Ring-Opening Polymerization of ε-Caprolactone by Using Aluminum Complexes Bearing Aryl Thioether Phenolates: Labile Thioether Chelation.

In this study, aluminum complexes bearing ferrocene-based and arylthiomethylphenolate ligands were synthesized, and their catalytic activity for ε-caprolactone (CL) polymerization was investigated. The catalytic activity of the reduced form of Al complexes was higher than that of the oxidized form. The CL polymerization rate of the reduced form fcO2AlMe (75 min, conversion = 100%) was higher than that of the oxidized form fcoxO2AlMe (4320 min, conversion = 45%), and the CL polymerization rate of fc(OAlMe2)2 (40 min, conversion = 100%) was higher than that of fcox(OAlMe2)2 (60 min, conversion = 97%). Electron deficiency substituents on phenolate decreased the catalytic activity of Al complexes bearing arylthiomethylphenolate ligands. Density functional theory calculations revealed that thioether coordination stabilized the transition state (TS1) and that the oxidized form fcox(OAlMe2)2 exhibited weaker thioether coordination and higher activation energy in TS1 compared with those of the reduced form fcO2AlMe. In addition, our study determined that the thioether group is a suitable chelating group for Al catalysts in CL polymerization due to its labile nature.

Collaboration between Trinuclear Aluminum Complexes Bearing Bipyrazoles in the Ring-Opening Polymerization of ε-Caprolactone.

Trinuclear aluminum complexes bearing bipyrazoles were synthesized, and their catalytic activity for ε-caprolactone (CL) polymerization was investigated. DBu2Al3Me5 exhibited higher catalytic activity than did the dinuclear aluminum complex LBu2Al2Me4 (16 times as high for CL polymerization; [CL]:[DBu2Al3Me5]:[BnOH] = 100:0.5:5, [DBu2Al3Me5] = 10 mM, conversion 93% after 18 min at room temperature). Density functional theory calculations revealed a polymerization mechanism in which CL first approached the central Al atom and then moved to an external Al. The coordinated CL ring was opened because the repulsion of two tert-butyl groups on the ligands pushed an alkoxide initiator on an external Al to initiate CL. In these trinuclear Al catalysts, the central Al plays a role in monomer capture and then collaborates with the external Al to activate CL, accelerating polymerization.

A broad-range variable-temperature solid state NMR spectral and relaxation investigation of the water state in Nafion 117.

Understanding the water state in Nafion is not only crucial for operating a proton-exchange membrane (PEM)-based fuel cell, but also intimately related to the elucidation of the proton transport mechanism in a PEM. Although many studies have been published on this subject, some controversies and ambiguities remain unresolved. In this work, we design three different types of Nafion samples by substituting protons with lithium or sodium cations. We also pay special attention to the preparation of samples for carrying out broad-range variable temperature solid state NMR experiments so that no membrane dehydration occurs during the long experimental time at low temperatures. With these precautions and improvements, clear and largely straightforward information could be obtained to ensure minimal ambiguity and complexity in the interpretation of the experimental data. Our results show that about 40-60% of water remains unfrozen at -70 °C, depending on the type of the substituting cation. Both the 1H and 2H spectral and relaxation results indicate that water freezing starts from the center of the nanopores inside Nafion and increases gradually as the temperature decreases. The protons remain dissociated with sulfonate groups even at the lowest temperature we reached (-70 °C), whereas both lithium and sodium are associated with sulfonate groups at most temperatures below 0 °C. The experimental data also suggest that besides frozen and unfrozen water, there is broad distribution of water state and dynamics in Nafion as the temperature is lowered from above zero down to -70 °C. The effect of the size of the substituting cation significantly affects the properties of supercooled water by modifying the cation-water interaction and impeding the rotation of sulfonate groups. These novel results not only help us in establishing a better understanding of the water state in Nafion and its performance as a proton exchange mebrane, but also provide insights into water freezing, antifreeze and supercooling in other nanoscopic environments.

Different submicellar solubilization mechanisms revealed by 1H NMR and 2D diffusion ordered spectroscopy (DOSY).

Different submicellar solubilization mechanisms of two systems, Triton X-100 (TX-100)/tetradecane and sodium dodecyl sulfate (SDS)/butyl methacrylate, are revealed on the molecular scale by 1H NMR spectroscopy and 2D diffusion ordered spectroscopy (DOSY). It is evident that the apparent solubilities of both tetradecane and butyl methacrylate are enhanced, even at much lower surfactant concentrations than the CMCs. Solubilized solutes also contribute to the early formation of surfactant micelles. In general, the molar solubilization ratios (MSRs) of both solutes linearly increase as the surfactant concentrations increase. However, variations in MSRs of the two systems are different below and above the CMC, which is probably related to the different solubilization mechanisms. For TX-100/tetradecane, as the TX-100 concentration increases, the tetradecane resonance in the independent state transforms into that of the aggregated state and the corresponding evolution of diffusions is shown in the 2D DOSY spectra. These results demonstrate that below the CMC, tetradecane is first solubilized in TX-100 solutions, and then solubilized in TX-100 micelles above the CMC. For SDS/butyl methacrylate, the appearance of oligomeric SDS resonances below the CMC indicates that butyl methacrylate is partially solubilized in SDS oligomers. Then, when the CMC is reached, the dominant, monomeric SDS molecules aggregate into oligomers, and the similar diffusivity trend of butyl methacrylate with that of SDS indicates that a proportion of butyl methacrylate molecules are solubilized in it. Finally, the fusion of SDS resonances in the two states and the tendency of co-diffusion of SDS and butyl methacrylate indicate that all the SDS molecules gradually aggregate into micelles, and almost all the butyl methacrylate molecules are solubilized in them. In conclusion, above the CMCs, the solubilization manners of these two systems are similar. However, they are different below CMCs. The solubilization of tetradecane by TX-100 is driven by the intermolecular hydrophobic interaction, i.e., molecular-pair formation. However, the polar interaction between functional groups of butyl methacrylate and the polar head of SDS contributes to the solubilization of butyl methacrylate. The different submicellar solubilization mechanisms are mainly caused by the different properties of solutes and surfactants, which also results in different MSRs and solubilization sites in the micelles.

A comparative study of titanium complexes bearing 2-(arylideneamino)phenolates and 2-((arylimino)methyl)phenolates as catalysts for ring-opening polymerization of ε-caprolactone and L-lactide.

Titanium complexes bearing 2-(arylideneamino)phenolates and 2-((arylimino)methyl)phenolates were synthesized, and their catalytic activities in the polymerization of ε-caprolactone and L-lactide were studied. Among five-membered ring Ti complexes bearing 2-(arylideneamino)phenolates, FCl-Ti exhibited the highest level of catalytic activity ([CL] : [FCl-Ti] = 100 : 1, where [CL] = 2 M, and conv. = 86% at 60 °C after 9 h). For six-membered ring Ti complexes bearing 2-((arylimino)methyl)phenolates, SCl-Ti exhibited the highest level of catalytic activity ([CL] : [SCl-Ti] = 100 : 1, where [CL] = 2 M, and conv. = 88% at 60 °C after 118 h). The five-membered ring Ti complexes bearing 2-(arylideneamino)phenolates exhibited a higher level of catalytic activity (6.1-12.8 fold for the polymerization of ε-caprolactone and 6.2-23.0 fold for the polymerization of L-lactide) than the six-membered ring Ti complexes bearing 2-((arylimino)methyl)phenolates. The density functional theory (DFT) results revealed that the free energy of the first transition state FH-Ti-TS1 is 36.49 kcal mol-1 which is lower than that of SH-Ti-TS1 (46.58 kcal mol-1), which was ascribed to the fact that the Ti-Nim bond (2.742 Å) of FH-Ti-TS1 is longer than that of SH-Ti-TS1 (2.229 Å) and reduces the repulsion between ligands.

Improvement of catalytic activity of aluminum complexes for the ring-opening polymerization of ε-caprolactone: aluminum thioamidate and thioureidate systems.

In this study, a series of Al complexes bearing amidates, thioamidates, ureidates, and thioureidates were synthesized and their catalytic activity for ε-caprolactone (CL) polymerization was evaluated. SPr-Al exhibited a higher catalytic activity than OPr-Al (3.2 times as high for CL polymerization; [CL] : [SPr-Al] : [BnOH] = 100 : 0.5 : 2; [SPr-Al] = 10 mM, conv. = 93% after 14 min at 25 °C), and USCl-Al exhibited a higher catalytic activity than UCl-Al (4.6 times as high for CL polymerization; [CL] : [USCl-Al] : [BnOH] = 100 : 0.5 : 2; [USCl-Al] = 10 mM, conv. = 90% after 15 min at 25 °C). Regardless of whether aluminum amidates or ureidates were present, thioligands improved the polymerization rate of aluminum catalysts. Density functional theory calculations revealed that the eight-membered ring [SPr-AlOMe2]2 decomposed into the four-membered ring SPr-AlOMe2. However, [OPr-AlOMe2]2 did not decompose because of its strong bridging Al-O bond. The overall activation energy required for CL polymerization was lower when using [SPr-AlOMe2]2 (18.1 kcal mol-1) as a catalyst than when using [OPr-AlOMe2]2 (23.9 kcal mol-1). This is because the TS2a transition state of SPr-AlOMe2 had a more open coordination geometry with a small N-Al-S angle (72.91°) than did TS3c of [OPr-AlOMe2]2, the crowded highest-energy transition state of [OPr-AlOMe2]2 with a larger N-Al-O angle (99.63°).

Denticity governs the formation of ß-thioketiminato tri-copper(I) and mono-copper(I) complexes.

Two classes of ß-thioketiminate ligands, SN chelators (HL1 and HL2) and SNN chelators (HL3 and HL4), were prepared to understand their coordination behavior in copper(I) complex formation. The formation of these copper(I) complexes bearing ß-thioketiminate ligands and their corresponding adducts toward isocyanide, PPh3, and CO was investigated to address two important issues. First, whether the denticity governs the copper(I) thiolate species formation between SN chelators and SNN chelators. Second, how the length of the pendant pyridyl arm affects the coordination and reactivity behaviors of copper(I) complexes. Based on the characterization results, it was found that the denticity of SN chelators and SNN chelators led to different nuclearity of copper(I)-thiolate species. The coordination modes of the pendant pyridyl arm were confirmed by FTIR measurements, which allow us to conclude that the electron donating ability of the LCu fragment is in the order of SNN-chelator (SNN bound) > SNN-chelators (SN bound) > SN-chelator.

A NMR-based metabolomic approach for differentiation of hagfish dental and somatic skeletal muscles.

The hagfish dental muscle is a large and specialized element of the feeding apparatus that helps ingest food. This muscle has enzymatic activities and contractile properties different from the hagfish somatic skeletal muscle. To verify the functional relevance of protein alterations, we examined the metabolomic differentiation of hagfish dental and somatic skeletal muscles using ¹H-nuclear magnetic resonance (NMR)-based metabolomics and multivariate analysis that separated hagfish dental and somatic muscles by principal component analysis and partial least squares for discriminant analysis. Our analysis of assigned metabolites showed that anserine and taurine levels were higher in dental muscle, but creatine, fructose, glucose, glycerate, pyruvate, and succinate levels were higher in somatic muscle. We concluded that the primary energy sources of dental and somatic muscles are related to the citric acid cycle and the anaerobic glycolysis and metabolism of creatine. Thus, ¹H-NMR-based metabolomics can be integrated with the previous proteomic approach to derive biochemical and physiological information about hagfish muscles.

A Polypeptide-Based, Membrane-Penetrating, Target-Specific Contrast Agent for Magnetic Resonance Molecular Imaging.

Molecular imaging based on magnetic resonance imaging (MRI) requires a contrast agent with high relaxivity and specificity. Much effort has been devoted to this goal over the past decades. In this work, we continue this endeavor by synthesizing an MRI contrast agent that can penetrate the cellular membrane and bind with specific proteins. It was characterized with one- and two-dimensional NMR spectroscopy, NMR micro-imaging, and mass spectroscopy. The target specificity has been further confirmed by both molecular dynamics simulation and micro-imaging on a living biological system. It is one of the largest of peptide-based bioactivated MRI contrast agents, and its relaxivity enhancement factor is among the highest of MRI contrast agents hitherto published. We envision interesting applications and extension of this smart MRI contrast agent with bio-specificity and high contrast for molecular imaging.

Breaking the T1 constraint for quantitative measurement in magic angle spinning solid-state NMR spectroscopy.

Quantitative solid-state NMR experimental schemes that break the conventional T(1) constraint are described. The combination of broad-band homonuclear recoupling techniques and the conventional single pulse or cross-polarization (CP) schemes (referred as QUSP or QUCP) render the long T(1) of low-gamma spins no longer a constraint for obtaining quantitative NMR spectra. During the mixing time when dipolar recoupling occurs, the nonuniformly CP enhanced or recovered spin magnetization is redistributed under the reintroduced homonuclear dipole-dipole interactions so that uniformly enhanced or recovered magnetization is achieved when the system reaches the quasi-equilibrium state. It is shown that quantitative NMR spectra can be obtained for the recycle delays substantially shorter than the conventionally required 5T(1). In addition, the high efficiency gain can be achieved in QUSP and QUCP experiments with a relatively short recycle delay.

Use of pyrazoles as ligands greatly enhances the catalytic activity of titanium iso-propoxide for the ring-opening polymerization of l-lactide: a cooperation effect.

Using TiOiPr4 with a pyrazole ligand for one-pot LA polymerization improved catalytic activity compared with using TiOiPr4 only. At 60 °C, TiOiPr4 with furPz exhibited a higher catalytic activity (approximately 3-fold) than TiOiPr4. At room temperature, TiOiPr4 with BuPz exhibited a higher catalytic activity (approximately 17-fold) than TiOiPr4. High molecular mass PLA (M nGPC = 51 100, and D = 1.10) could be produced by using TiOiPr4 with furPz in melt polymerization ([TiOiPr4] : [furPz] = 1000 : 1 : 1 at 100 °C, 240 min). The crystal structure of MePz2Ti2OiPr7 revealed the cooperative activation between two Ti atoms during LA polymerization.

Prevention of spinning induced sample deterioration during long time solid state NMR experiments of quadrupolar spin systems.

A simple solution is proposed to prevent a solid state polycrystalline sample from deterioration during long time high speed spinning experiments in solid state NMR. It is found that if a certain percentage ( approximately 40% volume) of polyethylene glycol (PEG, (HO-CH(2)-(CH(2)-O-CH(2)-)(n)-CH(2)-OH)(n)) is mixed with the sample that are subject to deterioration, the quality of the sample can be maintained for a long time under high speed spinning for a few days or longer, sufficient for multi-dimensional and/or low-sensitivity experiments. Both 1D and 2D experimental results are shown to support this idea.

High-resolution pure shift NMR spectroscopy offers better metabolite discrimination in food quality analysis.

PSYCHE homonuclear decoupling, a prominent pure shift NMR method, is successfully applied to adulteration discrimination of honey and geographical originality identification of tea. Effects of homonuclear couplings are efficiently suppressed, producing resolution-enhanced spectra. The pair wise honey and tea samples are well separated in OPLS-DA models with high predictability. Due to the well-resolved and accurate assignment of singlet resonances after decoupling, PSYCHE is advantageous in the identification of differential components and accurate quantification of compound concentrations presented by enhanced volcano and Beeswarm plots of honey samples, while the analysis of NOESY is easily interfered by overlapped resonances, which is further proved by the STOCSY analysis, displaying the spectral stability and reproducibility. Experimental results show that PSYCHE can improve the spectral resolution of natural complex products such as honey and tea and be combined with multivariate statistical analysis and serve as a supplementary technique to the standard methods, especially for samples systems composed by a few high-content compounds.

Author Correction: High Resolution 31P NMR Spectroscopy Generates a Quantitative Evolution Profile of Phosphorous Translocation in Germinating Sesame Seed.

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has not been fixed in the paper.

High Resolution 31P NMR Spectroscopy Generates a Quantitative Evolution Profile of Phosphorous Translocation in Germinating Sesame Seed.

Phosphorus metabolism and circulation are essential bio-physicochemical processes during development of a plant and have been extensively studied and known to be affected by temperature, humidity, lighting, hormones etc. However, a quantitative description of how various phosphorous species evolve over time has not been reported. In this work, a combined 31P liquid and solid state NMR spectroscopic methodology is employed, supported by a new extraction scheme and data analysis method, to carry out a quantitative investigation of phosphorous circulation in germinating sesame seeds in dark and under illumination with and without adding a growth hormone. The spectra show that only slight changes occur for phosphorous metabolism at the initial stage but a rapid change takes place between 48-96 hours after germination is started. The metabolism is found to be temperature dependent and affected by illumination and hormone. However, neither illumination nor hormone affects the final residual concentration of phytin. Moreover, phytin does not flow out of cotyledon and the phosphorous flowing to other parts of the plant is always in the inorganic form. The overall evolution profile of phytate consumption is found to be a Gaussian decaying function. These findings can be explained with a dynamic model on phytin conversion.

Macromolecular Crowding May Significantly Affect the Performance of an MRI Contrast Agent: A 1H†NMR Spectroscopy, Microimaging, and Fast-Field-Cycling NMR Relaxometry Study.

Contrast enhancement agents are often employed in magnetic resonance imaging (MRI) for clinical diagnosis and biomedical research. However, the current theory on MRI contrast generation does not consider the ubiquitous presence of macromolecular crowders in biological systems, which poses the risk of inaccurate data interpretation and misdiagnosis. To address this issue, herein the macromolecular crowding effects on MRI contrast agent are investigated with the 1H relaxation rate of water in aqueous solutions of Dotarem with different concentrations of macromolecules. Two representative macromolecular crowder systems are used: polyethylene glycol (with no specific secondary structure) and bovine serum albumin (with compact secondary and tertiary structures). The water 1H relaxation rates in various solutions are measured in a fixed magnetic field and in variable magnetic fields. The results show significant crowding effects for both crowders. The relaxation rate is proportional to the concentration of the MRI contrast agent but shows conspicuous superlinearity with respect to the concentration of the crowder. The size of polyethylene glycol does not affect the relaxivity of water in Dotarem solutions. The above effects are verified with T1- and T2-weighted NMR microimages. These results highlight the importance of the effect of macromolecular crowding on the MRI contrast agent and are valuable for understanding the mechanism of MRI contrast agents and designing new-generation MRI contrast agents.

Proton magnetic resonance spectroscopy and diffusion-weighted imaging in intracranial cystic mass lesions.

BACKGROUND: The differential diagnosis of various intracranial cystic lesions is sometimes difficult on the basis of CT or MRI findings. Our objective was to evaluate (1)H MRS and DWI in the differential diagnosis of these lesions and in comparison with conventional MRI. METHODS: Fifty patients with intracranial cystic lesions (21 pyogenic abscesses, 23 tumor cysts, 3 epidermoid cysts, and 3 arachnoid cysts) were evaluated with conventional MRI, DWI, and in vivo (1)H MRS. Preoperative diagnosis of the lesions was based on the results of DWI and in vivo MRS. All DWI and (1)H MRS studies were performed with a clinical 1.5-T system. The DWI was performed using single-shot spin-echo echo-planar pulse sequence with b = 1000 s/mm(2). The ADC value was measured. Diagnostic accuracy of conventional MRI, DWI, and in vivo (1)H MRS was calculated with respect to a final diagnosis of brain abscess vs nonabscess cystic tumor. RESULTS: Lactate and cytosolic amino acids with/without succinate, acetate, and alanine were observed in 18 of 21 cases of abscesses on MRS. In 3 cases of epidermoid cysts, lactate was observed and could be differentiated from 3 cases of arachnoid cysts, which showed only minimal lactate. Only lactate was seen in 14 of 23 patients with tumor cysts, whereas both lipid/lactate and choline were visible in 9 patients with tumor cysts. Increased signal was seen in 20 of 21 patients with abscesses and 3 patients with epidermoid cysts on DWI. Decreased signal was observed in 22 of 23 patients with tumor cysts and 3 patients with arachnoid cyst on DWI. Diagnostic accuracy, sensitivity, specificity, positive predictive value, and negative predictive value of conventional MRI for the differentiation of brain abscess from nonabscess cystic tumor were 61.4%, 61.9%, 60.9%, 59.1%, and 63.6%, respectively, whereas they were 93.2%, 85.7%, 100%, 100%, and 88.5% with MRS; 95.5%, 95.2%, 95.7%, 95.2%, and 95.7% with DWI; and 97.7%, 95.2%, 100%, 100%, and 95.8% with MRS and DWI. Magnetic resonance imaging, when combined with in vivo MRS and DWI, accurately predicted the diagnosis in 47 (94%) of 50 and 48 (96%) of 50 of the cases, respectively. CONCLUSIONS: Proton MRS and DWI are useful as additional diagnostic modalities in differentiating intracranial cystic lesions. Combination of DWI with calculated ADC values and metabolite spectrum acquired by MRS add more information to MRI in the differentiation of intracranial cystic mass lesions.

NMR-based untargeted metabolomic study of hydrogen peroxide-induced development and diapause termination in brine shrimp.

Artemia diapause has been extensively studied in embryonic biology for a long time. It has been demonstrated that hydrogen peroxide (H2O2) can increase the hatching rate in the development and diapause termination of Artemia cysts. This study used an untargeted 1H NMR-based metabolomic approach to explore the physiological regulation of H2O2 in initiating the development and terminating the diapause of Artemia cysts. This experiment was divided into two parts. In the first part, we analyzed three groups with or without H2O2 as control-0h, control-5h and H2O2 (180µM)-5h; in the second part, after 7-d incubation, the non-hatching cysts were treated with different H2O2 concentrations as low as 180µM and as high 1800µM. The results showed that arginine and proline metabolism were up-regulated after 5h, and H2O2 up-regulated valine, leucine and isoleucine biosynthesis in the development of cysts. In the second part, low H2O2 (180µM) showed alanine, aspartate and glutamate metabolism, but high H2O2 (1800µM) also up-regulated arginine and proline metabolism, as in the control group without H2O2 stimulus. These results suggest that enough H2O2 can catalyze cell transcription and translation in Artemia cysts, and it improves the cell growth rate, thus allowing embryo cells to grow again.

One and two dimensional 1H and 13C high resolution NMR investigation of lariat ethers and their alkali metal ionic complexes: a more tangible evidence for the presence of less common C-H***O hydrogen bonds.

The possible existence of less common hydrogen bonds in three lariat ethers and their alkali-metal ionic complexes have been investigated with one- and two-dimensional (1D and 2D) proton and carbon-13 high resolution liquid state NMR spectroscopy. The occurrence of hydrogen-bonding induced by the addition of metal ions has been identified with the observation of indirect dipolar coupling between the coupling partners involved in the hydrogen-bonding. The addition of metal ions, moreover, causes appreciable change of chemical shift of several protons and carbons. The chemical shift change depends on the ion radius, larger ions causing smaller change. Moreover, the change of chemical shift is in coincidence with the occurrence of hydrogen-bonding. The values of the coupling constants have been obtained for each of these hydrogen bonds and were used for evaluating the hydrogen-bond strength. An intriguing and surprising observation is that a C-H***O hydrogen bond identified in solution by this work was not found in the previous study with X-ray diffraction or other methods.

[Synthesis, characterization and relaxivity of dimeric DTPA-gadonium (III) complexes with long chain alkyl esters of L-lysine].

To look for new MRI (magnetic resonance imaging) contrast agents with higher relaxivity as well as liver-selecsivity, four novel ester-amino ligands were synthesized by bis-acylation of octadecanyl, hexadecanyl, tetradecanyl and dedecanyl L-lysine with diethylenetriaminepentaacetic acid mono-anhydride (DTPA-MA), respectively. The corresponding dimeric Gd(III) complexes were gained by the reaction of these ligands with GdCl3 x 6H2O. All ligands and complexes were characterized by FTIR, 1H NMR and elemental analysis. The longitudinal relaxation time (T1) was measured, and relevant longitudinal relaxivity (R1) of these neatral binuclear Gd(III) complexes is: 6.48, 6.02, 5.76 and 5.68 L x mmol(-1) x s(-1), respectively, and all are higher than that of Gd-DTPA (4.98 L x mmol(-1) x s(-1)) (300 MHz).