Self-assembly of β-turn forming synthetic tripeptides into supramolecular β-sheets and amyloid-like fibrils in the solid state

We have described here the self-assembling properties of the synthetic tripeptides Boc-Ala(1)-Aib(2)-Val(3)-OMe 1, Boc-Ala(1)-Aib(2)-Ile(3)-OMe 2 and Boc-Ala(1)-Gly(2)-Val(3)-OMe 3 (Aib=α-amino isobutyric acid, β-Ala=β-alanine) which have distorted β-turn conformations in their respective crystals. These turn-forming tripeptides self-assemble to form supramolecular β-sheet structures through intermolecular hydrogen bonding and other noncovalent interactions. The scanning electron micrographs of these peptides revealed that these compounds form amyloid-like fibrils, the causative factor for many neurodegenerative diseases including Alzheimer's disease, Huntington's disease and Prion-related encephalopathies.     

Syntheses and evaluation of fluorinated benzothiazole anilines as potential tracers for β-amyloid plaques in Alzheimer's disease

[¹¹C]2-(4′-(Methylamino)phenyl)-6-hydroxybenzothiazole ([¹¹C]PIB) is a most potential PET tracer for detecting the β-amyloid plaques in Alzheimer's disease. Here the syntheses of three fluorinated PIB, namely 2-(4′-(methylamino)phenyl)-6-fluoroethoxybenzothiazole (O-FEt-PIB), 2-(4′-(methylamino)phenyl)-6-fluoro-benzothiazole (F-N-Me) and 2-(4′-(dimethylamino)phenyl)-6-fluorobenzo-thiazole (F-N,N-Me), and the radiosynthesis of one corresponding ¹⁸F-labeled PIB compound, [¹⁸F]O-FEt-PIB, as well as their in vitro/in vivo biological characters were reported. The structures of the products were confirmed by IR, ¹H NMR, EI/ESI-MS, elemental analysis and HRMS techniques. The radiolabeled product was characterized by radio-TLC and radio-HPLC and purified by semi-preparative radio-HPLC. The suitable biological characters showed these tracers were potential to be developed as probes for detecting β-amyloid plaques in Alzheimer's disease.     

Synthesis of difluorinated β-ketosulfones and novel gem-difluoromethylsulfone-containing heterocycles as fluorinated building blocks

A series of new heterocyclic β-ketosulfides was prepared by the reaction of the corresponding heterocyclic thiols with α-bromoacetophenone and its derivatives. Oxidation of the products using m-CPBA gave the corresponding heterocyclic β-ketosulfones, which, on treatment with Selectfluor™ under anhydrous condition underwent electrophilic fluorination resulting in new heterocycles with difluoromethylene moiety adjacent to the sulfur atom and the carbonyl group. Base-induced cleavage of the five types of the resulting products, with different heterocyclic moieties as model compounds, afforded the difluoromethyl sulfones attached to the corresponding heterocycles. They can be considered as interesting fluorinated building blocks for further elaborations.     

Chemical states of radioactive lanthanide atoms in Pr and Nd fluorinated β-diketonates

An investigation has been carried out of the chemical behaviour of radioactive lanthanide atoms formed in Pr and Nd trifluoroacetylacetonates and in Pr and Nd hexafluoroacetylacetonates irradiated with 660 MeV protons, and of those resulting from the implantation of the Ln¹⁽¹⁺⁾ ions with an energy of 40 keV into crystalline Nd(HFA)₃. The experimental conditions, in particular, the chemical composition of the medium and the state of the crystalline lattice, appear to be important factors affecting the chemical reactions of the Ln¹ atoms and the forms in which they are stabilized. The behaviour of the Ln¹ atoms introduced into crystalline Nd(HFA)₃ in different ways (by proton irradiation or ion implantation) indicates that it is independent of their energy.     

Molecular Recognition of Immobilized Supramolecular Complex of Ironporphyrin and β-CD Polymer as an Analogue for Peroxidase

The immobilized supramolecular inclusion complex of FeTPPS4 and -CD polymer was applied as a mimesis of peroxidase, and its molecular recognition for twenty substrates was studied. As the space structure;CD interior cavities, high selectivity was obtained by immobilized mimetic enzyme. p-Chlorophenic acid was identified as an optimal substrate in the system tested.     

Difference in formation mechanism of inclusion complex between configuration isomers of gallate-type catechin and β-cyclodextrin

Journal of Thermal Analysis and Calorimetry - The formation mechanism of inclusion complex between (-)-epigallocatechin gallate (EGCg), which is the main catechin in tea leaves, or...     

Study on the Supramolecular Interaction of β-Cyclodextrin with Gemfibrozil by Spectrofluorimetry and Its Analytical Application

The supramolecular interaction of gemfibrozil with β-cyclodextrin （β-CD） was studied by spectrofluorimetry. The mechanism of the inclusion was discussed by spectrofluoremetry, infrared spectrum and ^1H NMR spectrum. The results showed that a 1 ： 1 （β-CD ： gemfibrozil） complex was formed with an apparent association constant of 3.844 × 10^3 L·mol^-1. Based on the enhancement of the fluorescent intensity of gemfibrozil, a spectrofluorimetric method for the determination of gemfibrozil in bulk aqueous solution in the presence of β-CD was developed. The linear range was 3.30 ng·mL^- 1 -6.00 ug·mL^-1 with the detection limit of 0.980 ng·mL^-1. There was no interference from the excipients normally used in tablet composition and the serum main compositions. The proposed method was then successfully applied to the determination of gemfibrozil in capsules and serum.     

Supramolecular helix and β-sheet through self-assembly of two isomeric tetrapeptides in crystals and formation of filaments and ribbons in the solid state

Single crystal X-ray diffraction studies show that the β-turn structure of tetrapeptide I, Boc-Gly-Phe-Aib-Leu-OMe (Aib: α-amino isobutyric acid) self-assembles to a supramolecular helix through intermolecular hydrogen bonding along the crystallographic a axis. By contrast the β-turn structure of an isomeric tetrapeptide II, Boc-Gly-Leu-Aib-Phe-OMe self-assembles to a supramolecular β-sheet-like structure via a two-dimensional (a, b axis) intermolecular hydrogen bonding network and π-π interactions. FT-IR studies of the peptides revealed that both of them form intermolecularly hydrogen bonded supramolecular structures in the solid state. Field emission scanning electron micrographs (FE-SEM) of the dried fibrous materials of the peptides show different morphologies, non-twisted filaments in case of peptide I and non-twisted filaments and ribbon-like structures in case of peptide II.     

Supramolecular assembly of a new squaraine and β-cyclodextrin for detection of thiol-containing amino acids in water

A new unsymmetrical aniline-based squaraine (SQ2) bearing binding unit of Hg²⁺ ion was designed and synthesised. SQ2 can form 1:2 inclusion complex with β-cyclodextrin, and the resulting complex, which undergoes absorption and fluorescence bleaching upon binding Hg²⁺, can serve as a turn-on colorimetric or fluorescent chemosensor in organic solvent-free aqueous solution for thiol-containing amino acids with high selectivity and tunable measuring range.     

Molybdenum(VI) dioxo and oxo-imido complexes of fluorinated β-ketiminato ligands and their use in OAT reactions

Substitution of a methyl by a trifluoromethyl moiety in well-known β-ketimines afforded the ligands (Ar)NC(Me)CH(2)CO(CF(3)) (HL(H), Ar = C(6)H(5); HL(Me), A r= 2,6-Me(2)C(6)H(3); HL(iPr), Ar = 2,6-(i)Pr(2)C(6)H(3)). Subsequent complexation to the [MoO(2)](2+) core leads to the formation of novel complexes of general formula [MoO(2)(L(R))(2)] (R = H, 1; R = Me, 2; R = iPr, 3). For reasons of comparison the oxo-imido complex [MoO(N(t)Bu)(L(Me))(2)] (4) has also been synthesized. Complexes 1-4 were investigated in oxygen atom transfer (OAT) reactions using the substrate trimethylphosphine. The respective products after OAT, the reduced Mo(IV) complexes [MoO(PMe(3))(L(R))(2)] (R = H, 5; R = Me, 6; R = iPr, 7) and [Mo(N(t)Bu)(PMe(3))(L(Me))(2)] (8), were isolated. All complexes have been characterized by NMR spectroscopy, and 1-4 also by cyclic voltammetry. A positive shift of the Mo(VI)-Mo(V) reduction wave upon fluorination was observed. Furthermore, molecular structures of complexes 2, 4, 5, and 8 have been determined via single crystal X-ray diffraction analysis. Complex 8 represents a rare example of a Mo(IV) phosphino-imido complex. Kinetic measurements by UV-vis spectroscopy of the OAT reactions from complexes 1-4 to PMe(3) showed them to be more efficient than previously reported nonfluorinated ones, with ligand L' = (Ar)NC(Me)CH(2)CO(CH(3)) [MoO(2)(L')(2)] (9) and [MoO(N(t)Bu)(L')(2)] (10), respectively. Thermodynamic activation parameters ΔH(?) and ΔS(?) of the OAT reactions for complexes 2 and 4 have been determined. The activation enthalpy for the reaction employing 2 is significantly smaller (12.3 kJ/mol) compared to the reaction with the nonfluorinated complex 9 (60.8 kJ/mol). The change of the entropic term ΔS(?) is small. The reaction of the oxo-imido complex 4 to 8 revealed a significant electron-donating contribution of the imido substituent.     

Influence of carbon and nitrogen sources and temperature on hyperproduction of a thermotolerant β-glucosidase from synthetic medium by Kluyveromyces marxianus

The effect of carbon source and its concentration, inoculum size, yeast extract concentration, nitrogen source, pH of the fermentation medium, and fermentation temperature on β-glucosidase production by Kluyveromyces marxianus in shake-flask culture was investigated. These were the independent variables that directly regulated the specific growth and β-glucosidase production rate. The highest product yield, specific product yield, and productivity of β-glucosidase occurred in the medium (pH 5.5) inoculated with 10% (v/v) inoculum of the culture. Cellobiose (20 g/L) significantly improved β-glucosidase production measured as product yield (Y _P/S ) and volumetric productivity (Q _P ) followed by sucrose, lactose, and xylose. The highest levels of productivity (144 IU/[L·h]) of β-glucosidase occurred on cellobiose in the presence of CSL at 35°C and are significantly higher than the values reported by other researchers on almost all other organisms. The thermodynamics and kinetics of β-glucosidase production and its deactivation are also reported. The enzyme was substantially stable at 60°C and may find application in some industrial processes.     

Steric Considerations in Supramolecular Inclusion of Modified β-cyclodextrins with Triton X-100 and α-bromonaphthalene

Supramolecular inclusion of modified β-cyclodextrin (β-CD) with Triton X-100 (TX) and α-bromonaphthalene (BN) was studied by fluorescence and phosphorescence measurements. Major differences were observed in the magnitude of the apparent stability constants and quenching constants of the inclusion complexes. Methyl substitution on the rims of β-CD increased the binding of TX with β-CD but was unfavorable to the protection of the phenyl group of TX from fluorescence quenching and further accommodation of BN for steric considerations. According to the overall molecular size of β-CD, TX and BN, further inclusion of BN in the cavity of β-CD occupied by TX may force the flexible tert-octyl chain of TX to deform to a greater extent and close packing complexes were obtained. Phosphorescence of BN arising from intermolecular energy transfer between BN and the phenyl group of TX was observed when the phenyl group of TX was irradiated. In the case of heptakis(2,6-di-O-methyl)-β-CD, BN failed to penetrate into the cavity because of the steric hindrance of the methyl substituents at the rim of the β-CD cavity.     

Antioxidant properties of β-carotene isomers and their role in photosystems: insights from Ab initio simulations

In this work we investigate the effect of cis isomerizations and conformational changes on the antioxidant activity of β-carotene, one of the most important pigments in nature. The electrodonating (ω(-)) and electroaccepting (ω(+)) powers of the most relevant isomers of β-carotene are first evaluated in polar and nonpolar solvents using density functional theory (DFT), and these quantities are then used to establish an antioxidant scale of the isomers. The electrodonating power, which is directly related to the antioxidant activity, is shown to provide a very good correlation with the experimental data. Next, we compute the intermediate twisted structures of the β-carotene isomers generated by partial rotation of every single bond in the polyenic chain. The electrodonating and electroaccepting powers are evaluated for each of these intermediate structures along with their maximum absorption wavelengths, which are computed using time-dependent DFT (TD-DFT). The trends observed for both the electrodonating power and the maximum absorption wavelength can be rationalized in terms of the effective conjugated chain length of the structure resulting from single bond rotations. The results obtained are used to analyze the conformational distribution of β-carotene in the well-resolved photosystem I (PS-I) of purple cyanobacteria. It is then shown that the isomers present in this photosystem are those having the lowest calculated relative energies and that those with enhanced antioxidant activity are preferentially located in the inner core of the protein complex.     

Chain termination in the oxidation of lipid models by α-tocopherol and its synthetic analog

The rate constants (k₁) and stoichiometric coefficients (f) of chain termination in the oxidation of lipid models (methyl esters of acids in the oleic series) by -tocopherol and its synthetic analog at 50°C were measured. The values of k₁ and f do not depend on the structure of the substituent R in the peroxide radical ROO of the lipid models undergoing oxidation, but the values of k₁ are determined by the conformation of the investigated phenols.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 29, No. 5, pp. 441–444, September–October, 1993.     

Self-assembly of flexible β-strands into immobile amyloid-like β-sheets in membranes as revealed by solid-state 19F NMR

The cationic peptide [KIGAKI](3) was designed as an amphiphilic β-strand and serves as a model for β-sheet aggregation in membranes. Here, we have characterized its molecular conformation, membrane alignment, and dynamic behavior using solid-state (19)F NMR. A detailed structure analysis of selectively (19)F-labeled peptides was carried out in oriented DMPC bilayers. It showed a concentration-dependent transition from monomeric β-strands to oligomeric β-sheets. In both states, the rigid (19)F-labeled side chains project straight into the lipid bilayer but they experience very different mobilities. At low peptide-to-lipid ratios ≤1:400, monomeric [KIGAKI](3) swims around freely on the membrane surface and undergoes considerable motional averaging, with essentially uncoupled φ/ψ torsion angles. The flexibility of the peptide backbone in this 2D plane is reminiscent of intrinsically unstructured proteins in 3D. At high concentrations, [KIGAKI](3) self-assembles into immobilized β-sheets, which are untwisted and lie flat on the membrane surface as amyloid-like fibrils. This is the first time the transition of monomeric β-strands into oligomeric β-sheets has been characterized by solid-state NMR in lipid bilayers. It promises to be a valuable approach for studying membrane-induced amyloid formation of many other, clinically relevant peptide systems.     

A piezo-resonator as a detector of α- and β-alanine in aqueous solutions

The results of analysis of aqueous solutions of α- and β-alanine using a piezo-resonator are presented. Analytical signals from analytes were found to significantly differ from each other, which is connected with the peculiarities of the piezo-resonator operation in the solution as a viscosimetric rather than a mass-sensitive device.     

A simple and effective strategy for labeling cysteine sulfenic acid in proteins by utilization of β-ketoesters as cleavable probes

β-ketoesters are robust probes for labeling sulfenic acid (-SOH) proteins allowing quantitative cleavage of the tag for improved analysis of the labeled peptides by MS.