Observation and Kinetic Characterization of Transient Schiff Base Intermediates by CEST NMR Spectroscopy

Synthesis of a reactive lignin-based flame retardant and its application in phenolic foam

To improve the flame retardancy of phenolic foam from the perspective of sustainable development, it is a feasible way to add bio-based flame retardants into phenolic foam. Lignin has a similar structure to phenol, which provides a possibility to replace part of phenol. In this paper, we prepared a kind of reactive bio-based flame retardant based on enzymatic hydrolyzed lignin, in which side chain was chemically grafted with phosphorus and nitrogen and benzene ring would participate in the phenolic condensation reaction. According to elemental analysis and ICP-OES data, the content of nitrogen and phosphorus in modified lignin (NP-L) increased to 2.95% and 3.55% respectively. Compared with original lignin, the carbon residue rate of NP-L increased from 3.25% to 12.13% because of the presence of flame retardant elements N and P. Then lignin-based flame retardant was used to replace phenol for modifying phenolic foams (NPLPFX). The limited oxygen index (LOI) and compressive strength of phenolic foam were improved effectively by adding modified lignin when the substitution rate was less than 25%. The LOI and compressive strength of the modified phenolic foam with 5% replacement amount (NPLPF5) are 55.6% and 0.24 MPa, which increased by 88% and 60% compared with pure phenolic foam. The cone calorimetric data also showed that NPLPF5 had good flame retardancy, and the peak heat release rate and total heat release were significantly lower than PF. This work suggests a novel green strategy for improving the flame retardancy performance of phenolic foam and promoting the utilization of lignin.

Injectable hydrogels as promising in situ therapeutic platform for cartilage tissue engineering

Injectable hydrogels are gaining prominence as a biocompatible, minimally invasive, and adaptable platform for cartilage tissue engineering. Commencing with their synthesis, this review accentuates the tailored matrix formulations and cross-linking techniques essential for fostering three-dimensional cell culture and melding with complex tissue structures. Subsequently, it spotlights the hydrogels' enhanced properties, highlighting their augmented functionalities and broadened scope in cartilage tissue repair applications. Furthermore, future perspectives are advocated, urging continuous innovation and exploration to surmount existing challenges and harness the full clinical potential of hydrogels in regenerative medicine. Such advancements are crucial for validating the long-term efficacy and safety of hydrogels, positioning them as a promising direction in regenerative medicine to address cartilage-related ailments.

Beyond slow two-state protein conformational exchange using CEST: applications to three-state protein interconversion on the millisecond timescale

Although NMR spectroscopy is routinely used to study the conformational dynamics of biomolecules, robust analyses of the data are challenged in cases where exchange is more complex than two-state, such as when a 'visible' major conformer exchanges with two 'invisible' minor states on the millisecond timescale. It is becoming increasingly clear that chemical exchange saturation transfer (CEST) NMR experiments that were initially developed to study systems undergoing slow interconversion are also sensitive to intermediate-fast timescale biomolecular conformational exchange. Here we investigate the utility of the amide 15N CEST experiment to characterise protein three-state exchange occurring on the millisecond timescale by studying the interconversion between the folded (F) state of the FF domain from human HYPA/FBP11 (WT FF) and two of its folding intermediates I1 and I2. Although 15N CPMG experiments are consistent with the F state interconverting with a single minor state on the millisecond timescale, 15N CEST data clearly establish an exchange process between F and a pair of minor states. A unique three-state exchange model cannot be obtained by analysis of 15N CEST data recorded at a single temperature. However, including the relative sign of the difference in the chemical shifts of the two minor states based on a simple two-state analysis of CEST data recorded at multiple temperatures, results in a robust three-state model in which the F, I1 and I2 states interconvert with each other on the millisecond timescale ( k e x , F I 1 ~ 550 s-1, k e x , F I 2 ~ 1200 s-1, k e x , I 1 I 2 ~ 5000 s-1), with I1 and I2 sparsely populated at ~ 0.15% and ~ 0.35%, respectively, at 15 °C. A computationally demanding grid-search of exchange parameter space is not required to extract the best-fit exchange parameters from the CEST data. The utility of the CEST experiment, thus, extends well beyond studies of conformers in slow exchange on the NMR chemical shift timescale, to include systems with interconversion rates on the order of thousands/second.

Azotriptycenes: Photoswitchable Molecular Brakes

The control of molecular motions is a central topic of molecular machine research. Molecular brakes are fundamental building blocks towards such goal as they allow deliberately decelerating specific motions after an outside stimulus is applied. Here we present azotriptycenes as structural framework for light-controlled molecular brakes. The intrinsic kinetics and their changes upon azotriptycene isomerization are scrutinized comprehensively by a mixed theoretical and variable temperature NMR approach. With azotriptycenes C-N bond rotation rates can be decelerated or accelerated reversibly by up to five orders of magnitude. Rate change effects are highly localized and are strongest for the C-N bond connecting a triptycene rotor fragment to the central diazo group. The detailed mechanistic insights provide a solid basis for further conscious design and applications in the future.

Studying micro to millisecond protein dynamics using simple amide 15N CEST experiments supplemented with major-state R2 and visible peak-position constraints

Over the last decade amide 15N CEST experiments have emerged as a popular tool to study protein dynamics that involves exchange between a 'visible' major state and sparsely populated 'invisible' minor states. Although initially introduced to study exchange between states that are in slow exchange with each other (typical exchange rates of, 10 to 400 s-1), they are now used to study interconversion between states on the intermediate to fast exchange timescale while still using low to moderate (5 to 350 Hz) 'saturating' B1 fields. The 15N CEST experiment is very sensitive to exchange as the exchange delay TEX can be quite long (~0.5 s) allowing for a large number of exchange events to occur making it a very powerful tool to detect minor sates populated ([Formula: see text]) to as low as 1%. When systems are in fast exchange and the 15N CEST data has to be described using a model that contains exchange, the exchange parameters are often poorly defined because the [Formula: see text] versus [Formula: see text] and [Formula: see text] versus exchange rate ([Formula: see text]) plots can be quite flat with shallow or no minima and the analysis of such 15N CEST data can lead to wrong estimates of the exchange parameters due to the presence of 'spurious' minima. Here we show that the inclusion of experimentally derived constraints on the intrinsic transverse relaxation rates and the inclusion of visible state peak-positions during the analysis of amide 15N CEST data acquired with moderate B1 values (~50 to ~350 Hz) results in convincing minima in the [Formula: see text] versus [Formula: see text] and the [Formula: see text] versus [Formula: see text] plots even when exchange occurs on the 100 μs timescale. The utility of this strategy is demonstrated on the fast-folding Bacillus stearothermophilus peripheral subunit binding domain that folds with a rate constant ~104 s-1. Here the analysis of 15N CEST data alone results in [Formula: see text] versus [Formula: see text] and [Formula: see text] versus [Formula: see text] plots that contain shallow minima, but the inclusion of visible-state peak positions and restraints on the intrinsic transverse relaxation rates of both states during the analysis of the 15N CEST data results in pronounced minima in the [Formula: see text] versus [Formula: see text] and [Formula: see text] versus [Formula: see text] plots and precise exchange parameters even in the fast exchange regime ([Formula: see text]~5). Using this strategy we find that the folding rate constant of PSBD is invariant (~10,500 s-1) from 33.2 to 42.9 °C while the unfolding rates (~70 to ~500 s-1) and unfolded state populations (~0.7 to ~4.3%) increase with temperature. The results presented here show that protein dynamics occurring on the 10 to 104 s-1 timescale can be studied using amide 15N CEST experiments.

Photo enhancement reveals (E,Z) and (Z,Z) configurations as additional intermediates in iminium ion catalysis

Imidazolidinone-based α,β-unsaturated iminium ions are the reactive species within countless synthetic protocols in asymmetric organocatalysis. However, (E,Z) and (Z,Z) imidazolidinone iminium ions, i.e. (Z)-CC configurations, have been elusive so far. Herein we describe how in situ photoisomerization enables the observation and assignment of high energetic (Z)-configured intermediates below the detection limit of NMR spectroscopy for (E,Z) and (Z,Z) iminium perchlorate complexes derived from MacMillan's 1st generation catalyst and cinnamaldehyde. Traces of (E,Z) could even be detected under synthetic conditions at 25 °C in MeCN. Using back isomerization studies and diffusion ordered spectroscopy, conditions were found to stabilize the (E,Z) and (Z,Z) isomers for several hours via ion pair aggregation. Thus, at least (E,Z) should be considered for future investigations in asymmetric iminium ion catalysis.

Role of Protein Damage Inflicted by Dopamine Metabolites in Parkinson's Disease: Evidence, Tools, and Outlook

Dopamine is an important neurotransmitter that plays a critical role in motivational salience and motor coordination. However, dysregulated dopamine metabolism can result in the formation of reactive electrophilic metabolites which generate covalent adducts with proteins. Such protein damage can impair native protein function and lead to neurotoxicity, ultimately contributing to Parkinson's disease etiology. In this Review, the role of dopamine-induced protein damage in Parkinson's disease is discussed, highlighting the novel chemical tools utilized to drive this effort forward. Continued innovation of methodologies which enable detection, quantification, and functional response elucidation of dopamine-derived protein adducts is critical for advancing this field. Work in this area improves foundational knowledge of the molecular mechanisms that contribute to dopamine-mediated Parkinson's disease progression, potentially assisting with future development of therapeutic interventions.

The A39G FF domain folds on a volcano-shaped free energy surface via separate pathways

Conformational dynamics play critical roles in protein folding, misfolding, function, misfunction, and aggregation. While detecting and studying the different conformational states populated by protein molecules on their free energy surfaces (FESs) remain a challenge, NMR spectroscopy has emerged as an invaluable experimental tool to explore the FES of a protein, as conformational dynamics can be probed at atomic resolution over a wide range of timescales. Here, we use chemical exchange saturation transfer (CEST) to detect "invisible" minor states on the energy landscape of the A39G mutant FF domain that exhibited "two-state" folding kinetics in traditional experiments. Although CEST has mostly been limited to studies of processes with rates between ∼5 to 300 s^-1 involving sparse states with populations as low as ∼1%, we show that the line broadening that is often associated with minor state dips in CEST profiles can be exploited to inform on additional conformers, with lifetimes an order of magnitude shorter and populations close to 10-fold smaller than what typically is characterized. Our analysis of CEST profiles that exploits the minor state linewidths of the 71-residue A39G FF domain establishes a folding mechanism that can be described in terms of a four-state exchange process between interconverting states spanning over two orders of magnitude in timescale from ∼100 to ∼15,000 μs. A similar folding scheme is established for the wild-type domain as well. The study shows that the folding of this small domain proceeds through a pair of sparse, partially structured intermediates via two discrete pathways on a volcano-shaped FES.

Measuring radiofrequency fields in NMR spectroscopy using offset-dependent nutation profiles

The application of NMR spectroscopy for studying molecular and reaction dynamics relies crucially on the measurement of the magnitude of radiofrequency (RF) fields that are used to nutate or lock the nuclear magnetization. Here, we report a method for measuring RF field amplitudes that leverages the intrinsic modulations observed in offset-dependent NMR nutation profiles of small molecules. Such nutation profiles are exquisitely sensitive to the magnitude of the RF field, and B₁ values ranging from 1 to 2000 Hz, as well the inhomogeneity in B₁ distributions, can be determined with high accuracy and precision using this approach. In order to measure B₁ fields associated with NMR experiments carried out on protein or nucleic acids, where these modulations are obscured by the large transverse relaxation rate constants of the analyte, our approach can be used in conjunction with a suitable external small molecule standard, expanding the scope of the method for large biomolecules.

A facile, versatile hydrogel bioink for 3D bioprinting benefits long-term subaqueous fidelity, cell viability and proliferation

Both of the long-term fidelity and cell viability of three-dimensional (3D)-bioprinted constructs are essential to precise soft tissue repair. However, the shrinking/swelling behavior of hydrogels brings about inadequate long-term fidelity of constructs, and bioinks containing excessive polymer are detrimental to cell viability. Here, we obtained a facile hydrogel by introducing 1% aldehyde hyaluronic acid (AHA) and 0.375% N-carboxymethyl chitosan (CMC), two polysaccharides with strong water absorption and water retention capacity, into classic gelatin (GEL, 5%)-alginate (ALG, 1%) ink. This GEL-ALG/CMC/AHA bioink possesses weak temperature dependence due to the Schiff base linkage of CMC/AHA and electrostatic interaction of CMC/ALG. We fabricated integrated constructs through traditional printing at room temperature and in vivo simulation printing at 37°C. The printed cell-laden constructs can maintain subaqueous fidelity for 30 days after being reinforced by 3% calcium chloride for only 20 s. Flow cytometry results showed that the cell viability was 91.38 ± 1.55% on day 29, and the cells in the proliferation plateau at this time still maintained their dynamic renewal with a DNA replication rate of 6.06 ± 1.24%. This work provides a convenient and practical bioink option for 3D bioprinting in precise soft tissue repair.

A structure-supporting, self-healing, and high permeating hydrogel bioink for establishment of diverse homogeneous tissue-like constructs

The ready-to-use, structure-supporting hydrogel bioink can shorten the time for ink preparation, ensure cell dispersion, and maintain the preset shape/microstructure without additional assistance during printing. Meanwhile, ink with high permeability might facilitate uniform cell growth in biological constructs, which is beneficial to homogeneous tissue repair. Unfortunately, current bioinks are hard to meet these requirements simultaneously in a simple way. Here, based on the fast dynamic crosslinking of aldehyde hyaluronic acid (AHA)/N-carboxymethyl chitosan (CMC) and the slow stable crosslinking of gelatin (GEL)/4-arm poly(ethylene glycol) succinimidyl glutarate (PEG-SG), we present a time-sharing structure-supporting (TSHSP) hydrogel bioink with high permeability, containing 1% AHA, 0.75% CMC, 1% GEL and 0.5% PEG-SG. The TSHSP hydrogel can facilitate printing with proper viscoelastic property and self-healing behavior. By crosslinking with 4% PEG-SG for only 3 min, the integrity of the cell-laden construct can last for 21 days due to the stable internal and external GEL/PEG-SG networks, and cells manifested long-term viability and spreading morphology. Nerve-like, muscle-like, and cartilage-like in vitro constructs exhibited homogeneous cell growth and remarkable biological specificities. This work provides not only a convenient and practical bioink for tissue engineering, targeted cell therapy, but also a new direction for hydrogel bioink development.

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A time-sharing structure-supporting (TSHSP) bioink based on gelation time difference between two gelling systems.

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The high permeability of TSHSP hydrogel is the basis for effective matter exchange.

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The TSHSP hydrogel facilitates room temperature printing with proper viscoelastic property and self-healing behavior.

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Cells manifest long-term viability and spreading morphology in bioprinted TSHSP constructs.

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In vitro tissue-like TSHSP constructs exhibit homogeneous cell growth and remarkable biological specificities.

Indirect detection of intermediate in decarboxylation reaction of phenylglyoxylic acid by hyperpolarized 13C NMR

The decarboxylation reaction of phenylglyoxylic acid with hydrogen peroxide is studied by real-time hyperpolarized carbon-13 nuclear magnetic resonance (13C NMR) spectroscopy at room temperature. A non-observable reaction intermediate is identified using blind selective saturation pulses in the expected chemical shift range, thereby revealing information on the reaction mechanism.

A CEST NMR experiment to obtain glycine 1Hα chemical shifts in 'invisible' minor states of proteins

Chemical exchange saturation transfer (CEST) experiments are routinely used to study protein conformational exchange between a 'visible' major state and 'invisible' minor states because they can detect minor states with lifetimes varying from ~ 3 to ~ 100 ms populated to just ~ 0.5%. Consequently several ¹H, ¹⁵N and ¹³C CEST experiments have been developed to study exchange and obtain minor state chemical shifts at almost all backbone and sidechain sites in proteins. Conspicuously missing from this extensive set of CEST experiments is a ¹H CEST experiment to study exchange at glycine (Gly) ¹H^α sites as the existing ¹H CEST experiments that have been designed to study dynamics in amide ¹H-¹⁵N spin systems and methyl ¹³CH₃ groups with three equivalent protons while suppressing ¹H-¹H NOE induced dips are not suitable for studying exchange in methylene ¹³CH₂ groups with inequivalent protons. Here a Gly ¹H^α CEST experiment to obtain the minor state Gly ¹H^α chemical shifts is presented. The utility of this experiment is demonstrated on the L99A cavity mutant of T4 Lysozyme (T4L L99A) that undergoes conformational exchange between two compact conformers. The CEST derived minor state Gly ¹H^α chemical shifts of T4L L99A are in agreement with those obtained previously using CPMG techniques. The experimental strategy presented here can also be used to obtain methylene proton minor state chemical shifts from protein sidechain and nucleic acid backbone sites.

Extending the Sensitivity of CEST NMR Spectroscopy to Micro-to-Millisecond Dynamics in Nucleic Acids Using High-Power Radio-Frequency Fields

Biomolecules undergo motions on the micro-to-millisecond timescale to adopt low-populated transient states that play important roles in folding, recognition, and catalysis. NMR techniques, such as Carr-Purcell-Meiboom-Gill (CPMG), chemical exchange saturation transfer (CEST), and R1ρ are the most commonly used methods for characterizing such transitions at atomic resolution under solution conditions. CPMG and CEST are most effective at characterizing motions on the millisecond timescale. While some implementations of the R1ρ experiment are more broadly sensitive to motions on the micro-to-millisecond timescale, they entail the use of selective irradiation schemes and inefficient 1D data acquisition methods. Herein, we show that high-power radio-frequency fields can be used in CEST experiments to extend the sensitivity to faster motions on the micro-to-millisecond timescale. Given the ease of implementing high-power fields in CEST, this should make it easier to characterize micro-to-millisecond dynamics in biomolecules.