Sub-Ångstrom structure of collagen model peptide (GPO)10 shows a hydrated triple helix with pitch variation and two proline ring conformations.

Collagens are large structural proteins that are prevalent in mammalian connective tissue. Peptides designed to include a glycine-proline-hydroxyproline (GPO) amino acid triad are biomimetic analogs of the collagen triple helix, a fold that is a hallmark of collagen-like sequences. To inform the rational engineering of collagen-like peptides and proteins for food systems, we report the crystal structure of the (GPO)10 peptide at 0.89-Å resolution, solved using direct methods. We determined that a single chain in the asymmetric unit forms a pseudo-hexagonal network of triple helices that have a pitch variation consistent with the model 7/2 helix (3.5 residues per turn). The proline rings occupied one of two states, while the helix was found to have a well-defined hydration shell involved in the stabilization of the inter-helix crystal network. This structure offers a new high-resolution basis for understanding the hierarchical assembly of native collagens, which will aid the food industry in engineering new sustainable food systems.

Selective Anion Recognition by a Dynamic Quadruple Helicate.

An M2 L4 quadruple helicate, formed by wrapping four molecules of 1,4-bis(3-pyridyloxy)benzene (L1 ) about two palladium(II) centers, is shown to bind anions within its internal cavity. 1 H NMR exchange experiments provide a quantitative measure of anion selectivity and reveal a preference for ClO4 - over the other tetrahedral anions BF4 - and ReO4 - and the octahedral anion PF6 - . X-ray crystal structures of [Pd2 (L1 )4 ]4+ helicates containing ClO4 , BF4 - and I- reveal that the cavity size can dynamically change in response to the size of the guest.

Synthesis and direct C2 functionalization of imidazolium and 1,2,4-triazolium N-imides.

Pd-catalyzed direct C2 arylation and Cu-catalyzed direct one-pot alkynylation/intramolecular cyclization of azolium N-imides are reported. Various acetylenes, aryl iodides, and 1-alkyl substituents were examined. The mild protocol allows direct C2 arylation of azolium N-imides without the use of specialized reagents together with novel one-pot regioselective preparations of imidazole-pyrazolo and pyrazolo-1,2,4-triazole ring systems. The electronic properties of selected examples were examined by fluorescence spectroscopy.

One-pot decarboxylative acylation of N-, O-, S-nucleophiles and peptides with 2,2-disubstituted malonic acids.

Monocarbonyl activation of 2,2-disubstituted malonic acids with benzotriazole leads to decarboxylation of one of the carboxy groups and formation of a CH bond. Intermediate carbonyl benzotriazoles then readily acylate nucleophilic reagents and peptides resulting in libraries of conjugates and peptidomimetics.

Gabapentin hybrid peptides and bioconjugates.

Synthetic approaches to gabapentin bioconjugates that overcome the tendency of gabapentin to cyclize into its γ-lactam are studied. Gabapentin was converted by N-acylation at its N-terminus into di-, tri-, and tetrapeptides (L-Ala-Gbp, L-Val-Gbp, L-Ala-L-Phe-Gbp, Gly-L-Ala-ß-Ala-Gbp). Carboxyl-activated Boc-protected gabapentin was used to N-, O-, and S-acylate small peptides and hormones to give conjugates that could also provide prodrugs containing conformationally constrained gabapentin units.

Oxyazapeptides: synthesis, structure determination, and conformational analysis.

Herein we report the synthesis, X-ray structure determination, and conformational analysis of a novel class of heteroatom-modified peptidomimetics, which we shall call "oxyazapeptides". Substituting the typical native N-C(α) bond with an O-N(α) bond creates a completely new, previously unknown family of peptidomimetics, which are hydrolytically stable and display very interesting conformational behavior. Force field calculations revealed that the barrier to rotation around the O-N(α) bond in oxyazapeptides is five times lower than that around the N-N(α) bond in azapeptides. Also, conformational analysis supported by X-ray suggests that the oxyaza moiety can effectively induce ß-turns, which can make the newly discovered oxyazapeptide scaffold a useful tool for drug discovery and for design of biologics.

Push-pull triazenes derived from 1-(benzylideneamino)- and 1-(sulfonimido)-azolylidenes.

In-situ-generated neutral 1-(benzylideneamino)- and novel anionic 1-(sulfonimido)-azolylidenes react with organic azides to afford diverse classes of push-pull triazenes and triazene salts. The scope of the heterocyclic core and substituents at the N1 and N3 positions of NHC precursors together with the thermal properties of resulting compounds were examined.

A new benzotriazole-mediated stereoflexible gateway to hetero-2,5-diketopiperazines.

Open chain Cbz-L-aa(1)-L-Pro-Bt (Bt=benzotriazole) sequences were converted into either the corresponding trans- or cis-fused 2,5-diketopiperazines (DKPs) depending on the reaction conditions. Thermodynamic tandem cyclization/epimerization afforded selectively the corresponding trans-DKPs (69-75%). Complementarily, tandem deprotection/cyclization led to the cis-DKPs (65-72%). A representative set of proline-containing cis- and trans-DKPs has been prepared. A mechanistic investigation, based on chiral HPLC, kinetics, and computational studies enabled a rationalization of the results.

Benzotriazole-mediated syntheses of depsipeptides and oligoesters.

Reactions of O-Pg(α-hydroxyacyl)benzotriazoles with (a) unprotected α-hydroxycarboxylic acids, (b) amino acids, and (c) amines afforded, respectively, chirally pure (a) oligoesters, (b) depsidipeptides, and (c) amide conjugates (yields 52-94%). N-Pg(α-Aminoacyl)benzotriazoles reacted with α-hydroxycarboxylic acids to yield depsidipeptides (47-87%). N-Pg(depsidipeptidoyl)benzotriazoles, obtained from depsidipeptides, gave depsitripeptides (yields 55-78%) on reaction with amino acids and α-hydroxycarboxylic acids. O-Acylation of α-hydroxycarboxylic acids with N-Pg(α-aminoacyl)benzotriazoles followed by deprotection produced unprotected depsides useful for the preparation of depsitripeptides.

Carbene-mediated transformations of 1-(benzylideneamino)benzimidazoles.

Carbene-mediated transformations of N-(3-butylbenzimidazol-3-ium-1-yl)-1-arylmethanimine iodides with carbon disulfide and benzoyl isothiocyanate gave the corresponding NHC·CS(2) betaines in 68-85% and benzoyl-[1-butyl-3-[(E)-(aryl)methyleneamino]benzimidazol-1-ium-2-carbothioyl]azanides, respectively, in 74-85% yields. However, reaction with excess isopropyl isothiocyanate in NaH/THF at room temperature yielded the 1-butyl-1',3'-diisopropyl-3-[(E)-(aryl)methyleneamino]spiro[benzimidazole-2,5'-imidazolidine]-2',4'-dithiones (74-77%).

Cycloaddition of homochiral dihydroimidazoles: a 1,3-dipolar cycloaddition route to optically active pyrrolo[1,2-a]imidazoles.

N-Alkylation of optically active 1-benzyl-4-phenyl-4,5-dihydroimidazole with active alkyl halides and treatment of the so-formed 4,5-dihydroimidazolium ions with DBU in the presence of a range of electron-deficient alkene dipolarophiles, constitutes a 'one-pot' cascade terminating in a 1,3-dipolar cycloaddition reaction that affords optically active pyrrolo[1,2-a]imidazoles. Three bonds of the so-formed pyrrolidine moiety are constructed in this cascade. The cycloaddition follows an endo approach of dipole and dipolarophile with anti geometry of the dipole and facial selectivity derived from the phenyl substituent. Inter- and intramolecular cycloaddition modes are observed.

Benzotriazol-1-yl-sulfonyl azide for diazotransfer and preparation of azidoacylbenzotriazoles.

Benzotriazol-1-yl-sulfonyl azide, a new crystalline, stable, and easily available diazotransfer reagent provides N-(α-azidoacyl)benzotriazoles convenient for N-, O-, C- and S-acylations. The efficient syntheses of various amides, azido protected peptides, esters, ketones and thioesters is reported together with a wide range of azides (including α-azido acids from α- amino acids in partially aqueous conditions) and diazo compounds.

NMR study of the tautomeric behavior of N-(α-aminoalkyl)tetrazoles.

N-(α-Aminoalkyl)tetrazoles exist in solution as equilibrium mixtures of N1 and N2 tautomers. The position of equilibrium depends significantly on the polarity of the solvent and the substituents in the tetrazole ring. Interconversion between individual tautomers is shown to proceed via tight ion-pair intermediates in which intramolecular recombination is faster than the intermolecular crossover since the latter probably requires solvent separation of ion-pair intermediates.

catena-Poly[[silver(I)-µ-4-aminopyridine] perchlorate]: a 1-D staircase coordination polymer.

Reaction of 4-amino-pyridine with silver(I) perchlorate leads to a one-dimensional coordination polymer, {[Ag(C(5)H(6)N(2))]ClO(4)}(n), in which the amino-pyridine binds through both N atoms. The perchlorate anion is hydrogen bonded to the amino H atoms and inter-acts weakly with the silver(I) atoms (Ag-O > 2.70 Å), both located on inversion centres, and some aromatic H atoms (O-H > 2.55 ÅA), thereby extending the dimensionality of the assembly. This is the first silver complex in which this ligand acts in a bridging mode.

Bis(2-acetylpyridine-κN,O)silver(I) tetra-fluoridoborate: a complex with silver in a seesaw coordination geometry.

The reaction of 2-acetylpyridine with silver(I) tetra-fluorido-borate leads to the discrete title complex, [Ag(C(7)H(7)NO)(2)]BF(4), in the cation of which the Ag atom is coordinated by two 2-acetylpyridine ligands, each of which is N,O-bidentate, albeit with stronger bonding to the N atoms [Ag-N = 2.2018 (15) and 2.2088 (14) Å; Ag-O = 2.5380 (13) and 2.5454 (13) Å]. The four-coordinate Ag atom has a seesaw coordination geometry with a τ(4) index of 0.51. The tetra-fluoridoborate anion is disordered over two orientations with 0.568 (10):0.432 (10) occupancies.

Tautomerism of guanidines studied by (15)N NMR: 2-hydrazono-3-phenylquinazolin-4(3H)-ones and related compounds.

2-Hydrazono-3-phenylquinazolin-4(3H)-ones 11a-i are shown by (15)N NMR to exist in DMSO solution predominantly as the imino tautomers B and not the amino tautomers A. 2-Hydrazino-benzimidazole derivative 12 and 2-hydrazino-4,6-dimethylpyrimidine derivative 13 were found to exist predominantly as the amino tautomers.

Selective synthesis and structural elucidation of S-acyl- and N-acylcysteines.

N-(Acyl)-1H-benzotriazoles 6a-f react with l-cysteine 5 at 20 degrees C to give exclusively (i) N-acyl-l-cysteines 8a-e in the presence of triethylamine in CH(3)CN-H(2)O (3:1), but (ii) S-acyl-l-cysteines 7a-e in CH(3)CN-H(2)O (5:1) in the absence of base. Structures 7b, 7d and 8b, 8d are supported by 2D NMR spectroscopic methods including gDQCOSY, gHMQC, gHMBC, and (1)H-(15)N CIGAR-gHMBC experiments. The structure of compound 8d was also supported by single-crystal X-ray diffraction.

4,5-di(2-pyridyl)-1,2,3-triazolate: the elusive member of a family of bridging ligands that facilitate strong metal-metal interactions.

The new click-adduct 4,5-di(2-pyridyl)-1,2,3-triazole acts as a doubly-chelating anionic bridging ligand that forms dinuclear ruthenium(II) complexes which exhibit strong metal-metal interactions.

Molecular fibers and wires in solid-state and solution self-assemblies of cyclodextrin [2]rotaxanes.

Cyclodextrin [2]rotaxanes have been prepared by coupling dimethylanilines with dicarboxylic acids using DMT-MM, in aqueous solutions of alpha-cyclodextrin, and the example illustrated shows unusual fluorescence emission and other spectroscopic behavior characteristic of the formation of molecular wires in solution, similar to the fibers observed in the solid state.

3,3'-Dibromo-5,5'-di-tert-butyl-2,2'-dimethoxy-biphen-yl.

The title compound, C(22)H(28)Br(2)O(2), crystallizes in a staggered arrangement to minimize the inter-actions of its ortho substituents, with a dihedral angle of 84.2 (3)° between the two aromatic rings. Short C-H⋯O hydrogen-bonding inter-actions between meth-oxy groups result in a one-dimensional polymeric chain of mol-ecules lying parallel to the b axis. One tert-butyl group is disordered equally over two positions.