can be converted to (κ(6)-P,N,N,N,C,P-(Ph2PPr)PDI)MoI upon iodomethane addition, while ((Ph2PPr)PDI)MoH2 is prepared from (κ(6)-P,N,N,N,C,P-(Ph2PPr)PDI)MoI in the presence of excess NaEt3BH. Similarly, (κ(6)-P,N,N,N,C,P-(Ph2PPr)PDI)MoI can be converted to (κ(6)-P,N,N,N,C,P-(Ph2PPr)PDI)MoH with 1 equiv. of NaEt3BH, while the opposite transformation occurs following iodomethane addition to (κ(6)-P,N,N,N,C,P-(Ph2PPr)PDI)MoH. Facile interconversion between [((Ph2PPr)PDI)MoI][I], (κ(6)-P,N,N,N,C,P-(Ph2PPr)PDI)MoI, (κ(6)-P,N,N,N,C,P-(Ph2PPr)PDI)MoH, and ((Ph2PPr)PDI)MoH2 is expected to guide future reactivity studies on this unique set of compounds.

Bio micro fuel cell grand challenge final report.

Christopher Apblett, Kent Schubert, Bruce Kelley, Andrew Walker, Blake Simmons, Ted Borek, Stephen Meserole, Todd Alam, Brian Cherry, Greg Roberts, Jim Novak, Jim Hudgens, Dave Peterson, Jason Podgorski, Susan Brozik, Jeb Flemming, Stan Kravitz, David Ingersoll, Steve Eisenbies, Randy Shul, Sarah Rich, Carrie Schmidt, Mike Beggans, Jeanne Sergeant, Chris Cornelius, Cy Fujimoto, Micheal Hickner, Swapnil Chabra, Suzanne Ma, Joanne Volponi, Micheal Kelly, Kevin Zavadil, Chad Staiger, Patricia Dolan, Monica Manginell, Jason Harper, Dan Doughty, Steve Casalnuovo

Hierarchical spidroin micellar nanoparticles as the fundamental precursors of spider silks

Significance The true physical form of the proteins within the silk glands of spiders that permits storage at very high concentrations rather than as precipitated material prior to being transformed into solid silk fibers remains one of the fundamental mysteries that has limited our ability to produce artificial silks of the quality of natural silks. Here we determine that spider silk proteins are stored as complex micellar nanoparticles composed of assembled subdomains. When extruded during the silk spinning process, these subdomains undergo fibrillization while remaining assembled in micelles. Knowledge of the nanostructured protein assemblies in the dope is critical to the basic understanding of the spinning process and to our ability to mimic the natural material properties in synthetic analogues. Many natural silks produced by spiders and insects are unique materials in their exceptional toughness and tensile strength, while being lightweight and biodegradable–properties that are currently unparalleled in synthetic materials. Myriad approaches have been attempted to prepare artificial silks from recombinant spider silk spidroins but have each failed to achieve the advantageous properties of the natural material. This is because of an incomplete understanding of the in vivo spidroin-to-fiber spinning process and, particularly, because of a lack of knowledge of the true morphological nature of spidroin nanostructures in the precursor dope solution and the mechanisms by which these nanostructures transform into micrometer-scale silk fibers. Herein we determine the physical form of the natural spidroin precursor nanostructures stored within spider glands that seed the formation of their silks and reveal the fundamental structural transformations that occur during the initial stages of extrusion en route to fiber formation. Using a combination of solution phase diffusion NMR and cryogenic transmission electron microscopy (cryo-TEM), we reveal direct evidence that the concentrated spidroin proteins are stored in the silk glands of black widow spiders as complex, hierarchical nanoassemblies (∼300 nm diameter) that are composed of micellar subdomains, substructures that themselves are engaged in the initial nanoscale transformations that occur in response to shear. We find that the established micelle theory of silk fiber precursor storage is incomplete and that the first steps toward liquid crystalline organization during silk spinning involve the fibrillization of nanoscale hierarchical micelle subdomains.

High-resolution NMR reveals secondary structure and folding of amino acid transporter from outer chloroplast membrane.

Solving high-resolution structures for membrane proteins continues to be a daunting challenge in the structural biology community. In this study we report our high-resolution NMR results for a transmembrane protein, outer envelope protein of molar mass 16 kDa (OEP16), an amino acid transporter from the outer membrane of chloroplasts. Three-dimensional, high-resolution NMR experiments on the 13C, 15N, 2H-triply-labeled protein were used to assign protein backbone resonances and to obtain secondary structure information. The results yield over 95% assignment of N, HN, CO, Cα, and Cβ chemical shifts, which is essential for obtaining a high resolution structure from NMR data. Chemical shift analysis from the assignment data reveals experimental evidence for the first time on the location of the secondary structure elements on a per residue basis. In addition T 1Z and T2 relaxation experiments were performed in order to better understand the protein dynamics. Arginine titration experiments yield an insight into the amino acid residues responsible for protein transporter function. The results provide the necessary basis for high-resolution structural determination of this important plant membrane protein.