Time Optimal Control of Coupled Qubits Under Non-Stationary Interactions

In this article, we give a complete characterization of all the unitary transformations that can be synthesized in a given time for a system of coupled spin-1/2 in presence of general time varying coupling tensor. Our treatment is quite general and our results help to characterize the reachable set at all times for a class of bilinear control systems with time varying drift and unbounded control amplitude. These results are of fundamental interest in geometric control theory and have applications to control of coupled spins in solid state NMR spectroscopy.Comment: 4 page

Conformationally selective multidimensional chemical shift ranges in proteins from a PACSY database purged using intrinsic quality criteria

We have determined refined multidimensional chemical shift ranges for intra-residue correlations ([superscript 13]C–[superscript 13]C, [superscript 15]N–[superscript 13]C, etc.) in proteins, which can be used to gain type-assignment and/or secondary-structure information from experimental NMR spectra. The chemical-shift ranges are the result of a statistical analysis of the PACSY database of >3000 proteins with 3D structures (1,200,207 [superscript 13]C chemical shifts and >3 million chemical shifts in total); these data were originally derived from the Biological Magnetic Resonance Data Bank. Using relatively simple non-parametric statistics to find peak maxima in the distributions of helix, sheet, coil and turn chemical shifts, and without the use of limited “hand-picked” data sets, we show that ~94 % of the [superscript 13]C NMR data and almost all [superscript 15]N data are quite accurately referenced and assigned, with smaller standard deviations (0.2 and 0.8 ppm, respectively) than recognized previously. On the other hand, approximately 6 % of the [superscript 13]C chemical shift data in the PACSY database are shown to be clearly misreferenced, mostly by ca. −2.4 ppm. The removal of the misreferenced data and other outliers by this purging by intrinsic quality criteria (PIQC) allows for reliable identification of secondary maxima in the two-dimensional chemical-shift distributions already pre-separated by secondary structure. We demonstrate that some of these correspond to specific regions in the Ramachandran plot, including left-handed helix dihedral angles, reflect unusual hydrogen bonding, or are due to the influence of a following proline residue. With appropriate smoothing, significantly more tightly defined chemical shift ranges are obtained for each amino acid type in the different secondary structures. These chemical shift ranges, which may be defined at any statistical threshold, can be used for amino-acid type assignment and secondary-structure analysis of chemical shifts from intra-residue cross peaks by inspection or by using a provided command-line Python script (PLUQin), which should be useful in protein structure determination. The refined chemical shift distributions are utilized in a simple quality test (SQAT) that should be applied to new protein NMR data before deposition in a databank, and they could benefit many other chemical-shift based tools.National Institutes of Health (U.S.) (Grant GM066976

Structure of the amantadine binding site of influenza M2 proton channels in lipid bilayers

The M2 protein of influenza A virus is a membrane-spanning tetrameric proton channel targeted by the antiviral drugs amantadine and rimantadine1. 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 amino-terminal half of the pore2, indicating a physical occlusion mechanism for inhibition. However, a solution NMR structure of M2(18–60) showed four rimantadines bound to the carboxy-terminal lipid-facing surface of the helices3, 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

Nanoscale morphology of polyanhydride copolymers

The microphase separation in polyanhydride random copolymers composed of 1,6-bis(p-carboxyphenoxy)hexane and sebacic acid is described. Though the copolymers are random, the monomers are sufficiently long and the segment-segment interaction parameter is sufficiently high to promote microphase separation when the copolymer is rich in one component. Solid-state NMR spin diffusion experiments and synchrotron small-angle X-ray scattering are used to discern the length scales of the microphase separation. Both techniques reveal a nanostructure with domain sizes less than 25 Ã…. This nanostructure is compared to approximate calculations of chain dimensions based on a random coil model and discussed in the context of the rational design of these materials for drug delivery applications

Exploring water-soluble organic aerosols structures in urban atmosphere using advanced solid-state 13C NMR spectroscopy

Water-soluble organic matter (WSOM) in air particles has profound effects on climate and human health. At the heart of this environmental significance of WSOM lies a complex set of compounds, of which a major fraction still often remains undeciphered. Yet, not all environmental problems require delving into the molecular-level identification of WSOM constituents. Understanding the contribution of different functional groups to whole aerosol WSOM composition offers a highly important structural dataset that enables a better representation of WSOM in climate studies. For the first time, advanced solid-state 13C nuclear magnetic resonance (NMR) techniques, including nearly quantitative 13C multiple cross polarization/magic angle spinning (multiCP/MAS), multiCP/MAS with dipolar dephasing, multiCP/MAS with 13C chemical shift anisotropy filter, and two-dimensional 1H−13C heteronuclear correlation (2D HETCOR), are applied to acquire an accurate quantitative structural description of whole aerosol WSOM collected in an urban atmosphere. Two urban aerosol WSOM samples collected in two short periods of time, under different wintry weather conditions, were investigated. NMR data successfully pinpointed the variability of whole aerosol WSOM composition, allowing to suggest source-specific structural characteristics for each sample in two short periods of time. A new structural model of urban aerosol WSOM was build based on this compositional data, showing the presence of three independent classes of compounds that vary both in content and molecular diversity within short periods of time: heteroatom-rich aliphatic (either chain or branched), carbohydrate-like moieties, and highly substituted aromatic units. These findings establish advanced solid-state NMR as a promising tool for probing the chemical structures of inhomogeneous aerosol WSOM in rapidly changing atmospheric conditions, allowing to resolve discrepancies between modeled and measured aerosol WSOM.Fulbright Scholar Program; NSF (Award No. 1726346).publishe

Synthesis and characterization of ionic block copolymer templated calcium phosphate nanocomposites

Self-assembling thermo-reversibly gelling anionic and zwitterionic pentablock copolymers were used as templates for precipitation of calcium phosphate nanostructures, controlling their size and ordered structural arrangement. Calcium and phosphate ions were dissolved in a block-copolymer micellar dispersion at low temperatures. Aging at ambient temperature produced inorganic nanoparticles, presumably nucleated by ionic interactions. The self-assembled nanocomposites were characterized by small-angle X-ray and neutron scattering (SAXS/SANS), nuclear magnetic resonance (NMR), thermogravimetric analysis (TGA), and transmission electron microscopy (TEM). 1H-31P NMR with 1H spin diffusion from polymer to phosphate proved the formation of nanocomposites, with inorganic particle sizes from ∼2 nm, characterized by 1H-31P dipolar couplings, to \u3e 100 nm. TEM analysis showed polymer micelles surrounded by calcium phosphate. SAXS attested that a significant fraction of the calcium phosphate was templated by the polymer micelles. SANS data indicated that the order of the polymer was enhanced by the inorganic phase. The nanocomposite gels exhibited higher moduli than the neat polymer gels. The calcium phosphate was characterized by TGA, X-ray diffraction, high-resolution TEM, and various NMR techniques. An unusual crystalline phase with \u3e2 chemically and \u3e3 magnetically inequivalent HPO4 2- ions was observed with the zwitterionic copolymer, highlighting the influence of the polymer on the calcium phosphate crystallization. The inorganic fraction of the nanocomposite was around 30 wt % of the dried hydrogel. Thus, a significant fraction of calcium phosphate has been templated by the tailored self-assembling ionic block copolymers, providing a bottom-up approach to nanocomposite synthesis

Mechanism for Selective Binding of Aromatic Compounds on Oxygen-Rich Graphene Nanosheets Based on Molecule Size/Polarity Matching

Selective binding of organic compounds is the cornerstone of many important industrial and pharmaceutical applications. Here, we achieved highly selective binding of aromatic compounds in aqueous solution and gas phase by oxygen-enriched graphene oxide (GO) nanosheets via a previously unknown mechanism based on size matching and polarity matching. Oxygen-containing functional groups (predominately epoxies and hydroxyls) on the nongraphitized aliphatic carbons of the basal plane of GO formed highly polar regions that encompass graphitic regions slightly larger than the benzene ring. This facilitated size match–based interactions between small apolar compounds and the isolated aromatic region of GO, resulting in high binding selectivity relative to larger apolar compounds. The interactions between the functional group(s) of polar aromatics and the epoxy/hydroxyl groups around the isolated aromatic region of GO enhanced binding selectivity relative to similar-sized apolar aromatics. These findings provide opportunities for precision separations and molecular recognition enabled by size/polarity match–based selectivity