Proton Positions Research Articles

Chiral Supramolecular Proton Conductors: Harnessing Highly Charged Zirconium-Amino Acid Oxo-Clusters.

Incorporation of amino acid capping molecules (alanine (Ala), methionine (Met), phenylalanine (Phe), tryptophan (Trp), tyrosine (Tyr), and valine (Val)) in their zwitterionic form into archetypal [Zr6(μ3-O)4(μ3-OH)4]12+ clusters creates supramolecular frameworks in which the assembly of these highly charged discrete units with chloride counterions provides a unique combination of porosity, chirality, and proton conductivity. The supramolecular frameworks assembled from these cluster entities (i.e., ZrAla, ZrMet, ZrPhe, ZrTrp, ZrTyr, and ZrVal) are based on the counterbalancing of the cationic hexanuclear entities by chloride anions. The resulting structures provide porous structures (except ZrVal) with variability of chemical and structural stability based on their supramolecular interactions. Among these compounds, ZrPhe and ZrVal remain stable due to the presence of a double chelation-like interaction involving four hydrogen bonds formed between a chloride anion, two ammonium groups, and two coordinated water molecules from two adjacent hexanuclear units. The presence of multiple acidic proton positions and strong hydrogen-bond donor/acceptor groups on the water channels gives rise to easy proton conduction pathways within the structures. In fact, ZrPhe and ZrVal, together with ZrTyr, exhibit high proton conductivity values with varying dependencies on the atmospheric humidity. Finally, the correlation between proton conduction and porosity is discussed.

Vanadium Substitution Dictates H Atom Uptake at Lindqvist-type Polyoxotungstates.

Understanding how modification of molecular structures changes the thermochemistry of H atom uptake can provide design criteria for the formation of highly active catalysts for reductive transformations. Herein, we describe the effect of doping an atomically precise polyoxotungstate with vanadium on proton-coupled electron transfer (PCET) reactivity. The Lindqvist-type polyoxotungstate [W6O19]2- displays reversible redox chemistry, which was found to be unchanged in the presence of acid, indicating an inability to couple reduction with protonation. However, the incorporation of a single vanadium center into the structure significantly changes the reactivity, and the potential required for one-electron reduction of [VW5O19]3- was shown to vary with the strength of the acid added. Construction of a potential-pKa diagram allowed assessment of the thermodynamics of H atom uptake, indicating BDFE(O-H) ≈ 64 kcal/mol, while chemical synthesis of the reduced/protonated derivative (TBA)3[VW5O19H] was used to probe the position of protonation.

Modeling Interfacial Proton-Coupled Electron Transfer Reactions Involving Imidazolium Proton Donors

Proton-coupled electron transfer (PCET) is an elementary reaction that plays a pivotal role in various electrochemical energy conversion and storage processes. PCET reactions involving nonaqueous proton donors are of interest for applications in energy storage such as redox flow batteries, as well as in CO2 and O2 reduction catalysis. Yet, there remains much to be understood about the molecular reactivity of these systems. In this talk, I will describe periodic density functional theory (DFT) studies of interfacial PCET reactions involving different imidazolium proton donors on Pt(111) electrode surfaces.The imidazole conjugate bases adsorb strongly to the electrode surface; however, this adsorption behavior varies with isomers that feature distinct proton positions and with different functional substituent groups. Because the adsorption of imidazole conjugate bases could affect reactivity, these models are used as a foundation to understand the intrinsic PCET kinetics of these systems. The Volmer and Heyrovsky steps of hydrogen evolution reaction are two of the most basic heterogeneous electrochemical PCET reactions and are therefore chosen as model interfacial PCET reactions in this study. Potential-dependent reaction energies and activation barriers are calculated using the charge extrapolation method, where the electrode potential is tuned by modifying proton coverage for different-sized unit cells. This approach is used to develop relationships between PCET reaction thermodynamics and kinetics through Brønsted–Evans–Polanyi (BEP) relationships for different imidazoles, where BEP slopes are analogs to charge transfer coefficients in the Butler–Volmer equation. Investigating reaction parameters influencing charge transfer kinetics at the electrode-electrolyte interface is crucial for designing efficient energy storage systems, impacting overall device performance. These studies show trends in the kinetic behavior of different functionalized imidazoles, which can aid in the design of catalytic and energy storage systems.

Cryo-electron microscopy reveals hydrogen positions and water networks in photosystem II.

Photosystem II starts the photosynthetic electron transport chain that converts solar energy into chemical energy and thus sustains life on Earth. It catalyzes two chemical reactions: water oxidation to molecular oxygen and plastoquinone reduction. Coupling of electron and proton transfer is crucial for efficiency; however, the molecular basis of these processes remains speculative owing to uncertain water binding sites and the lack of experimentally determined hydrogen positions. We thus collected high-resolution cryo-electron microscopy data of fully hydrated photosystem II from the thermophilic cyanobacterium Thermosynechococcus vestitus to a final resolution of 1.71 angstroms. The structure reveals several previously undetected partially occupied water binding sites and more than half of the hydrogen and proton positions. This clarifies the pathways of substrate water binding and plastoquinone B protonation.

Proton beam spot size and position measurements using a multi-strip ionization chamber

PurposeTo develop and characterize a large-area multi-strip ionization chamber (MSIC) for efficient measurement of proton beam spot size and position at a synchrotron-based proton therapy facility. Methods and MaterialsA 420 mm x 320 mm MSIC was designed with 240 vertical strips and 180 horizontal strips at 1.75 mm pitch. The MSIC was characterized by irradiating a grid of proton spots across 17 energies from 73.5 MeV to 235 MeV and comparing to simultaneous measurements made with a reference Gafchromic EBT3 film. Beam profiles, spot sizes, and positions were analyzed. Short term measurement stability and sensitivity were evaluated. ResultsExcellent agreement was demonstrated between the MSIC and EBT3 film for both spot size and position measurements. Spot sizes agreed within ± 0.18 mm for all energies tested. Measured beam spot positions agreed within ± 0.17 mm. The detector showed good short term measurement stability and low noise performance. ConclusionThe large-area MSIC enables efficient and accurate proton beam spot characterization across the clinical energy range. The results indicate the MSIC is suitable for pencil beam scanning proton therapy commissioning and quality assurance applications requiring fast spot size and position quantification.

Porphyrin photosensitizers compatible with both aqueous and non-aqueous conditions: Acid dependent spectral, solubility and photocatalytic properties of meso-tetra(pyridyl)porphyrins

The aerobic photooxidation of 1,3-diphenylisobenzofuran (DPBF) as a specific singlet oxygen quencher was studied in the presence of meso-tetra(aryl)porphyrins (aryl = 2-pyridyl, 3-pyridyl and 4-pyridyl) and their diprotonated and hexaprotonated derivatives to investigate the influence of the position of pyridine nitrogen atom, solvent, core protonation (with HCl or dichloroacetic acid) and full protonation of the porphyrins on their photocatalytic activity and photooxidative stability. The pseudo-first order rate constants (kobs) for the oxidation of DPBF with singlet oxygen were determined to compare the relative activity of the photosensitizers. 1,4-benzoquinone as a scavenger of superoxide anion radical provided evidence for the absence of this species in these photocatalytic systems. The singlet oxygen quantum yields (φΔ) of the porphyrin photosensitizers were estimated by determining the initial rate constants for the oxidation of DPBF. H4T(2-Py)P(Cl)2 and H4T(2-Py)P(CHCl2COO)2 were the most efficient photosensitizers of the series of the meso-tetra(2-, 3- or 4-pyridyl)porphyrins and the protonated derivatives.

Mapping Hydrogen Positions along the Proton Transfer Pathway in an Organic Crystal by Computational X-ray Spectra.

Understanding proton transfer (PT) dynamics in condensed phases is crucial in chemistry. We computed a 2D map of N 1s X-ray photoelectron/absorption spectroscopy (XPS/XAS) for an organic donor-acceptor salt crystal against two varying N-H distances to track proton motions. Our results provide a continuous spectroscopic mapping of O-H···N↔O-··· H+-N processes via hydrogen bonds at both nitrogens, demonstrating the sensitivity of N 1s transient XPS/XAS to hydrogen positions and PT. By reducing the O-H length at N1 by only 0.2 Å, we achieved excellent theory-experiment agreement in both XPS and XAS. Our study highlights the challenge in refining proton positions in experimental crystal structures by periodic geometry optimizations and proposes an alternative scaled snapshot protocol as a more effective approach. This work provides valuable insights into X-ray spectra for correlated PT dynamics in complex crystals, benefiting future experimental studies.

Examining the influence of hadronic interactions on the directed flow of identified particles in RHIC Beam Energy Scan energies using UrQMD model

The directed flow of identified particles can serve as a sensitive tool for investigating the interactions during initial and final states in heavy ion collisions. This study examines the rapidity distribution ([Formula: see text]), rapidity-odd directed flow ([Formula: see text]) and its slope ([Formula: see text]) for [Formula: see text], [Formula: see text], p, and [Formula: see text] in Au+Au collisions at different collision centralities and beam energies ([Formula: see text], 11.5, 14.5, 19.6, 27, and 39[Formula: see text]GeV) using the UrQMD model. We investigate the impact of late-stage hadronic interactions on charge-dependent [Formula: see text] and its slope by modifying the duration of the hadronic cascade lifetime ([Formula: see text]). The energy dependence of [Formula: see text] for p ([Formula: see text]) exhibits distinct pattern compared to [Formula: see text] and [Formula: see text]. Notably, we observe a change in the sign reversal position of proton [Formula: see text] at different beam energies with varying [Formula: see text] in central and mid-central collisions. Moreover, the difference in [Formula: see text] between positively and negatively charged hadrons ([Formula: see text]) demonstrates a stark centrality dependence for different particle species. The deuteron displays a significant increase in [Formula: see text] with increasing [Formula: see text] compared to p and n. This investigation underscores the importance of considering the temporal evolution and duration of the hadronic phase when interpreting the sign reversal, charge splitting of [Formula: see text] and light nuclei formation at lower RHIC energies.

Highly acidic N-triflylphosphoramides as chiral Brønsted acid catalysts: the effect of weak hydrogen bonds and multiple acceptors on complex structures and aggregation.

N-Triflylphosphoramides (NTPAs) represent an important catalyst class in asymmetric catalysis due to their multiple hydrogen bond acceptor sites and acidity, which is increased by several orders of magnitude compared to conventional chiral phosphoric acids (CPAs). Thus, NTPAs allow for several challenging transformations, which are not accessible with CPAs. However, detailed evidence on their hydrogen bonding situation, complex structures and aggregation is still lacking. Therefore, this study covers the hydrogen bonding behavior and structural features of binary NTPA/imine complexes compared to their CPA counterparts. Deviating from the single-well potential hydrogen bonds commonly observed in CPA/imine complexes, the NTPA/imine complexes exhibit a tautomeric equilibrium between two proton positions. Low-temperature NMR at 180 K supported by computer simulations indicates a OHN hydrogen bond between the phosphoramide oxygen and the imine, instead of the mostly proposed NHN H-bond. Furthermore, this study finds no evidence for the existence of dimeric NTPA/NTPA/imine complexes as previously suggested for CPA systems, both synthetically and through NMR studies.

A rotational spectroscopy study of microsolvation effects on intramolecular proton transfer in trifluoroacetylacetone-(H2O)1-3.

Trifluoroacetylacetone (TFAA) has two enol forms, which can switch to each other via proton transfer. While much attention has been paid to their conformational preferences, the influence of microsolvation on regulating the proton position remains unexplored. Herein, we report the rotational spectra of trifluoroacetylacetone-(water)n (n = 1-3) investigated by chirped pulse Fourier transform microwave spectroscopy in the 2-8 GHz frequency range. Two conformers were identified for both TFAA-H2O and TFAA-(H2O)2, while only one conformer was characterized for TFAA-(H2O)3. The results indicate that water binding on the CH3 side stabilizes the enolF form, whereas water binding on the CF3 side stabilizes the enolH form. The enolF form predominates over the enolH form in these hydrated complexes, which contrasts with the fact that only enolH exists in isolated TFAA. EnolH becomes preferred only when water inserts itself into the intramolecular hydrogen bond. Instanton theory calculations reveal that the proton transfer reaction is dominated by quantum tunneling at low temperatures, leading to the stable existence of only one enol form in each configuration of the hydrated clusters.

Protonation of Homocitrate and the E1 State of Fe-Nitrogenase Studied by QM/MM Calculations.

Nitrogenase is the only enzyme that can cleave the strong triple bond in N2, making nitrogen available for biological life. There are three isozymes of nitrogenase, differing in the composition of the active site, viz., Mo, V, and Fe-nitrogenase. Recently, the first crystal structure of Fe-nitrogenase was presented. We have performed the first combined quantum mechanical and molecular mechanical (QM/MM) study of Fe-nitrogenase. We show with QM/MM and quantum-refinement calculations that the homocitrate ligand is most likely protonated on the alcohol oxygen in the resting E0 state. The most stable broken-symmetry (BS) states are the same as for Mo-nitrogenase, i.e., the three Noodleman BS7-type states (with a surplus of β spin on the eighth Fe ion), which maximize the number of nearby antiferromagnetically coupled Fe-Fe pairs. For the E1 state, we find that protonation of the S2B μ2 belt sulfide ion is most favorable, 14-117 kJ/mol more stable than structures with a Fe-bound hydride ion (the best has a hydride ion on the Fe2 ion) calculated with four different density-functional theory methods. This is similar to what was found for Mo-nitrogenase, but it does not explain the recent EPR observation that the E1 state of Fe-nitrogenase should contain a photolyzable hydride ion. For the E1 state, many BS states are close in energy, and the preferred BS state differs depending on the position of the extra proton and which density functional is used.

Deep Variational Free Energy Approach to Dense Hydrogen.

We developed a deep generative model-based variational free energy approach to the equations of state of dense hydrogen. We employ a normalizing flow network to model the proton Boltzmann distribution and a fermionic neural network to model the electron wave function at given proton positions. By jointly optimizing the two neural networks we reached a comparable variational free energy to the previous coupled electron-ion MonteCarlo calculation. The predicted equation of state of dense hydrogen under planetary conditions is denser than the findings of abinitio molecular dynamics calculation and empirical chemical model. Moreover, direct access to the entropy and free energy of dense hydrogen opens new opportunities in planetary modeling and high-pressure physics research.

Analysis of the Ground Level Enhancement GLE 60 on 15 April 2001, and Its Space Weather Effects: Comparison With Dosimetric Measurements

AbstractAs a result of notable solar activity observed in April 2001, one of the strongest ground level enhancements (GLE) of solar cycle 23 occurred, namely GLE # 60 on 15 April 2001. In this paper, we derived the spectral and angular characteristics, and apparent source position of the solar protons during the GLE # 60, using a verified by direct measurements model and employing the calibrated neutron monitor yield function. Subsequently, employing the updated and verified by balloon measurements dosimetric model: Oulu CRAC:DOMO (Cosmic Ray Atmospheric Cascade: Dosimetric Model) we computed the dose rates throughout the event at several altitudes using the obtained spectra as an input. A global map of the ambient dose at an altitude of 35 kft is computed. A comparison with direct dosimetric measurements obtained by a Liulin device during an intercontinental flight is performed and good agreement is achieved.

On Integral INICS Aromaticity of Pyridodiazepine Constitutional Isomers and Tautomers.

The structure, energetics, and aromaticity of c.a. 100 constitutional isomers and tautomers of pyrido[m,n]diazepines (m = 1, 2; n = 2, 3, 4, 5; m ≠ n) were studied at the B3LYP/cc-pVTZ level. The pyrido[1,3]diazepines appear the most, while pyrido[2,4]diazepines are the least stable (ca. 26 kcal/mol). In the pyrido[1,n]diazepine group (n = 2-5), the [1,5] isomers are higher in energy by ca. 4.5 kcal/mol and the [1,4] ones by ca. 7 kcal/mol, and the pyrido[1,2]diazepines are the least stable (ca. 20 kcal/mol). All the most stable pyrido[1,n]diazepines have N-atoms near the ring's junction bond but on opposite sites. The most stable [2,n]-forms are also those with the pyridine ring N6-atom near the junction bond. Surprisingly, for the [1,2]-, [1,3]-, and [1,4]-isomer condensation types of pyridine and diazepine rings, the same N9 > N7 > N6 > N8 stability pattern obeys. The stability remains similar in a water medium simulated with the Polarizable Continuum Model of the solvent and is conserved when calculated using the CAM-B3LYP or BHandHlyp functionals. The ring's aromaticity in the pyridine[m,n]diazepines was established based on the integral INICS index resulting from the NICSzz-scan curves' integration. The integral INICS index is physically justified through its relation to the ringcurrent as demonstrated by Berger, R.J.F., et al. Phys. Chem. Chem. Phys. 2022, 24, 624. The six-membered pyrido rings have negative INICSZZ indices and can be aromatic only if they are not protonated at the N-atom. All protonated pyrido and seven-membered rings exhibit meaningful positive INICSZZ values and can be assigned as antiaromatic. However, some non-protonated pyrido rings also have substantial positive INICSZZ indices and are antiaromatic. A weak linear correlation (R2 = 0.72) between the INICSZZ values of the pyridine I(6) and diazepine I(7) rings exists and is a consequence of the communication between the π-electron systems of the two rings. The juxtaposition of the INICS descriptor of the six- and seven-membered rings and diverse electron density parameters at the Ring Critical Points (RCP) revealed good correlations only with the Electrostatic Potentials from the electrons and nuclei (ESPe and ESPn). The relationships with other RCP parameters like electron density and its Laplacian, total energy, and the Hamiltonian form of kinetic energy density were split into two parts: one nearly constant for the six-membered rings and one linearly correlating for the seven-membered rings. Thus, most of the electron density parameters at the RCP of the six-membered rings of pyridodiazepines practically do not change with the diazepine type and the labile proton position. In contrast, those of the seven-membered rings display aromaticity changes in the antiaromatic diazepine with its ring structural modifications.

Perfect Crystals: microgravity capillary counterdiffusion crystallization of human manganese superoxide dismutase for neutron crystallography

The NASA mission Perfect Crystals used the microgravity environment on the International Space Station (ISS) to grow crystals of human manganese superoxide dismutase (MnSOD)—an oxidoreductase critical for mitochondrial vitality and human health. The mission’s overarching aim is to perform neutron protein crystallography (NPC) on MnSOD to directly visualize proton positions and derive a chemical understanding of the concerted proton electron transfers performed by the enzyme. Large crystals that are perfect enough to diffract neutrons to sufficient resolution are essential for NPC. This combination, large and perfect, is hard to achieve on Earth due to gravity-induced convective mixing. Capillary counterdiffusion methods were developed that provided a gradient of conditions for crystal growth along with a built-in time delay that prevented premature crystallization before stowage on the ISS. Here, we report a highly successful and versatile crystallization system to grow a plethora of crystals for high-resolution NPC.

Path-integral molecular dynamics predictions of equilibrium H and O isotope fractionations between brucite and water

Precise prediction of hydrogen (H) isotope fractionations among different substances is a long-standing challenge in isotope geochemistry, as it needs treatments beyond the harmonic approximation. Path-integral molecular dynamics (PIMD) simulations have recently been proved to be valid in predicting equilibrium isotope fractionations for light elements. However, the lack of reliable force fields hinders the application of PIMD to the condensed phases. In this work, the deep potential models trained on the first-principles molecular dynamics (FPMD) data are applied to PIMD simulations to determine the D/H and 18O/16O isotope fractionation factors between brucite and water. To quantitatively assess the influence of quantum effects, the D/H and 18O/16O isotope fractionations between brucite and water are also determined under the framework of harmonic approximation.By comparing the results of the PIMD and those of harmonic calculations, we find that H and O atoms in brucite and water are strongly affected by the anharmonic effects. The accuracy of the harmonic isotope fractionations mainly depends on the cancellation percentage of the anharmonic effects on the reduced partition function ratios (RPFRs) of the two substances. The isotope fractionations predicted by PIMD simulations are close to the experimental results at high temperatures. However, at low temperatures, the predicted isotope fractionations are different from those of experiments. The discrepancies are attributed to the approximations in the density functional theory (DFT) functionals used in this work, i.e., the inaccuracy in predicting the proton positions in brucite and water. Moreover, we find that the H isotope fractionations are extremely sensitive to the change of pressure. The direction of H isotope fractionation between brucite and water (at 300 K) will be even inversed as the pressure increases to more than 3 GPa. This strong pressure sensitivity may be a common characteristic of hydrous minerals and water systems. Therefore, the extrapolated H isotope fractionations used in the studies of dehydration of subducting slabs or deep Earth H distribution based on the isotope fractionations under low pressures may need to be rechecked.

NMR-assisted crystallography of the tryptophan synthase α-aminoacrylate intermediate: Proton positions and role in inhibition.

NMR-assisted crystallography - the integrated application of solid-state NMR, X-ray crystallography, and first-principles computational chemistry - holds significant promise for mechanistic enzymology: by providing atomic-resolution characterization of stable intermediates, including hydrogen atom locations and tautomeric equilibria, NMR crystallography offers insight into structure and chemical dynamics. Here, we make use of this combined approach to characterize the tryptophan synthase α-aminoacrylate intermediate, a defining species for pyridoxal-5′-phosphate-dependent enzymes that catalyze β-elimination and replacement reactions. For this intermediate, NMR-assisted crystallography identifies the protonation states of the ionizable sites on the cofactor, substrate, and catalytic side chains, and the location and orientation of crystallographic waters within the active site. From this, a detailed three-dimensional picture of structure and reactivity emerges, highlighting the fate of the substrate L-serine hydroxyl leaving group and the reaction pathway back to the preceding transition state. Reaction of the α-aminoacrylate intermediate with benzimidazole, an isostere of the natural substrate, indole, shows benzimidazole bound in the active site and poised for, but unable to initiate, the subsequent bond formation step. Here, the chemically detailed, three-dimensional structure from NMR-assisted crystallography is key to understanding why benzimidazole does not react.

Salt or Cocrystal? Using XRPD to Infer Proton Transfer in Three Adducts of Iodoxybenzoic Acid by Analyzing Iodine–Oxygen Bond Lengths

2-Iodoxybenzoic acid (IBX) is a versatile oxidant, which has been used for a plethora of reactions through the years. Its explosive properties and low solubility, the major limitations to a more widespread use of this reactant, have led to the search for derivatives, adducts, or mixtures with comparable oxidative properties but better characteristics. In this paper, different adducts of IBX, in particular with pyridine (PIBX), quinoline (QUIBX), and nicotinamide (B3-IBX), have been structurally characterized using state-of-the-art X-ray powder diffraction (XRPD) methods. Their oxidative properties have been tested and compared to those of pristine IBX. While generally structure determination from XRPD does not allow us to investigate the proton position, a strategy was devised based on the investigation of iodine–oxygen exocyclic bond lengths to discriminate between single and double iodine–oxygen bonds. The final aim was to attempt to use XRPD to answer an elusive question: salt or cocrystal?

Unraveling the Nature of Hydrogen Bonds of "Proton Sponges" Based on Car-Parrinello and Metadynamics Approaches.

The nature of intra- and intermolecular non-covalent interactions was studied in four naphthalene derivatives commonly referred to as "proton sponges". Special attention was paid to an intramolecular hydrogen bond present in the protonated form of the compounds. The unsubstituted "proton sponge" served as a reference structure to study the substituent influence on the hydrogen bond (HB) properties. We selected three compounds substituted by methoxy, amino, and nitro groups. The presence of the substituents either retained the parent symmetry or rendered the compounds asymmetric. In order to reveal the non-covalent interaction properties, the Hirshfeld surface (HS) was computed for the crystal structures of the studied compounds. Next, quantum-chemical simulations were performed in vacuo and in the crystalline phase. Car-Parrinello molecular dynamics (CPMD), Path Integral Molecular Dynamics (PIMD), and metadynamics were employed to investigate the time-evolution changes of metric parameters and free energy profile in both phases. Additionally, for selected snapshots obtained from the CPMD trajectories, non-covalent interactions and electronic structure were studied. Quantum theory of atoms in molecules (QTAIM) and the Density Overlap Regions Indicator (DORI) were applied for this purpose. It was found based on Hirshfeld surfaces that, besides intramolecular hydrogen bonds, other non-covalent interactions are present and have a strong impact on the crystal structure organization. The CPMD results obtained in both phases showed frequent proton transfer phenomena. The proton was strongly delocalized in the applied time-scale and temperature, especially in the PIMD framework. The use of metadynamics allowed for tracing the free energy profiles and confirming that the hydrogen bonds present in "proton sponges" are Low-Barrier Hydrogen Bonds (LBHBs). The electronic and topological analysis quantitatively described the temperature dependence and time-evolution changes of the electronic structure. The covalency of the hydrogen bonds was estimated based on QTAIM analysis. It was found that strong hydrogen bonds show greater covalency, which is additionally determined by the proton position in the hydrogen bridge.

Description of peptide bond planarity from high-resolution neutron crystallography.

Neutron crystallography is a highly effective method for visualizing hydrogen atoms in proteins. In our recent study, we successfully determined the high-resolution (1.2 Å) neutron structure of high-potential iron-sulfur protein, refining the coordinates of some amide protons without any geometric restraints. Interestingly, we observed that amide protons are deviated from the peptide plane due to electrostatic interactions. Moreover, the difference in the position of the amide proton of Cys75 between reduced and oxidized states is possibly attributed to the electron storage capacity of the iron-sulfur cluster. Additionally, we have discussed about the rigidity of the iron-sulfur cluster based on the results of the hydrogen-deuterium exchange. Our research underscores the significance of neutron crystallography in protein structure elucidation, enriching our understanding of protein functions at an atomic resolution.