Final structure model and hydrogen bond networks of dihydrate β-chitin.

<p>It is seen along the chain direction and ab projection.</p

Superposition of dihydrate β-chitin (cyan backbone and cell) and anhydrous structure (green).

<p>Superposition of dihydrate β-chitin (cyan backbone and cell) and anhydrous structure (green).</p

Hydrogen bonding geometry.

<p>Hydrogen bonding geometry.</p

Direct Determination of the Hydrogen Bonding Arrangement in Anhydrous β-Chitin by Neutron Fiber Diffraction

The hydrogen bonding arrangement in anhydrous β-chitin, a homopolymer of <i>N</i>-acetylglucosamine, was directly determined by neutron fiber diffraction. Data were collected from a sample prepared from the bathophilous tubeworm Lamellibrachia satsuma in which all labile hydrogen atoms had been replaced by deuterium. Initial positions of deuterium atoms on hydroxyl and acetamide groups were directly located in Fourier maps synthesized using phases calculated from the X-ray structure and amplitudes measured from the neutron data. The hydrogen bond arrangement in the refined structure is in general agreement with predictions based on the X-ray structure: O3 donates a hydrogen bond to the O5 ring oxygen atom of a neighboring residue in the same chain; N2 and O6 donate hydrogen bonds to the same carbonyl oxygen O7 of an adjacent chain. The intramolecular O3···O5 hydrogen bond has the most energetically favorable geometry with a hydrogen to acceptor distance of 1.77 Å and a hydrogen bond angle of 171°

X-ray Fourier omit maps.

<p>Section through the X-ray σ_A map (showing positive density in blue) and the X-ray Fo-Fc omit map (showing positive and negative density in green and red, respectively). (a) Calculated using only the rigid backbone (omitting O6, C7, C8 and O7) as phasing model. Density indicated by circles can be associated with the hydroxymethyl oxygen O6 and two water molecules Ow1 and Ow2. (b) Calculated after introduction of O6 and the water oxygen atoms. Density indicated by circles can be associated with acetamide C7. (c) Calculated after further addition of C7. Density indicated by circles can be associated with C8, and O7. (d) Calculated with the complete molecular model (excluding hydrogen).</p

Conformational parameters of anhydrous β-chitin and dihydrate from the X-ray model.

<p>Conformational parameters of anhydrous β-chitin and dihydrate from the X-ray model.</p

Neutron Fourier omit maps.

<p>Section through the neutron σ_A map (showing positive density in blue) and the Fo-Fc omit map (showing positive and negative density in green and red, respectively) after (a) removing all deuterium atoms (b) adding DO3, DN2 and DO6 at position A (c) further adding the deuterium atoms attached to Ow2 (d) further adding the deuterium atoms attached to Ow1 (e) adding DO3, DN2 and DO6 at position B (f) further adding the deuterium atoms attached Ow1 and Ow2. In (f) small residual peaks can still be seen near Ow2.</p

On the Reliability of C−H···O Interactions in Crystal Engineering: Synthesis and Structure of Two Hydrogen Bonded Phosphonium Bis(aryloxide) Salts

Two related phosphonium aryloxides, [Ph3PCH3]+2[CH2(C6H2-3,5-tert-butyl-4-O)2]2-, 1a, and [Ph3PC2H5]+2[CH2(C6H2-3,5-tert-butyl-4-O)2]2-, 1b have been synthesized by protonation of nonstabilized phosphorus ylides with 4,4‘-methylenebis(2,6-di-tert-butylphenol) LH2. The crystal structures of 1a and 1b have been determined by single-crystal X-ray diffraction, and that of 1b has also been determined by low-temperature (20 K) neutron diffraction. Both crystallize as CH3CN solvates, and both exhibit polymeric supramolecular structures via extensive C−H···O hydrogen bonding. The predominant structural motif is chelation of the aryloxide oxygen atom of the anion by phosphonium alkyl and aryl C−H groups, such as has been previously observed for phosphonium monoaryloxide salts. The use of this motif as a supramolecular synthon in crystal engineering is explored. In the case of 1a, in addition to the expected intermolecular C−H···O hydrogen bonding pattern, C−H···π interactions between the anion and this solvent are observed. In the structure of 1b, C−H···π interactions between the cation and anion lead to an unpredicted supramolecular structure. The possible significance of this feature is discussed the context of the general use of weak hydrogen bonds in crystal engineering

Neutron Diffraction Study of a Phenol·Nitroxide Radical Adduct: A Structural Model for Hydrogen Atom Abstraction by Peroxyl Radicals from Vitamin E and Related Phenolic Antioxidants

Neutron Diffraction Study of a Phenol·Nitroxide Radical Adduct:  A Structural Model for Hydrogen Atom Abstraction by Peroxyl Radicals from Vitamin E and Related Phenolic Antioxidant

Self-Assembly of an Aspartate-Rich Sequence from the Adenovirus Fiber Shaft: Insights from Molecular Dynamics Simulations and Experiments

The self-assembly of short peptides into fibrous nanostructures (such as fibrils and tubes) has recently become the subject of intense theoretical and experimental scrutiny, as such assemblies are promising candidates for nanobiotechnological applications. The sequences of natural fibrous proteins may provide a rich source of inspiration for the design of such short self-assembling peptides. We describe the self-assembly of the aspartate-rich undecapeptide (NH₃⁺-LSG­SDS­DTL­TV-NH₂), a sequence derived from the shaft of the adenovirus fiber. We demonstrate that the peptide assembles experimentally into amyloid-type fibrils according to widely accepted diagnostic criteria. In addition, we investigate an aqueous solution of undecapeptides by molecular dynamics simulations with an implicit (GB) solvent model. The peptides are frequently arranged in intermolecular β-sheets, in line with their amyloidogenic propensity. On the basis of both experimental and theoretical insights, we suggest possible structural models of the fibrils and their potential use as scaffolds for templating of inorganic materials