Slow Structural Rearrangements Research Articles

Radical-Doped Metal-Organic Framework: Route to Nanoscale Defects and Magnetostructural Functionalities.

Nanosized structural defects in metal-organic frameworks (MOFs) attract growing attention and often remarkably enhance functional properties of these materials for various applications. In this work, a series of MOFs [Cu2(TPTA)1- x(BDPBTR) x] (H4TPTA, [1,1':3',1″-terphenyl]-3,3'',5,5''-tetracarboxylic acid; H4BDPBTR, 1,3-bis(3,5-dicarboxyphenyl)-1,2,4-benzotriazin-4-yl radical)) with a new stable radical linker doped into the structure has been synthesized and investigated using Electron Paramagnetic Resonance (EPR). Mixed linkers H4TPTA and H4BDPBTR were used to bridge copper(II) paddle-wheel units into a porous framework, where H4BDPBTR is the close structural analogue of H4TPTA. MOFs with various x = 0-0.4 were investigated. EPR studies indicated that the radical linker binds to the copper(II) units differently compared to diamagnetic linker, resulting in the formation of nanosized structural defects. Moreover, remarkable kinetic phenomena were observed upon cooling of this MOF, where slow structural rearrangements and concomitant changes of magnetic interactions were induced. Thus, our findings demonstrate that doping of structurally mimicking radical linkers into MOFs represents an efficient approach for designing target nanosized defects and introducing new magnetostructural functionalities for a variety of applications.

TMEM266 is a functional voltage sensor regulated by extracellular Zn2.

Voltage-activated ion channels contain S1-S4 domains that sense membrane voltage and control opening of ion-selective pores, a mechanism that is crucial for electrical signaling. Related S1-S4 domains have been identified in voltage-sensitive phosphatases and voltage-activated proton channels, both of which lack associated pore domains. hTMEM266 is a protein of unknown function that is predicted to contain an S1-S4 domain, along with partially structured cytoplasmic termini. Here we show that hTMEM266 forms oligomers, undergoes both rapid (µs) and slow (ms) structural rearrangements in response to changes in voltage, and contains a Zn2+ binding site that can regulate the slow conformational transition. Our results demonstrate that the S1-S4 domain in hTMEM266 is a functional voltage sensor, motivating future studies to identify cellular processes that may be regulated by the protein. The ability of hTMEM266 to respond to voltage on the µs timescale may be advantageous for designing new genetically encoded voltage indicators.

Heat Effects of the Thermal Decomposition of Amidoboranes of Potassium, Calcium, and Strontium

Thermal effects of the decomposition of potassium, calcium, and strontium amidoboranes at 354, 421, and 483 K are determined via drop calorimetry. The processes of decomposition are weakly exothermic and accompanied by the evolution of hydrogen. Upon the decomposition of calcium amidoborane at 421 K, a prolonged exothermic process is first observed; it is then followed by an endothermic effect, due possibly to the slow structural rearrangement of the product of decomposition. The solid products of decomposition are characterized by solid-state 11В NMR, FTIR spectroscopy, and mass spectrometry.

Sample-Based Models of Protein Energy Landscapes and Slow Structural Rearrangements.

Proteins often undergo slow structural rearrangements that involve several angstroms and surpass the nanosecond timescale. These spatiotemporal scales challenge physics-based simulations and open the way to sample-based models of structural dynamics. This article improves an understanding of current capabilities and limitations of sample-based models of dynamics. Borrowing from widely used concepts in evolutionary computation, this article introduces two conflicting aspects of sampling capability and quantifies them via statistical (and graphical) analysis tools. This allows not only conducting a principled comparison of different sample-based algorithms but also understanding which algorithmic ingredients to use as knobs via which to control sampling and, in turn, the accuracy and detail of modeled structural rearrangements. We demonstrate the latter by proposing two powerful variants of a recently published sample-based algorithm. We believe that this work will advance the adoption of sample-based models as reliable tools for modeling slow protein structural rearrangements.

From Aβ Filament to Fibril: Molecular Mechanism of Surface-Activated Secondary Nucleation from All-Atom MD Simulations.

Secondary nucleation pathways in which existing amyloid fibrils catalyze the formation of new aggregates and neurotoxic oligomers are of immediate importance for the onset and progression of Alzheimer's disease. Here, we apply extensive all-atom molecular dynamics simulations in explicit water to study surface-activated secondary nucleation pathways at the extended lateral β-sheet surface of a preformed Aβ9-40 filament. Calculation of free-energy profiles allows us to determine binding free energies and conformational intermediates for nucleation complexes consisting of 1-4 Aβ peptides. In addition, we combine the free-energy profiles with position-dependent diffusion profiles to extract complementary kinetic information and macroscopic growth rates. Single monomers bind to the β-sheet surface in a disordered, hydrophobically collapsed conformation, whereas dimers and larger oligomers can retain a cross-β conformation resembling a more ordered fibril structure. The association processes during secondary nucleation follow a dock/lock mechanism consisting of a fast initial encounter phase (docking) and a slow structural rearrangement phase (locking). The major driving forces for surface-activated secondary nucleation are the release of a large number of hydration water molecules and the formation of hydrophobic interface contacts, the latter being in contrast to the elongation process at filament tips, which is dominated by the formation of stable and highly specific interface hydrogen bonds. The calculated binding free energies and the association rates for the attachment of Aβ monomers and oligomers to the extended lateral β-sheet surface of the filament seed are higher compared to those for elongation at the filament tips, indicating that secondary nucleation pathways can become important once a critical concentration of filaments has formed.

TBid Undergoes Multiple Conformational Changes at the Membrane Required for Bax Activation

Bid is a Bcl-2 family protein that promotes apoptosis by activating Bax and eliciting mitochondrial outer membrane permeabilization (MOMP). Full-length Bid is cleaved in response to apoptotic stimuli into two fragments, p7 and tBid (p15), that are held together by strong hydrophobic interactions until the complex binds to membranes. The detailed mechanism(s) of fragment separation including tBid binding to membranes and release of the p7 fragment to the cytoplasm remain unclear. Using liposomes or isolated mitochondria with fluorescently labeled proteins at physiological concentrations as in vitro models, we report that the two components of the complex quickly separate upon interaction with a membrane. Once tBid binds to the membrane, it undergoes slow structural rearrangements that result in an equilibrium between two major tBid conformations on the membrane. The conformational change of tBid is a prerequisite for interaction with Bax and is, therefore, a novel step that can be modulated to promote or inhibit MOMP. Using automated high-throughput image analysis in cells, we show that down-regulation of Mtch2 causes a significant delay between tBid and Bax relocalization in cells. We propose that by promoting insertion of tBid via a conformational change at the mitochondrial outer membrane, Mtch2 accelerates tBid-mediated Bax activation and MOMP. Thus the interaction of Mtch2 and tBid is a potential target for therapeutic control of Bid initiated cell death.

Aggregation Kinetics of Interrupted Polyglutamine Peptides

Abnormally expanded polyglutamine domains are associated with at least nine neurodegenerative diseases, including Huntington's disease. Expansion of the glutamine region facilitates aggregation of the impacted protein, and aggregation has been linked to neurotoxicity. Studies of synthetic peptides have contributed substantially to our understanding of the mechanism of aggregation because the underlying biophysics of polyglutamine-mediated association can be probed independent of their context within a larger protein. In this report, interrupting residues were inserted into polyglutamine peptides (Q20), and the impact on conformational and aggregation properties was examined. A peptide with two alanine residues formed laterally aligned fibrillar aggregates that were similar to the uninterrupted Q20 peptide. Insertion of two proline residues resulted in soluble, nonfibrillar aggregates, which did not mature into insoluble aggregates. In contrast, insertion of a β-turn template DPG rapidly accelerated aggregation and resulted in a fibrillar aggregate morphology with little lateral alignment between fibrils. These results are interpreted to indicate that (a) long-range nonspecific interactions lead to the formation of soluble oligomers, while maturation of oligomers into fibrils requires conformational conversion and (b) that soluble oligomers dynamically interact with each other, while insoluble aggregates are relatively inert. Kinetic analysis revealed that the increase in aggregation caused by the DPG insert is inconsistent with the nucleation–elongation mechanism of aggregation featuring a monomeric β-sheet nucleus. Rather, the data support a mechanism of polyglutamine aggregation by which monomers associate into soluble oligomers, which then undergo slow structural rearrangement to form sedimentable aggregates.

Adsorption Kinetics of Laponite and Ludox Silica Nanoparticles onto a Deposited Poly(diallyldimethylammonium chloride) Layer Measured by a Quartz Crystal Microbalance and Optical Reflectometry

A quartz crystal microbalance with dissipation (QCM-D) and an optical reflectometer (OR) have been used to investigate the adsorption behavior of Laponite and Ludox silica nanoparticles at the solid-liquid interface. The adsorption of both Laponite and Ludox silica onto poly(diallyldimethylammonium chloride) (PDADMAC)-coated surfaces over the first few seconds were studied by OR. Both types of nanoparticles adsorbed rapidly and obtained a stable adsorbed amount after only a few minutes. The rate of adsorption for both nanoparticle types was concentration dependent. The maximum adsorption rate of Ludox nanoparticles was found to be approximately five times faster than that for Laponite nanoparticles. The QCM data for the Laponite remained stable after the initial adsorption period at each concentration tested. The observed plateau values for the frequency shifts increased with increasing Laponite particle concentration. The QCM data for the Ludox nanoparticles had a more complex long-time behavior. In particular, the dissipation data at 3 ppm and 10 ppm Ludox increased slowly with time, never obtaining a stable value within the duration of the experiment. We postulate here that this is caused by slow structural rearrangements of the particles and the PDADMAC within the surface adsorbed layer. Furthermore, the QCM dissipation values were significantly smaller for Laponite when compared with those for Ludox for all nanoparticle concentrations, suggesting that the Laponite adsorbed layer is more compact and more rigidly bound than the Ludox adsorbed layer.

Time evolution of the thermotropic behavior of spontaneous liposomes and disks of the DMPC–DTAC aqueous system

In this work, solubilization of the phospholipid 1,2-dimyristoyl- sn-glycero-3-phosphocholine (DMPC) by the cationic detergent n-dodecyltrimethylammonium chloride (DTAC) was studied in aqueous solution, at a fixed DMPC concentration and variable detergent:lipid (D:L) molar ratios. The colloidal nanostructures present in different stages of the solubilization process were characterized using micro-differential scanning calorimetry (DSC) and dynamic light scattering (DLS) techniques. For total (analytical) D:L molar ratios below ≈1, DTAC monomers incorporate into the DMPC liposome bilayers, forming smaller and more fluid vesicles than pure DMPC liposomes. At D:L ≈ 1–2, vesicles begin to rupture, coexisting with intact vesicles and bilayer fragments. At D:L ≈ 2–12.5, discoidal and spherical micelles are formed and coexist with vesicles; a slow structural rearrangement of the system, monitored in successive DSC heating/cooling cycles, was observed, and is reported for the first time. Finally, for D:L above ≈15–20, the bilayers are completely dissolved, and the main aggregates in solution become spherical micelles, which slowly evolve to cylindrical (threadlike) micelles. Based on the dependence of the temperature and enthalpy of transitions on the total D:L molar ratio, at constant DMPC concentration, a schematic model, showing the different colloidal nanostructures present in the solubilization process, is proposed.

Structural Rearrangements Linked to Global Folding Pathways of the Azoarcus Group I Ribozyme

Stable RNAs must fold into specific three-dimensional structures to be biologically active, yet many RNAs form metastable structures that compete with the native state. Our previous time-resolved footprinting experiments showed that Azoarcus group I ribozyme forms its tertiary structure rapidly (τ < 30 ms) without becoming significantly trapped in kinetic intermediates. Here, we use stopped-flow fluorescence spectroscopy to probe the global folding kinetics of a ribozyme containing 2-aminopurine in the loop of P9. The modified ribozyme was catalytically active and exhibited two equilibrium folding transitions centered at 0.3 and 1.6 mM Mg 2+, consistent with previous results. Stopped-flow fluorescence revealed four kinetic folding transitions with observed rate constants of 100, 34, 1, and 0.1 s − 1 at 37 °C. From comparison with time-resolved Fe(II)-ethylenediaminetetraacetic acid footprinting of the modified ribozyme under the same conditions, these folding transitions were assigned to formation of the I C intermediate, tertiary folding and docking of the nicked P9 tetraloop, reorganization of the P3 pseudoknot, and refolding of nonnative conformers, respectively. The footprinting results show that 50–60% of the modified ribozyme folds in less than 30 ms, while the rest of the RNA population undergoes slow structural rearrangements that control the global folding rate. The results show how small perturbations to the structure of the RNA, such as a nick in P9, populate kinetic folding intermediates that are not observed in the natural ribozyme.

Energetics and Kinetics of a Conformational Switch in G-Quadruplex DNA

Circular dichroism and differential scanning calorimetry were used to determine the energetics of the conformational switch of the human telomere quadruplex formed by the sequence d[AGGG(TTAGGG)3] between the sodium basket form and the potassium hybrid form. The energy barrier separating the two conformations was found to be modest, only 1.4-2.4 kcal mol(-1). The kinetics of exchange of bound K+ for Na+ cations and the concomitant conformational switch was assessed by measuring time-dependent changes in the circular dichroism spectrum accompanying the cation exchange reaction. The time course of these changes was found to consist of three distinct kinetic processes: a rapid phase that was complete in less than 5 ms followed by two slower phases with relaxation times of 40-50 s and 600-800 s at 25 degrees C and pH 7.0. We interpret these kinetics in terms of a model in which the bound Na+ cations are rapidly replaced by K+ followed by relatively slow structural rearrangements to generate the final K(+)-bound product(s). Circular dichroism studies showed that addition of the porphyrin TmPyP4 promoted conversion of the basket to the hybrid form. The kinetics of the TmPyP4-induced conformational change were the same as those observed for the cation exchange reaction.

Simulation Study of Noncovalent Hybridization of Carbon Nanotubes by Single-Stranded DNA in Water

Recent discovery that single-stranded DNA (ssDNA) binds to carbon nanotubes with high affinity to form soluble hybrids has received great attention as a promising approach to solving the long-standing problem of nanotube solubilization and separation. The mechanism of this process, including the nature of the DNA-nanotube interactions and the molecular structure of the hybrids is still not well understood. Here, we use all-atom replica-exchange molecular dynamics simulations to study the association of several ssDNA decamers with single-walled carbon nanotubes of different chirality in an aqueous environment. The oligonucleotides are found to readily adsorb onto the nanotube surface, after which they undergo a slow structural rearrangement. Cluster analysis of bound DNA conformations as well as population distribution maps computed as a function of several local and global order parameters show that the hybrids exhibit a complex morphology with DNA strands assuming a number of distinct backbone geometries, which depend on both DNA sequence and nanotube diameter. In contrast, the nucleotide bases are found to align parallel to the nanotube surface with a high degree of orientational order. While the binding appears to be primarily driven by energetically favorable pi-stacking of DNA bases onto the nanotube surface, equilibrium distribution of hybrid conformations is modulated by a complex interplay of forces, including the DNA conformational strain and solvent interactions. As a result, the hybrid free-energy landscapes are found to be rugged, with multiple low-lying minima separated by high barriers, several of which are significantly populated at room temperature. Qualitative differences are observed in free energy profiles of purine- and pyrimidine-based oligonucleotide sequences and are attributed to the difference in self-stacking propensity of the bases.

Synergetic mechanisms of structural regulation of the electron transfer and other reactions of biological macromolecules

A general concept of self-regulation of biomolecule function due to the interaction of fast reaction (in particular, electron transfer) events and slow structural re-arrangements is expounded. It proceeds from the fact that the structural changes produced by each elementary reaction act can be accumulated, up to drastic modification of the reaction turnover. Thus, conformational control of biomolecule functioning is realized along synergetic scenarios at the single-molecule level. A proper theoretical formalism and its application to the analysis of electron transfer reactions in photosynthetic reaction centers (RCs) are described. The peculiarities of bistability and hysteresis phenomena as well as of corresponding effects of RC protein memory, revealed both theoretically and experimentally, are discussed.

Cooperativity in the Thermosome

The thermosome from Thermoplasma acidophilum is a type II chaperonin composed of eight alpha and eight beta subunits. The genes encoding the two types of subunit were co-expressed in Escherichia coli and the alpha8/beta8 complex purified from the cell extract. The isolated complex showed steady-state ATPase properties characteristic of the thermosome purified from the native organism and was capable of enhancing the folding yield of a thermostable enzyme at elevated temperature (55 degrees C). To compare the nucleotide response of this double-ring structure with the type I and more compositionally heterogeneous type II chaperonins, the tryptophan residue within the alpha subunit was used as a fluorescence reporter of the conformational changes within the thermosome induced by the binding of nucleotides. Stopped-flow measurements of indole fluorescence at 55 degrees C showed that there is a fast (approximately 350 s(-1)) and a slow (approximately 0.6 s(-1)) structural rearrangement when ATP binds to the thermosome. Further examination of the fast rearrangement showed that the associated rate constant followed a two-phase saturation profile, as it does for GroEL and for the type II chaperonin from the eukaryotic cytoplasm. This result, in keeping with these precedents, reveals that the thermosome is also a negatively cooperative system with respect to inter-ring communications, i.e. the first ring loads with higher affinity than the second. As in the case of GroEL, the loading of the second ring is weakened by ADP, implying that asymmetric ATP/ADP complexes are favoured over symmetric ones. Despite the difference in co-protein involvement in the type I and II chaperonins, these observations show that negative cooperativity is a common feature of all chaperonins thus far examined. This property results in a strong preference for asymmetry in nucleotide occupancy and implies at least some commonality with the reciprocating encapsulation mechanism shown for the GroE chaperonins.

Rheological properties of whey protein/gum arabic coacervates

Complex coacervation in whey protein (WP)/gum arabic (GA) mixtures occurred in a specific pH window between 2.5 and 4.8. After phase separation, a concentrated polymer (also called coacervate) phase was obtained, whose viscoelastic properties were studied at various pH values. The viscosity of the WP/GA coacervate exhibited a surprisingly low shear-rate dependence, especially at pH 4.0, from 0.3 to 30 s−1 and a shear thinning above 30 s−1, indicating a structural change. Hysteresis in the flow curve was measured at the pH values at which the electrostatic interactions were the strongest. Hysteresis was due to a slow structural rearrangement of the coacervate phase and, with time, the initial viscosity was completely recovered, showing that structural changes were reversible. In frequency sweep experiments, the values of the viscous modulus G″ were up to ten times higher than the values of the elastic modulus G′, indicating the highly viscous character of the coacervates. pH 4.0 appeared to be the pH at which the coacervate phase was the most concentrated in biopolymer (Cp=32%), and at which the highest viscosity was measured. By decoupling the effect of biopolymer concentration and electrostatic interactions, it appeared that the high viscosity of the WP/GA coacervates was mainly due to the strong electrostatic interactions between WP and GA at low pH. The weaker the electrostatic interaction was, the lower the viscosity, especially at pH 4.5 and 3.0. The viscous behavior of the coacervates showed parallels with that of concentrated latex dispersions. WP/GA particles would consist of a GA polymer chain (as in latex) but now crosslinked by the electrostatic interactions with WP.

Structure, Dynamics and Properties of Materials with Polymers Having Complex Architectures

AbstractVarious non‐linear highly branched polymers such as multiarm stars, block copolymer micelles and bottlebrush‐like polymers have been studied in order to analyze their intramolecular structure and effects of spatial ordering resulting from their specific macromolecular architecture. These polymers constitute a class of compact macromolecules which, due to the high intramolecular density, interact strongly, excluding each other in space. Investigations of the structure and dynamics in such systems, using various experimental methods as well as computer simulations, have been performed. Small angle X‐ray scattering is used to characterize the structure and mechanical spectroscopy is used for the detection of the dynamic behavior of the systems. Simulations have been performed using the cooperative motion algorithm with lattice polymers equivalent to the considered macromolecules. Results of both experiments and the simulation seem to support the concept of slow structural cooperative rearrangements controlling the flow in such systems. The effects are analogous to those related to flow in low molecular liquids but take place on another size scale. The new slow relaxation processes creates new super soft‐states which are characterized by shear modulus plateau lower than 104 Pa.

Vibrational echo correlation spectroscopy probes of hydrogen bond dynamics in water and methanol

Multidimensional vibrational echo correlation spectroscopy with full phase resolution is used to measure hydrogen bond dynamics in water and methanol. The OD hydroxyl stretches of methanol-OD oligomers in CCl 4 and HOD in H 2O are studied using the shortest mid-IR pulses (<50 fs, <4 cycles of light) produced to date. The pulses have sufficient spectral bandwidth to span the very broad (>400 cm −1) spectrum of the 0–1 and 1–2 transitions. Hydrogen bond population dynamics are extricated with exceptional detail in MeOD oligomers because the different hydrogen bonded species are spectrally distinct. The experimental results along with detailed calculations indicate the strongest hydrogen bonds are selectively broken through a non-equilibrium relaxation pathway following vibrational relaxation of the hydroxyl stretch. The correlation spectra are also a sensitive probe of the fluctuations in water and provide a stringent test of water models that are widely used in simulations of aqueous systems. The analysis of the 2D band shapes demonstrates that different hydrogen bonded species are subject to distinct (wavelength dependent) ultrafast (∼100 fs) local fluctuations and essentially identical slow (0.4 and ∼2 ps) structural rearrangements. Observation of wavelength dependent dynamics demonstrates that standard theoretical approaches assuming Gaussian fluctuations cannot adequately describe water dynamics.

Chaperone-like activity of alpha-crystallin is enhanced by high-pressure treatment.

alpha-Crystallin, an oligomeric protein in vertebrate eye lens, is a member of the small heat-shock protein family. Several papers pointed out that its chaperone-like activity could be enhanced by increasing the temperature. We demonstrate in the present study that structural perturbations by high hydrostatic pressures up to 300 MPa also enhance this activity. In contrast with temperature-induced changes, the pressure-induced enhancement is reversible. After pressure release, the extra activity is lost with a relaxation time of 2.0+/-0.5 h. Structural alterations contributing to the higher activity were studied with IR and fluorescence spectroscopy, and light-scattering measurements. The results suggest that while the secondary structure barely changes under pressure, the interactions between the subunits weaken, the oligomers dissociate, the area of accessible hydrophobic surfaces significantly increases and the environment of tryptophan residues becomes slightly more polar. It seems that structural flexibility and the total surface area of the oligomers are the key factors in the chaperone capacity, and that the increase in the chaperone activity does not require the increase in the oligomer size as was assumed previously [Burgio, Kim, Dow and Koretz (2000) Biochem. Biophys. Res. Commun. 268, 426-432]. After pressure release, the structure of subunits are reorganized relatively quickly, whereas the oligomer size reaches its original value slowly with a relaxation time of 33+/-4 h. In our interpretation, both the fast and slow structural rearrangements have an impact on the functional relaxation.

Kinetics of rapid structural changes during heat setting of preoriented PEEK/PEI blend films as followed by spectral birefringence technique

The influence of composition and preorientation on the development of structural hierarchy during heat setting of PEEK/PEI films was investigated using on-line birefringence, and off-line wide-angle and small-angle X-ray scattering, infrared dichroism and thermal analysis techniques. When the PEEK/PEI blends are drawn to deformation levels below the onset of strain hardening, the subsequent heat setting at high temperature starts with a large relaxation process followed by a fast crystallization and a long-term slow structural rearrangement stages. When the films are predrawn beyond a critical structural level (crystallinity and orientation), the initial relaxation stage disappears. This signifies that beyond a critical structural order a long-range physical network, where the nodes consist of crystallized domains and chain—self and cross-entanglements—are formed. This physical network allows the entropy driven shrinkage stresses to be maintained that results in the development of oriented crystalline phase. The addition of non-crystallizable PEI chains was found to retard the formation of this ‘network structure’ resulting in lower orientation levels.

Cooperative two-photon effects in chalcogenide photoresists

Photosensitive chalcogenide glasses are very promising materials for pulsed holography, pulsed photolithography and pulsed optical information recording due to their exceptional sensitivity to pulsed light excitation. We present a comprehensive study of the dynamics of the pulse photoresponse of 0.3–1.0 μm thick chalcogenide glassy As 50Se 50 thin films using a transient-grating method. Both the decrease of transparency and a change of the dissolution rate were recorded after single Nd : YAG and ArF laser pulse and about (1–5)×1000 times larger energy was necessary to obtain the same changes using CW radiation. We show that when excited by a short laser pulse, two different time scales behavior and different intensity dependence of short- and long-time scale signals are present: the short-time signal behaves linearly with the input power but the long-time signal behaves quadratically with the input power. Such behavior reflects a fast electronic process followed by a slow structural rearrangement (the after-pulse effect). The obtained data indicate that the strong increase of photosensitivity following a short intense pulsed light excitation is due to a two-photon effect that aids the process of structural rearrangement as when two photons weaken or break the neighboring bonds, the probability of structural transformation increases significantly.