Stable Pairs Research Articles

First Principles Study of Bismuth Vacancy Formation in (111)-Strained BiFeO3

Epitaxial strain is known to significantly influence the structural and functional properties of oxide thin films. However, its impact on point defect concentration has been less explored. Due to the challenges in experimentally measuring thin-film stoichiometry, computational studies become crucial. In this work, we use first-principles calculations based on density functional theory to investigate the formation and stability of Bi vacancies and Bi-O vacancy pairs in BiFeO3 (BFO) under (111) epitaxial strain. Our results demonstrate that compressive strain (−4%) decreases the formation enthalpy of Bi vacancies by 0.88 eV, whereas tensile strain (4%) increases it by 0.39 eV. Out-of-plane (OP) Bi-O vacancy pairs exhibit enhanced stability under both compressive and tensile strain, with formation enthalpy reductions of 1.49 eV and 1.05 eV, respectively. In contrast, in-plane (IP) vacancy pairs are stabilized under compressive strain but are insensitive to tensile strain. Finally, we discuss how these findings influence Bi stoichiometry during thin-film growth and the role of local strain fields in the formation of conducting domain walls.

Radiation protection of W–Al composite films/coatings for aviation using genetic algorithms

This study evaluated electron shielding capabilities of various materials using Geant4 Monte Carlo software. By analyzing materials with different atomic numbers, we designed and tested composite films and coatings for radiation protection. Under 1.2 MeV electron irradiation, these materials reduced the dose deposited in electronic components by over 70 %. The fabricated composite films (W–Al) reduced the actual total dose by 100 % with a 30.67 % mass increase, closely matching the simulation result of 75.36 %. The composite coatings (W–Al) reduced the dose by 89.2 % with a 33 % mass increase, matching the simulation result of 70 %. Cascade collision simulations revealed that higher PKA energies lead to longer times to reach the thermal peak, more peak defects, and more stable defect pairs. This is due to the displacement threshold energy of the atoms in aluminum and tungsten. These results demonstrate the effectiveness of our composite films and coatings in enhancing electron shielding performance and validate our design methods.

Logarithmic Pandharipande–Thomas spaces and the secondary polytope

Maulik and Ranganathan have recently introduced moduli spaces of logarithmic stable pairs. In the case of toric surfaces we recast this theory using three ingredients: Gelfand, Kapranov and Zelevinsky secondary polytopes, Hilbert schemes of points, and tautological vector bundles. In particular, logarithmic stable pairs spaces are expressed as the zero set of an explicit section of a vector bundle on a logarithmically smooth space, thus providing an explicit description of their virtual fundamental class. A key feature of our construction is that moduli spaces are completely canonical, unlike the existing construction, which is only well-defined up to logarithmic modifications. We calculate the Euler–Satake characteristics of our moduli spaces in a number of basic examples. These computations indicate the complexity of the spaces we construct.

Zn2+ ions improve the fidelity of metal-mediated primer extension while suppressing intrinsic and Mn2+-induced mutagenic effects by DNA polymerases.

While Mn2+ ions are well-established for reducing the fidelity of DNA polymerases, leading to the misincorporation of nucleotides, our investigation of the effects of metal ions revealed a contrasting role of Zn2+. Here, we demonstrate that Zn2+ ions enhance the fidelity of DNA polymerases (the 3' → 5' exonuclease-deficient Klenow fragment and Taq DNA polymerase) by suppressing misincorporation during primer extension reactions. Remarkably, Zn2+ ions inhibit both intrinsic misincorporation and Mn2+-induced misincorporation of nucleotides. Furthermore, Zn2+ ions also effectively suppressed misincorporation during metal-mediated primer extension reactions, which involved forming Ag+ and Hg2+ ion-mediated base pairs. These findings suggest that Zn2+ ions inhibit both intrinsic and Mn2+-induced mismatched base pair formation. Consequently, the combined use of Mn2+ and Zn2+ ions may offer a strategy for precisely regulating the fidelity of DNA polymerases. Remarkably, Zn2+ ions even suppress misincorporation in primer extension reactions that rely on metal-mediated base pairs, and conversely, this suggests that DNA polymerases recognize metal-mediated base pairs such as T-Hg2+-T, C-Ag+-A, and C-Ag+-T as relatively stable base pairs. These results imply that Zn2+ ions may also enhance the fidelity of DNA polymerases when incorporating non-canonical nucleobases, potentially paving the way for the expansion of the genetic alphabet.

Direct observation of the complex S(IV) equilibria at the liquid-vapor interface

The multi-phase oxidation of S(IV) plays a crucial role in the atmosphere, leading to the formation of haze and severe pollution episodes. We here contribute to its understanding on a molecular level by reporting experimentally determined pKa values of the various S(IV) tautomers and reaction barriers for SO2 formation pathways. Complementary state-of-the-art molecular-dynamics simulations reveal a depletion of bisulfite at low pH at the liquid-vapor interface, resulting in a different tautomer ratio at the interface compared to the bulk. On a molecular-scale level, we explain this with the formation of a stable contact ion pair between sulfonate and hydronium ions, and with the higher energetic barrier for the dehydration of sulfonic acid at the liquid-vapor interface. Our findings highlight the contrasting physicochemical behavior of interfacial versus bulk environments, where the pH dependence of the tautomer ratio reported here has a significant impact on both SO2 uptake kinetics and reactions involving NOx and H2O2 at aqueous aerosol interfaces.

Modeling High Concentration Bisalt-in-Sulfolane Electrolytes and the Observation of Ligand-Bridged Cation-Pair Complexes.

Despite the abundance of sodium over lithium in Earth's crust and the copious amounts of expensive lithium salt required to make Li-ion high-concentration electrolytes (HCEs), studies of HCEs made from sodium salts remain sparse. A comparative molecular-level study of Li- and Na-ion HCEs and mixed cation or bisalt HCEs in an organic solvent is missing. To fill this gap, we studied model HCEs of pure and mixed Li and Na salts of bis(fluorosulfonyl)amide (FSI) in sulfolane using a confluence of classical molecular dynamics (MD), ab initio MD (AIMD) simulations, and quantum chemical cluster calculations. While Li-ion HCEs display transport properties superior to those of Na-ion HCEs, the latter's performance can be considerably improved by replacing even 25% of Na-ions with Li-ions. While the effects of doping are largely systemic, a larger sensitivity of the identity of solvation shells of Li-ions to the Li-content of the HCE is observed; in contrast, those of Na-ions are more oblivious to it. Fascinating ligand-bridged, short-distance cation pairs observed in the classical MD simulations are confirmed using density functional theory-based AIMD simulations. Quantum chemical calculations in the gas phase reveal the thermodynamic stability of such cation pairs complexed with multiple anions and solvent molecules.

Twist along Central C-C Bonds in a Series of Fully π-Fused Propellanes.

Without stereogenic carbon centers, organic molecules can be chiral when they have large energy barriers for conformational inversion. In this work, conformational behaviors are investigated for a series of tricyclic propellane skeletons with increasing 6-membered-ring peripheral moieties fused with aromatic rings. According to theoretical calculations, trinaphtho[3.3.3]propellane has three vertical naphthalene rings like triptycene shape without torsion along the central C-C single bond. On the other hand, hexabenzo[4.4.4]propellane shows hexaphenylethane-like ca. 60° twist along the bond with large activation energy of 64 kcal mol-1 for twist inversion because of the high congestion caused by three 6-membered-ring loops. Indeed, the [4.4.4]propellane gives a stable pair of chiroptical enantiomers toward heating at 146 °C. By contrast, a hybrid [4.3.3]propellane exhibits fast interconversion between two twisted conformations even at -80 °C.

Computation of Deformable Interface Two‐Phase Flows: A Semi‐Lagrangian Finite Element Approach

ABSTRACTThis work aims at presenting a new computational approach to study two and three dimensional two‐phase flows and two dimensional coalescence phenomenon using direct numerical simulation. The flows are modeled by the incompressible Navier–Stokes equations, which are approximated by the finite element method. The Galerkin formulation is used to discretize the Navier–Stokes equations in the spatial domain and the semi‐Lagrangian method is used to discretize the material derivative. In order to satisfy the Ladyzhenskaya–Babuška–Brezzi condition, high‐order stable pair of elements are used, with pressure and velocity fields being calculated on different degrees of freedom in the unstructured mesh nodes. The interface is modeled by an unfitted adaptive moving mesh, where interface nodes are tracked in a Lagrangian fashion and moved with the velocity solution of the fluid motion equations. The surface tension is computed using the interface curvature and the gradient of a Heaviside function, and added in the momentum equations as a body force. In order to avoid undesired spurious modes at the interface due to high property ratios, a smooth transition between fluid properties is defined on the interface region. Several benchmark tests have been carried out to validate the proposed approach, and the obtained results have demonstrated agreement with analytical solutions and numerical results reported in the literature. A coalescence model is also proposed based on geometric criteria and results show interesting dynamics.

Spin transfer torque and field driven 360° domain wall to bimeron conversion

In this work, using micromagnetic simulations we study the mechanism involved in the conversion of a 360° domain wall into stable pairs of bimeron (Q = +1) and antibimeron (Q = -1) in a ferromagnetic background when it is subjected to Zhang Li type of spin transfer torque. We show that the time scales associated with the conversion process can be significantly reduced by applying out of plane bias field. In particular, choosing a constant inclination angle and increasing the intensity of bias field aids in reducing the energy barrier required to stabilize the spin textures. Similarly, for a constant bias field intensity, varying the inclination angle between 0° to 40° reveals that the time required to stabilize the first bimeron is reduced by ∼ 3 ns. Further, tailoring the strength of in-plane current density as well as effective uniaxial anisotropy leads to faster conversion of domain wall to bimerons.

Stick-slip unfolding favors self-association of expanded HTT mRNA

In Huntington’s Disease (HD) and related disorders, expansion of CAG trinucleotide repeats produces a toxic gain of function in affected neurons. Expanded huntingtin (expHTT) mRNA forms aggregates that sequester essential RNA binding proteins, dysregulating mRNA processing and translation. The physical basis of RNA aggregation has been difficult to disentangle owing to the heterogeneous structure of the CAG repeats. Here, we probe the folding and unfolding pathways of expHTT mRNA using single-molecule force spectroscopy. Whereas normal HTT mRNAs unfold reversibly and cooperatively, expHTT mRNAs with 20 or 40 CAG repeats slip and unravel non-cooperatively at low tension. Slippage of CAG base pairs is punctuated by concerted rearrangement of adjacent CCG trinucleotides, trapping partially folded structures that readily base pair with another RNA strand. We suggest that the conformational entropy of the CAG repeats, combined with stable CCG base pairs, creates a stick-slip behavior that explains the aggregation propensity of expHTT mRNA.

Oligonucleotides Containing 8,1′‐Ethano‐2′‐deoxyadenosine: Synthesis and Properties

This article presents the synthesis and properties of oligonucleotides containing 8,1′‐ethano‐2′‐deoxyadenosine (ethano‐dA) in which the adenine base (ethA) is fixed to the syn conformation. Ethano‐dA phosphoramidite was synthesized by using a 1,5‐hydrogen‐atom transfer/radical cyclization process and introduced into oligonucleotides, which were successfully obtained by employing the 4,4′‐dimethoxytrityl‐off synthesis mode. The duplex‐forming ability of the oligonucleotides containing ethano‐dA was evaluated. In the antiparallel Watson–Crick duplex, the ethA formed stable base pairs with thymine and guanine, in which two Hoogsteen‐type hydrogen bonds could be formed. By contrast, the parallel Hoogsteen duplex containing the ethA:thymine base pair showed thermal stability comparable to that containing a natural adenine:thymine base pair because ethA and thymine can form two Watson–Crick‐type hydrogen bonds. These results indicated that ethano‐dA may be helpful for elucidating the function of adenine bases via the syn conformation in DNA.

Concept of electrons pairing during the formation of covalent chemical bonds

This research studies the forces acting in an electron pair with antiparallel spins participating in forming a covalent chemical bond between atoms. The first is the force of electrostatic repulsion of electrons. The second is the gravitational attraction of these electrons. The third is the force of electromagnetic attraction between the electron pair. From calculations it follows, that the force of gravitational attraction is negligible, and therefore it is not able to withstand the force of electrostatic repulsion between electrons. However, the force of electromagnetic attraction between a pair of electrons with antiparallel spins turned out to be hundreds of times greater than the force of their electrostatic repulsion. As a result, the formation of stable electron pairs in molecular orbitals becomes possible. Thus, valence electrons of neighboring atoms interact with each other like femto-electromagnets, as a result of which a stable bond is formed between these electrons. Calculations have shown that in hydrogen molecule the force of electrostatic attraction between nuclei and paired electrons of a molecular orbital significantly exceeds the force of electrostatic repulsion of nuclei, which ensures the formation of a strong covalent chemical bond. To form covalent bonds in other molecules, the force of electrostatic attraction between the nuclei and paired electrons of the molecular orbitals must be much greater than the forces of electrostatic repulsion between both the positively charged nuclei and the negatively charged electrons of the atomic orbitals of different atoms.

Investigation of the Nonradiative Photoprocesses of Unnatural DNA Base: 7-(2-Thienyl)-imidazo[4,5-b]pyridine (Ds)─A Computational Study.

7-(2-Thienyl)-imidazo[4,5-b]pyridine (Ds) is an unnatural nucleic acid that forms a stable pair with pyrrole-2-carbaldehyde (Pa) in DNA. This Ds-Pa pair gets stabilized via van der Waals interaction and shape fitting. In our previous study [Ghosh, P. J. Phys. Chem. A 2021, 125, 5556-5561], we investigated the nonradiative photoprocesses of the unnatural DNA base Pa, and also there are some studies on its stability and reactivity in the ground state. But, to consider it as a good unnatural base pair, one has to understand its stability not only in the ground state but also in the excited states after absorbing ultraviolet (UV) radiation. Therefore, in this study, the excited-state photoprocesses of Ds on UV irradiation and its nonradiative decay channels have been investigated using state-of-the-art multireference methods, and this investigation finally leads the molecule to access the minimum energy crossing point (MECP) via a downhill pathway.

Stereo-Active Lone Pairs Induced Second Harmonic Generation Responses and Electrocatalytic Activity in Hybrid Material.

The lone pair electrons in the electronic structure of molecules have been a prominent research focus in chemistry for more than a century. Stable s2 lone pair electrons significantly influence material properties, including thermoelectric properties, nonlinear optical properties, ferroelectricity, and electro(photo)catalysis. While major advances have been achieved in understanding the influence of lone pair electrons on material characteristics, research on this effect in organic-inorganic hybrid materials is in its initial stage. In this work, we successfully obtained a novel organic-inorganic hybrid multifunctional material incorporating Ge with 4s2 lone pair electrons, (MeHDabco)2[GeBr3]4-H2O (MeHDabco=N-methyl-1,4-diazabicyclo[2.2.2]octane) (1). Driven by the stereochemically active lone pair electrons on the Ge2+, 1 crystallizes in the noncentrosymmetric space group P21 at room temperature and exhibits good second harmonic generation (SHG) responses. Interestingly, 1 also shows electrocatalytic activity for the hydrogen evolution reaction (HER) due to the existence of lone pair electrons on Ge2+ cations. The electrochemical experiment combined with the density functional theory (DFT) calculations revealed that the lone pair electrons act as both an active site for proton adsorption and facilitate the ionization of water. This work not only emphasizes the important role of lone pair electrons in material properties and functions but also provides new insight for designing novel Ge-based multifunctional hybrid materials.

SmithHunter: a workflow for the identification of candidate smithRNAs and their targets

BackgroundSmithRNAs (Small MITochondrial Highly-transcribed RNAs) are a novel class of small RNA molecules that are encoded in the mitochondrial genome and regulate the expression of nuclear transcripts. Initial evidence for their existence came from the Manila clam Ruditapes philippinarum, where they have been described and whose activity has been biologically validated through RNA injection experiments. Current evidence on the existence of these RNAs in other species is based only on small RNA sequencing. As a preliminary step to characterize smithRNAs across different metazoan lineages, a dedicated, unified, analytical workflow is needed.ResultsWe propose a novel workflow specifically designed for smithRNAs. Sequence data (from small RNA sequencing) uniquely mapping to the mitochondrial genome are clustered into putative smithRNAs and prefiltered based on their abundance, presence in replicate libraries and 5′ and 3′ transcription boundary conservation. The surviving sequences are subsequently compared to the untranslated regions of nuclear transcripts based on seed pairing, overall match and thermodynamic stability to identify possible targets. Ample collateral information and graphics are produced to help characterize these molecules in the species of choice and guide the operator through the analysis. The workflow was tested on the original Manila clam data. Under basic settings, the results of the original study are largely replicated. The effect of additional parameter customization (clustering threshold, stringency, minimum number of replicates, seed matching) was further evaluated.ConclusionsThe study of smithRNAs is still in its infancy and no dedicated analytical workflow is currently available. At its core, the SmithHunter workflow builds over the bioinformatic procedure originally applied to identify candidate smithRNAs in the Manila clam. In fact, this is currently the only evidence for smithRNAs that has been biologically validated and, therefore, the elective starting point for characterizing smithRNAs in other species. The original analysis was readapted using current software implementations and some minor issues were solved. Moreover, the workflow was improved by allowing the customization of different analytical parameters, mostly focusing on stringency and the possibility of accounting for a minimal level of genetic differentiation among samples.

Asymptotic estimates for concentrated vortex pairs

Burton-Lopes Filho-Nussenzveig Lopes (2013) [6] studied the existence and stability of slowly traveling vortex pairs as maximizers of the kinetic energy penalized by the impulse relative to a prescribed rearrangement class. In this paper, we prove that after a suitable scaling transformation the maximization problem studied by Burton-Lopes Filho-Nussenzveig Lopes in fact gives rise to a family of stable concentrated vortex pairs approaching a pair of point vortices with equal magnitude and opposite signs. The key ingredient of the proof is to deduce a uniform bound for the size of the supports of the scaled maximizers.

7-Deaza-2'-deoxyisoguanosine, a Noncanonical Nucleoside for Nucleic Acid Code Expansion and New DNA Constructs: Nucleobase Functionalization of Inverse Watson-Crick and Purine-Purine Base Pairs.

7-Deaza-2'-deoxyisoguanosine forms stable inverse Watson-Crick base pairs with 5-methyl-2'-deoxyisocytidine and purine-purine base pairs with 2'-deoxyguanosine or 5-aza-7-deaza-2'-deoxyguanosine. Both base pairs expand the genetic coding system. The manuscript reports on the functionalization of these base pairs with halogen atoms and clickable side chains introduced at 7-position of the 7-deazapurine base. Oligonucleotides containing the functionalized base pairs were prepared by solid-phase synthesis. To this end, a series of phosphoramidites were synthesized and clickable side chains with short and long linkers were incorporated in oligonucleotides. Fluorescent pyrene conjugates were obtained by postmodification. Functionalization of DNA with a single inverse Watson-Crick base pair by halogens or clickable residues has only a minor impact on duplex stability. Pyrene click adducts increase (long linker) or decrease (short linker) the double helix stability. Stable hybrid duplexes were constructed containing three consecutive purine-purine pairs of 7-functionalized 7-deaza-2'-deoxyisoguanine with guanine or 5-aza-7-deazaguanine in the center and Watson-Crick pairs at both ends. The incorporation of a hybrid base pair tract of 7-deaza-2'-deoxyisoguanosine/5-aza-7-deaza-2'-deoxyguanosine pairs stabilizes the double helix strongly. Fluorescence intensity of pyrene short linker adducts increased when the 7-deazapurine base was positioned opposite to 5-methylisocytosine (inverse base pair) compared to purine-purine base pairs with guanine or 5-aza-7-deazaguanine in opposite positions. For long liker adducts, the situation is more complex. Circular dichroism (CD) spectra of purine DNA differ to those of Watson-Crick double helices and are indicative for the new DNA constructs. The impact of 7-deaza-2'-deoxyisoguanine base pair functionalization is studied for the first time and all experimental details are reported to prepare DNA functionalized at the 7-deazaisoguanine site. The influence of single and multiple incorporations on DNA structure and stability is shown. Clickable residues introduced at the 7-position of the 7-deazaisoguanine base provide handles for Huisgen-Sharpless-Meldal click cycloadditions without harming the stability of purine-pyrimidine and purine-purine base pairs. Other chemistries might be used for bioconjugation. Our investigation paves the way for the functionalization of a new DNA related recognition system expanding the common Watson-Crick regime.

7-Deazapurine and Pyrimidine Nucleoside and Oligonucleotide Cycloadducts Formed by Inverse Diels-Alder Reactions with 3,6-Di(pyrid-2-yl)-1,2,4,5-tetrazine: Ethynylated and Vinylated Nucleobases for Functionalization and Impact of Pyridazine Adducts on DNA Base Pair Stability and Mismatch Discrimination.

The manuscript reports on 7-deazapurine and pyrimidine nucleoside and oligonucleotide cycloadducts formed by the inverse electron demand Diels-Alder (iEDDA) reaction with 3,6-di(pyrid-2-yl)-1,2,4,5-tetrazine. Cycloadducts were constructed from ethynylated and vinylated nucleobases. Oligonucleotides were synthesized containing iEDDA modifications, and the impact on duplex stability was investigated. iEDDA reactions were performed on nucleoside triple bond side chains. Oxidation was not required in these cases as dihydropyridazine intermediates are not formed. In contrast, oxidation is necessary for reactions performed on alkenyl compounds. This was verified on 5-vinyl-2'-deoxyuridine. A diastereomeric mixture of 1,2-dihydropyridazine cycloadduct intermediates was isolated, characterized, and later oxidized. 12-mer oligonucleotides containing 1,2-pyridazine inverse Diels-Alder cycloadducts and their precursors were hybridized to short DNA duplexes. For that, a series of phosphoramidites was prepared. DNA duplexes with 7-functionalized 7-deazaadenines and 5-functionalized pyrimidines display high duplex stability when spacer units are present between nucleobases and pyridazine cycloadducts. A direct connectivity of the pyridazine moiety to nucleobases as reported for metabolic labeling of vinyl nucleosides reduced duplex stability strongly. Oligonucleotides bearing linkers with and without pyridazine cycloadducts attached to the 7-deazaadenine nucleobase significantly reduced mismatch formation with dC and dG.

Theoretical Study of an Authentic Hydrocarbon Ion Pair.

For many years researchers believed that hydrocarbons only contain covalent bonds. However, since 1985 Okamoto et al. demonstrated the formation of hydrocarbon salts in several systems, demolishing the structural principle that hydrocarbons only contain covalent bonds. Despite the great importance of this outcome to the study of chemical bonds, quantum chemical calculations on these systems are essentially nonexistent. The stability of the hydrocarbon ions along with the steric hindrance associated with the formation of the covalent bond contribute to their occurrence either in solution (dissociated) or in the solid state. These facts along with the common formation of ion pairs in solvents of low polarity motivated us to search for hydrocarbon ion pairs in the gas phase. Its energetics has also been studied in four nonprotic solvents, through a continuum solvation model (CPCM). DFT and CASSCF calculations indicate a metastable and highly polar ion pair between the tricyclopropylcyclopropenylium cation and a simplified Kuhn's anion. The barrier to the covalent structure varies from ∼4.8 to 14.4 kcal/mol, while the energy difference between the ion pair and the covalent form varies from ∼4.3 to 25.4 kcal/mol. The obtained theoretical results along with previous experimental results suggest the following strategy to obtain kinetically and thermodynamically stable hydrocarbon ion pairs: choose very stable hydrocarbon ions and systematically increase the steric hindrance between them.

Role of ion pairs in model glycosylation reactions of permethylated glucosyl and xylosyl triflates

Elucidating the molecular mechanisms of chemical O-glycosylation remains a significant challenge in glycochemistry. This study examines the mechanism of the nucleophilic substitution reaction between glycosyl triflates, which are extensively used in studies of glycosylation mechanisms, and several acceptor alcohols. The investigation was conducted through a comparative analysis of permethylated glucosyl triflate GTf and its xylosyl counterpart XTf. The glycosylation reactions, conducted in dichloromethane using GTf and XTf with EtOH, tBuOH, and CF3CH2OH, exhibited diverse α/β selectivities depending on the types of donor and acceptor molecules used. Identifying a unified mechanism to explain this range of selectivities proved challenging. Notably, we observed a distinct trend wherein the addition of excess triflate salt (Bu4NOTf) had a more pronounced effect on the α/β selectivity in glycosylation reactions utilizing XTf compared to those using GTf. Quantum chemical calculations performed at the SCS-MP2//DFT(M06-2X) level, with explicit inclusion of five solvent molecules, showed that contact ion pairs arising from XTf were significantly more stable than those from GTf. These experimental and computational results strongly suggest that ion pairs play a more crucial role in the glycosylation process involving XTf than GTf. Additionally, our quantum chemical analyses clarified that the enhanced stability of the ion pairs from XTf was attributed not to the strength of the C-1−OTf bond within XTf but to the flexibility in the conformational changes of XTf's pyranosyl ring.