Structural and dynamical characterization of fibrils from a disease-associated alanine expansion domain using proteolysis and solid-state NMR spectroscopy

The nuclear poly(A) binding protein PABPN1 possesses a natural 10 alanine stretch that can be extended to 17 Ala by codon expansion. The expansions are associated with the disease oculopharyngeal muscular dystrophy (OPMD), which is characterized histopathologically by intranuclear fibrillar deposits. Here, we have studied the Ala extended fibrillar N-terminal fragment of PABPN1, (N-(+7)Ala), comprising 152 amino acids. At natural abundance, cross-polarized 13C MAS NMR spectra are dominated by the three Ala signals with characteristic beta-sheet chemical shifts. In contrast, directly polarized 13C MAS spectra show a multitude of narrow lines, suggesting a large portion of highly mobile sites. Proteolytic cleavage of the protein combined with MALDI-TOF mass spectrometry revealed a protease-resistant peptide encompassing residues 13/14 to 50-52 with the poly-Ala stretch in the center. Measurements of the 1H-13Calpha dipolar couplings of 13C/15N-labeled N-(+7)Ala revealed high order parameters of 0.77 for the poly-Ala stretch of the fibril, while the majority of the residues of N-(+7)Ala exhibited very low order parameters between 0.06 and 0.15. Only some Gly residues that are flanking the Ala-rich region had significant order parameters of 0.47. Thus, site-specific dynamic mapping represents a useful tool to identify the topology of fibrillar proteins.     

Total synthesis and structural revision of the marine macrolide neopeltolide

The total synthesis and structural revision of the marine natural product neopeltolide is reported. The key bond-forming step involves a Lewis acid-catalyzed intramolecular cyclization to install the tetrahydropyran ring and the macrocycle simultaneously. This type of cyclization is the first of its kind and assembles the carbon backbone of the natural product efficiently. The synthesis of the reported structure revealed differences in the data between the natural and synthetic material. After significant investigation, the diastereomeric molecule with the C11 and C13 configurations inverted was synthesized using the initial route. This compound matches the data reported for neopeltolide (1H, 13C, HRMS, IR, NOESY, [alpha]), thereby establishing the correct overall structure for this potent macrolide natural product, including the relative and absolute stereochemistry.     

Hydrogen bonding effects on the electronic configuration of five-coordinate high-spin iron(II) porphyrinates

The characterization of a new five-coordinate derivative of (2-methylimidazole)(tetraphenylporphinato)iron(II) provides new and unique information about the effects of forming a hydrogen bond to the coordinated imidazole on the geometric and electronic structure of iron in these species. The complex studied has two crystallographically distinct iron sites; one site has an axial imidazole ligand modified by an external hydrogen bond, and the other site has an axial imidazole ligand with no external interactions. The iron atoms at the two sites have distinct geometric features, as revealed in their molecular structures, and distinct electronic structures, as shown by M?ssbauer spectroscopy, although both are high spin (S = 2). The molecule with the external hydrogen bond has longer equatorial Fe-N(p) bonds, a larger displacement of the iron atom out of the porphyrin plane, and a shorter axial bond compared to its counterpart with no hydrogen bonding. The M?ssbauer features are distinct for the two sites, with differing quadrupole splitting and isomer shift values and probably differing signs for the quadrupole splitting as shown by variable-temperature measurements in applied magnetic field. These features are consistent with a significant change in the nature of the doubly populated d orbital and are all in the direction of the dichotomy displayed by related imidazole and imidazolate species where deprotonation leads to major differences. The results points out the possible effects of strong hydrogen bonding in heme proteins.     

Lewis acid-catalyzed conjugate additions of silyloxyallenes: a selective solution to the intermolecular Rauhut-Currier problem

Silyloxyallenes serve as highly useful alpha-acylvinyl anion equivalents. These latent allenolates undergo conjugate additions to alkylidene malonates in the presence of 10 mol % Sc(OTf) 3. The reaction delivers intermolecular Rauhut-Currier products in excellent yields and regioselectivities for a wide scope of substrates. Notably, the formal cross-coupling of two different alpha,beta-unsaturated carbonyl compounds (a cross Rauhut-Currier reaction) is achieved. Preliminary investigations have demonstrated good levels of enantioselectivity for the addition of a racemic silyloxyallene with a chiral Lewis acid.     

Enantioselective Friedel-Crafts alkylations catalyzed by bis(oxazolinyl)pyridine-scandium(III) triflate complexes

The enantioselective Friedel-Crafts addition of a variety of indoles catalyzed by bis(oxazolinyl)pyridine-scandium(III) triflate complexes (Sc(III)-pybox) was accomplished utilizing a series of beta-substituted alpha,beta-unsaturated phosphonates and alpha,beta-unsaturated 2-acyl imidazoles. The acyl phosphonate products were efficiently transformed into esters and amides, whereas the acyl imidazole adducts were converted to a broader spectrum of functionalities such as esters, amides, carboxylic acids, ketones, and aldehydes. The sense of stereoinduction and level of enantioselectivity were found to be functions of the size of the substrate employed, the substitution on the ligand, and the catalyst loading. Molecular modeling of the catalyst with the bound substrates was performed based on the crystal structures of the catalyst complexes and the sense of stereoinduction observed in the addition reaction. Nonlinear effects over a range of catalyst concentrations implicate a mononuclear complex as the active catalyst.     

Anion chelation by amido acid functionalized fused quartz/water interfaces studied by nonlinear optics

We report resonantly enhanced surface second harmonic generation (SHG) measurements to track the interaction of the EPA priority toxic metal pollutant chromium(VI) with fused quartz/water interfaces containing tailor-made amino acids that serve as model systems for environmental and biological interfaces. chi3 measurements of amido acid functionalized fused quartz/water interfaces are consistent with two acid-base equilibria, suggesting the formation of a laterally hydrogen-bonded environment similar to what is observed for aliphatic carboxylic acids. Chromate adsorption isotherms recorded at pH 7 are suggestive of an intramolecular chelation mechanism that becomes important when four or more hydrogen-bonding moieties are displayed toward the incoming chromate. The strong binding affinities of the amido acid functionalized fused quartz/water interfaces toward chromate are consistent with nearly 50% slower transport rates with respect to free-flowing groundwater, indicating that, in the absence of redox processes, peptide materials in heterogeneous geochemical environments can significantly increase chromate residence times. The strong evidence for synergistic effects dominating the interactions of chromate with surface-bound amido acids indicates that chemical complexity can be systematically addressed using tailor-made organic surfaces and interfaces.