Hydrogen Acceptor Research Articles

In situ electrochemical regeneration of active 1,4-NADH for enzymatic lactic acid formation via concerted functions on Pt-modified TiO2/Ti.

Nicotinamide adenine dinucleotide (NAD+) and its reduced form (NADH) are key cofactors serving as essential hydrogen acceptors and donors to facilitate energy and material conversions under mild conditions. We demonstrate direct electrochemical conversion to achieve highly efficient regeneration of enzymatically active 1,4-NADH using a Pt-modified TiO2 catalyst grown directly on a Ti mesh electrode (Pt-TOT). Spectral analyses revealed that defects formed by the inclusion of Pt species in the lattice of TiO2 play a critical role in the regeneration process. In particular, Pt-TOT containing approximately 3 atom% of Pt exhibited unprecedented efficiency in the electrochemical reduction of NAD+ at the lowest overpotential to date. This exceptional performance led to the production of active 1,4-NADH with a significantly high yield of 86 ± 3% at -0.6 V vs. Ag/AgCl (-0.06 V vs. RHE) and an even higher yield of 99.5 ± 0.4% at a slightly elevated negative potential of -0.8 V vs. Ag/AgCl (-0.2 V vs. RHE). Furthermore, the electrochemically generated NADH was directly applied in the enzymatic conversion of pyruvic acid to lactic acid using lactate dehydrogenase.

One-pot aerobic conversion of fructose to 2,5-diformylfuran using silver-decorated carbon materials

The synthesis of 2,5-diformylfuran (DFF) from fructose is a simultaneous process using two distinct catalysts, including fructose dehydration to 5-hydroxymethylfurfural (5-HMF) accelerated by an acid catalyst, followed by 5-HMF oxidation to DFF mediated by a redox catalyst. In this study, various catalysts with Ag metal decorated on sulfonated amorphous carbon supports (AC-SO3H@Ag) were performed, and their structures were characterized via different characterization techniques such as X-ray diffraction, scanning electron microscopy, X-ray fluorescence spectroscopy, thermogravimetric analysis, Boehm's titration, inductively coupled plasma mass spectroscopy, energy-dispersive X-ray spectroscopy, elemental mapping analysis and Fourier-transform infrared spectroscopy. The catalytic performance of the prepared AC-SO3H@Ag bifunctional catalysts with various wt% of Ag/AC-SO3H was determined for the one-pot transformation of fructose into DFF. The influence of solvent, substrates, reaction time, and catalyst loading were also examined thoroughly to optimize the reaction, and a mechanism was proposed. 5-HMF formed by the dehydration of fructose was converted entirely to DFF, and the highest DFF yield of 77.9 % was obtained after 18 h of reaction time at 150 °C in dimethyl sulfoxide with 3 wt% of the AC-SO3H@Ag catalyst and without the use of any additives, traditional hydrogen acceptors, or oxidants. The reusability of the catalysts was also tested.

Engineering functionalized Zr-MOFs as facile removal of indole: Experimental studies and first-principles modeling

Zr-based metal-organic frameworks (Zr-MOFs) are emerging as potential adsorbents for removing nitrogen contaminants in fuel and wastewater. Functionalized Zr-MOFs exhibited superior indole (IND) adsorption capacity than the original MOF, but the activity of the multiple adsorption sites has not been discussed. Through experiment measurement and Density Functional Theory (DFT) calculations, this research investigated the IND adsorption mechanism at various sites on Zr-BDC-NH2 and Zr-BDC-NO2. Based on the adsorption isotherm, the adhesion of IND with Zr-MOFs was determined via the Freundlich model. DFT results suggest that the IND adsorption occurred at highly active sites such as O-linker, μ3-O-cluster, N-amine, and O-nitro via hydrogen and π-hydrogen bonds. Furthermore, the binding energy at each adsorption site in Zr-BDC-NH2 is higher than in Zr-BDC-NO2. In addition to the hydrogen acceptor bond, the π-hydrogen bond has also been analyzed through the electron density difference.

How Modulator Binding at the Amyloidβ-γ-Secretase Interface Enhances Substrate Binding and Attenuates Membrane Distortion.

Inhibition of γ-secretase, an intramembrane protease, to reduce secretion of Amyloid-β (Aβ) peptides has been considered for treating Alzheimer's disease. However, γ-secretase inhibitors suffer from severe side effects. As an alternative, γ-secretase modulators (GSM) reduce the generation of toxic peptides by enhancing the cleavage processivity without diminishing the enzyme activity. Starting from a known γ-secretase structure without substrate but in complex with an E2012 GSM, we generated a structural model that included a bound Aβ43 peptide and studied interactions among enzyme, substrate, GSM, and lipids. Our result suggests that E2012 binding at the enzyme-substrate-membrane interface attenuates the membrane distortion by shielding the substrate-membrane interaction. The model predicts that the E2012 modulation is charge-dependent and explains the preserved hydrogen acceptor and the aromatic ring observed in many imidazole-based GSM. Predicted effects of γ-secretase mutations on E2012 modulation were confirmed experimentally. We anticipate that the study will facilitate the future development of effective GSMs.

Analysis of stabilization mechanisms in β-lactoglobulin-based amorphous solid dispersions by experimental and computational approaches

Our previous work shows that β-lactoglobulin-stabilized amorphous solid dispersion (ASD) loaded with 70 % indomethacin remains stable for more than 12 months. The stability is probably due to hydrogen bond networks spread throughout the ASD, facilitated by the indomethacin which has both hydrogen donors and acceptors. To investigate the stabilization mechanisms further, here we tested five other drug molecules, including two without any hydrogen bond donors. A combination of experimental techniques (differential scanning calorimetry, X-ray power diffraction) and molecular dynamics simulations was used to find the maximum drug loadings for ASDs with furosemide, griseofulvin, ibuprofen, ketoconazole and rifaximin. This approach revealed the underlying stabilization factors and the capacity of computer simulations to predict ASD stability. We searched the ASD models for crystalline patterns, and analyzed diffusivity of the drug molecules and hydrogen bond formation. ASDs loaded with rifaximin and ketoconazole remained stable for at least 12 months, even at 90 % drug loading, whereas stable drug loadings for furosemide, griseofulvin and ibuprofen were at a maximum of 70, 50 and 40 %, respectively. Steric confinement and hydrogen bonding to the proteins were the most important stabilization mechanisms at low drug loadings (≤ 40 %). Inter-drug hydrogen bond networks (including those with induced donors), ionic interactions, and a high Tg of the drug molecule were additional factors stabilizing the ASDs at drug loading greater than 40 %.

Acceptorless or Transfer Dehydrogenation of Glycerol Catalyzed by Base Metal Salt Cobaltous Chloride - Facile Access to Lactic Acid and Hydrogen or Isopropanol.

The dehydrogenation of glycerol to lactic acid (LA) under both acceptorless and transfer dehydrogenation conditions using readily available, inexpensive, environmentally benign and earth-abundant base metal salt CoCl2 is reported here. The CoCl2 (0.5 mol %) catalyzed acceptorless dehydrogenation of glycerol at 160 °C in the presence of 0.75 equiv. of KOH, gave up to 33 % yield of LA in 44 % selectivity apart from hydrogen. Alternatively, with acetone as a sacrificial hydrogen acceptor, the CoCl2 (0.5 mol %) catalyzed dehydrogenation of glycerol at 160 °C in the presence of 1.1 equiv. of NaOt Bu resulted in up to 93 % LA with 96 % selectivity along with another value-added product isopropanol. Labelling studies revealed a modest secondary KIE of 1.68 which points to the involvement of C-H bond activation as a part of the catalytic cycle but not as a part of the rate-determining step. Catalyst poisoning experiments with PPh3 and CS2 are indicative of the homogeneous nature of the reaction mixture involving molecular species that are likely to be in-situ formed octahedral Co(II) as inferred from EPR, HRMS and Evans magnetic moment studies. The net transfer dehydrogenation activity is attributed to exclusive contribution from the alcoholysis step.

Herringbone Helical Foldamers from Aromatic Ether Derived ϵ-Amino Acid Peptides.

Oligomers based on an aromatic ether derived ϵ-amino acid peptides folded into herringbone helical structures, induced by successive NH-O-NH & O-NH-O bifurcated hydrogen bonding interactions and reinforced by π-π stacking between aryls from adjacent layers. The diaryl ether bonds -O- worked both as structural units to provide turn motifs for changing the amplitude of the slope along the axis of helix for herringbone formation, and also as acceptors for hydrogen bonding. Attachment of a single chiral carbon to the C-termini of the peptides induced excess of single-handed screw sense and amplification through the chain propagation as exemplified by chain length dependent circular dichroism (CD) investigations.

Elucidating the Structure, Dynamics, and Interaction of a Choline Chloride and Citric Acid Based Eutectic System by Spectroscopic and Molecular Modeling Investigations.

Eutectic solvent systems are versatile solvents that have found widespread use in numerous applications. Traditional solvents are homogeneous, having only one component, and their chemistry is relatively simple, with some exceptions. On the other hand, deep eutectic solvents (DESs) comprise binary components, generally a donor and an acceptor in hydrogen bonding with varying ratios. The interaction chemistry among the donor and acceptor involved in hydrogen bonding in DESs is complicated. Although numerous research is focused on the synthesis and application of DESs, few studies are reported to elucidate the complex structure and dynamic and interaction behavior of DESs. In this study, we employed calorimetry, vibrational spectroscopy techniques including FTIR and Raman, and nuclear magnetic resonance to derive insight into the structural feature and noncovalent contact of choline chloride (ChCl) and citric acid (CA) while they formed DESs. The 1:1 ChCl/CA eutectic system showed phase transitions and melting peaks with the most pronounced peak at 156.22 °C, suggesting the DESs melting at a lower temperature than the melting temperatures of ChCl and CA. In addition to IR and Raman findings, 1H NMR investigations demonstrate hydrogen bonding intermolecular interactions between ChCl and CA, supporting the formation of 1:1 ChCl/CA DESs based on the deshielded chemical shifts of the proton for Ch. The interaction of the chloride anion with the methyl protons (H4) and methylene protons (H3) of ChCl as well as the strong hydrogen bonding interactions between the hydroxyl hydrogen (H1) of ChCl with one of CA's carbonyl oxygens both supported the formation of conformer E. In addition, molecular dynamics followed by the density functional theory (DFT) was employed to visualize the structure and interaction of DESs using the ωB97XD theory and 6-311++G (d,p) basis set. Both experimental and theoretical IR, Raman, and structural analyses provided evidence of the formation of DESs by possessing hydrogen bonds. These multifaceted experimental and computational investigations provide details of structural and intermolecular interactions of ChCl/CA DESs.

Effect of secondary gas-phase reactions (SGR) in pyrolysis of carbon feedstocks for anisotropic carbon materials production – 3: Modifying non-coking coal tar through co-pyrolysis of coal with linear low-density polyethylene and high-impact polystyrene

This work focuses on co-pyrolysis experiments of Utah Sufco coal with linear low-density polyethylene (LLDPE) and high-impact polystyrene (HIPS). These two plastic types have differing pyrolysis chemistries and have different hydrogen transfer behavior. Similar to earlier work in this series of papers, controlled secondary gas-phase reactions during pyrolysis were used to induce cracking and condensation reactions among the pyrolytic tar species. Co-pyrolysis tests were performed with feed plastic percentages ranging from 10 to 20 wt % and pyrolysis SGR temperatures ranging from 800 to 900° C. Analyses of the intermediate tar products showed that oxygen contents and aromaticity were substantially different, depending on the plastic, and resulting pitch softening points and mesophase contents also varied greatly depending on the starting plastic feedstock used. Most of the synergies observed in the co-pyrolysis results were negative, except for the oxygen content. Oxygen contents were higher than expected when LLDPE was used, resulting in reactive pitches with softening points >350° C. On the other hand, oxygen contents were lower than expected when HIPS was used, resulting in less reactive pitches. Ultimately, only the HIPS/coal samples created at the SGR temperature of 900° C had reasonable softening points at or under 350° C, making them the only samples created in this work potentially suitable for mesophase pitch-based carbon fiber production. The success in creating fusible mesophase pitches from co-pyrolyzing HIPS with Utah Sufco coal is likely attributed to the fact that polystyrene is a stronger hydrogen acceptor rather than donor, which should facilitate more cracking rather than stabilizing tar oxygen functional groups, making the tar species ultimately less reactive during thermal conversion to mesophase pitch.

Hydrogen Sulfide Ligation in Hemoglobin I of Lucina pectinata─A QM/MM and Local Mode Study.

In this study, we investigated the interaction between the H2S ligand and the heme pocket of hemoglobin I (HbI) of Lucina pectinata for the wild-type protein; three known mutations where distal glutamine is replaced by hydrophobic valine (Gln64Val) and hydrophilic histidine in both protonation forms (Gln64Hisϵ and Gln64Hisδ); five known mutations of the so-called phenyl cage, replacing the hydrophobic phenylalanines Phe29 and Phe43 with tyrosine (Tyr), valine (Val), or leucine (Leu); and two additional mutations, Phe68Tyr and Phe68Val, in order to complement previous studies with new insights about the binding mechanism at the molecular level. A particular focus was on the intrinsic strengths of the chemical bonds involved, utilizing local vibrational force constants based on combined quantum mechanical-molecular mechanical calculations. Wild-type protein and mutations clustered into two distinct groups: Group 1 protein systems with a proton acceptor in the distal protein pocket, close to one of the H2S bonds, and Group 2 protein systems without a hydrogen acceptor close by in the active site of the protein. According to our results, the interactions between H2S and HbI of Lucina pectinata involve two important elements, namely, binding of H2S to Fe of the heme group, followed by the proton transfer from the HS bond to the distal residue. The distal residue is additionally stabilized by a second proton transfer from the distal residue to COO- of the propionate group in heme. We could identify the FeS bond as a key player and discovered that the strength of this bond depends on two mutual factors, namely, the strength of the HS bond involved in the proton transfer and the electrostatic field of the protein pocket qualifying the FeS bond as a sensitive probe for monitoring changes in H2S ligation upon protein mutations. We hope our study will inspire and guide future experimental studies, targeting new promising mutations such as Phe68Tyr, Phe68Val, or Phe43Tyr/Phe68Val.

A laser flash photolysis study of the free radical chemistry of lipoic acid and dihydrolipoic acid.

The free radical chemistry of lipoic acid (LA) and dihydrolipoic acid (DHLA) intersect at the point where DHLA loses hydrogen to a good hydrogen abstracting radical, while LA reacts with strongly reducing ketyl radicals capable of donating a hydrogen atom. While aliphatic thiyl radicals have an absorbance at ~ 330nm, the resulting radical, formally also a thiyl radical has distinct spectroscopic properties with a maximum at 385nm, suggesting that the two sulphur centres interact strongly with each other as part of the chromophore. The reactions that form these radicals were studied by laser flash photolysis that revealed DHLA as an excellent hydrogen donor, while LA is an excellent hydrogen acceptor. The results support earlier evidence that the real antioxidant is DHLA, while LA is not; yet, the reported facile interconversion of the two molecules suggests that LA may be a better supplement, given its shelf stability, compared with a far more difficult-to-handle DHLA.

Impact of Hydrogen Plasma on Electrical Properties and Deep Trap Spectra in Ga2O3 Polymorphs

In this study, the results of hydrogen plasma treatments of β-Ga2O3, α-Ga2O3, κ-Ga2O3 and γ-Ga2O3 polymorphs are analyzed. For all polymorphs, the results strongly suggest an interplay between donor-like hydrogen configurations and acceptor complexes formed by hydrogen with gallium vacancies. A strong anisotropy of hydrogen plasma effects in the most thermodynamically stable β-Ga2O3 are explained by its low-symmetry monoclinic crystal structure. For the metastable, α-, κ- and γ-polymorphs, it is shown that the net result of hydrogenation is often a strong increase in the density of centers supplying electrons in the near-surface regions. These centers are responsible for prominent, persistent photocapacitance and photocurrent effects.

DFT studies of solvent effect in hydrogen abstraction reactions from different allyl-type monomers with benzoyl radical

Inert allyl-type monomers have been widely documented due to reduce degradation chain transfer. Recently, we and others discovered that the [3 + 2] cyclization reaction process by a photo-driven radical reaction, which can accelerate the polymerization. It was discovered that allyl ether monomers had much higher reactivity than other allyl monomers in the suspension photopolymerization initiated by Type I photoinitiator. Since the hydrogen abstraction reaction (HAR) is the initial step of cyclization, and in order to clarify the influence of solvents effect, three allyl-type monomers were employed, containing “O”, “N” and “S” atom as hydrogen donors. The benzoyl radical obtained from cleavage of photoinitiator was chosen as hydrogen acceptors. We explored the hydrogen abstraction reaction in different solvents (methanol, water and DMSO) by quantum chemistry for geometry and energy. An investigation was undertaken regarding the structural orbital by electrostatic potential (ESP) and topological analysis (ELF and LOL). The findings were also combined with the distortion model and transition state theory. We obtained the molecular interactions used independent gradient method in the Hirshfeld molecular density partition (IGMH). The Eckart’s correction allowed to examine the driving factors of the hydrogen abstraction reaction tunnels and these reactions constant rates are determined in the range of 500–2500 K depending on the modified Arrhenius form in different solvents effect. Our results can provide an answer for the different reactivities.

Intriguing Thiazolidinediones as PPAR γ Agonists: A Review

Diabetes mellitus is a chronic metabolic disorder marked by persistently elevated blood sugar levels. left untreatedover a long duration can cause multiple body disorders and may cause a person's early death. Though a traditional disorder, typeII diabetes prevalence is increasing daily, especially in the adolescent population worldwide. Peroxisome proliferator-activatedreceptor (PPAR) is a group of receptors consisting of three isoforms (PPAR α, PPAR β/δ, and PPAR γ. PPAR γ is involved inglucose metabolism by facilitating insulin's actions. Thiazolidinedione is a heterocyclic moiety standing pre-eminent in treatingdiabetes mellitus as a PPAR Gamma activator. Thiazolidinedione is a five-membered heterocyclic organic compound, athiazolidine derivative consisting of two carbonyl groups at positions 2 and 4 of the thiazolidine ring. Thiazolidinediones possessan idiosyncratic scaffold featuring a hydrogen bond acceptor region and hydrogen bond donating region at the third and fifthpositions. Thiazolidinedione is an indispensable pharmacophore with many pharmacological activities like antiproliferative,antiviral, antibacterial, tyrosine kinase inhibitory, aldose reductase inhibitory, alpha-glucosidase inhibitory, anti-inflammatory,antioxidant, antitubercular, antihyperlipidemic, etc. Many drugs have been introduced but later have been reticent because ofserious side effects like liver toxicity, CVS toxicity, etc. Pioglitazone and Rosiglitazone have been marketed medications fortreating type II diabetes. This review article deliberates all the cardinal points of thiazolidinediones as PPAR agonists in treatingdiabetes mellitus, which were precluded in some articles. We aim to have an all-embracing review of thiazolidinediones as PPARgamma agonists. The review's objective is to inspire researchers to develop a more superior, secure, and efficient anti-diabeticmedication by thoroughly understanding the molecular mechanisms of thiazolidinediones at the PPAR gamma receptors, theirrisks, and the effect of the various substitutions on the thiazolidinedione.

Proton Activity and Pathway in Aqueous Organic Redox Flow Batteries

Aqueous soluble organic (ASO) redox-active materials have recently shown great promise as alternatives to transition metal ions employed as energy-bearing active materials in redox flow batteries for large-scale energy storage because of their structural tunability, cost-effectiveness, availability, and safety features. However, development so far has been limited to a small palette of organics that are aqueous soluble. This presentation will use fluorenone as an example to showcase how a natively redox-inactive molecule can be tuned to possess two-electron redox reversibility through hydrogenation and dehydrogenation. The modified fluorenone molecules demonstrated high energy density and recorded stable cycling. Furthermore, research has shown the kinetics of the redox reactions; thus, the system rate capabilities can be improved by incorporating suitable hydrogen acceptors that regulate the proton pathway for a faster redox reaction.Reference: Feng et al., Science 372, 836–840 (2021), Feng et al., submitted, 2022

Chemical features and machine learning assisted predictions of protein-ligand short hydrogen bonds

There are continuous efforts to elucidate the structure and biological functions of short hydrogen bonds (SHBs), whose donor and acceptor heteroatoms reside more than 0.3 Å closer than the sum of their van der Waals radii. In this work, we evaluate 1070 atomic-resolution protein structures and characterize the common chemical features of SHBs formed between the side chains of amino acids and small molecule ligands. We then develop a machine learning assisted prediction of protein-ligand SHBs (MAPSHB-Ligand) model and reveal that the types of amino acids and ligand functional groups as well as the sequence of neighboring residues are essential factors that determine the class of protein-ligand hydrogen bonds. The MAPSHB-Ligand model and its implementation on our web server enable the effective identification of protein-ligand SHBs in proteins, which will facilitate the design of biomolecules and ligands that exploit these close contacts for enhanced functions.

Hydrogen Bonds in Lead Halide Perovskites: Insights from Ab Initio Molecular Dynamics.

Hydrogen bonds (HBs) play an important role in the rotational dynamics of organic cations in hybrid organic/inorganic halide perovskites, thus affecting the structural and electronic properties of the perovskites. However, the properties and even the existence of HBs in these perovskites are not well established. In this study, we investigate HBs in perovskites MAPbBr3 (MA+ = CH3NH3+), FAPbI3 (FA+ = CH(NH2)2+), and their solid solution with composition (FAPbI3)7/8(MAPbBr3)1/8, using ab initio molecular dynamics and electronic structure calculations. We consider HBs donated by X-H fragments (X = N and C) of the organic cations and accepted by the halides (Y = Br and I) and characterize their properties based on pair distribution functions and on a combined distribution function of the hydrogen-acceptor distance with the donor-hydrogen-acceptor angle. By analyzing these functions, we establish geometrical criteria for HB existence based on the hydrogen-acceptor (H-Y) distance and donor-hydrogen-acceptor angle (X-H-Y). The distance condition is defined as d(H - Y) < 3 Å for N-H-donated HBs and d(H - Y) < 4 Å for C-H-donated HBs. The angular condition is 135° < (X - H - Y) < 180° for both types of HBs. A HB is considered to be formed when both angular and distance conditions are simultaneously satisfied. At the simulated temperature (350 K), the HBs dynamically break and form. We compute the time correlation functions of HB existence and HB lifetimes, which range between 0.1 and 0.3 ps at that temperature. The analysis of HB lifetimes indicates that N-H-Br bonds are relatively stronger than N-H-I bonds, while C-H-Y bonds are weaker, with a minimal influence from the halide and cation. To evaluate the impact of HBs on the vibrational spectra, we present the power spectrum in the region of N-H and C-H stretching modes, comparing them with the normal mode frequencies of isolated cations. We show that the peaks associated with N-H stretching modes in perovskites are redshifted and asymmetrically deformed, while the C-H peaks do not exhibit these effects.

Theory of reactions between hydrogen and group-III acceptors in silicon

Theory of reactions between hydrogen and group-III acceptors in silicon

Solubility measurement of syringic acid, trans-cinnamic acid, p-coumaric acid, and vanillin constituent of Deverra DC. genus in (ethyl acetate + ethyl alcohol) mixtures: Thermodynamic modeling and preferential solvation

This work aims firstly to measure the solubility of four natural phenolic compounds (syringic acid, trans-cinnamic acid, p-coumaric acid, and vanillin) present in Deverra DC. Genus in (ethyl alcohol + ethyl acetate) using the gravimetric method from 283.15 to 323.15 K at normal pressure and then to analyze the solubility using the Jouyban-Acree and van't Hoff-Jouyban-Acree models. Secondly, based on the obtained solubility data, the preferential solvation parameters (δx1,3) of four natural phenolic compounds (PhC) in double solvent mixtures of ethyl alcohol (1) + ethyl acetate (2) were calculated using the inverse Kirkwood-Buff integrals method. The δx1,3 numbers differ indirectly with the co-solvent (1) fraction in all the binary mixtures. For the ethyl alcohol (1) + ethyl acetate (2) systems, the δx1,3 values are negative in ethyl acetate-rich mixtures and positive in constituents (x1) ranging from 0.59 to 1 for syringic acid, 0.51 to 0.63 trans-cinnamic acid, and 0.48 to 0.63 for vanillin in the mole fraction of ethyl alcohol. However, for the ethyl alcohol (1) + ethyl acetate (2) +p-coumaric acid (3) mixtures comprising 0 <x1 < 0.63, the values of δx1,3 are positive. This indicates that the co-solvent preferentially dissolved p-coumaric acid. The positive amounts of δx1,3 obtained are because the phenolic molecules that interact with the hydrogen acceptor groups existing in the co-solvent have higher acidic properties.

Overall protein structure quality assessment using hydrogen-bonding parameters.

Atomic model refinement at low resolution is often a challenging task. This ismostly because the experimental data are not sufficiently detailed to be described by atomic models. To make refinement practical and ensure that a refined atomic model is geometrically meaningful, additional information needs to be used such as restraints on Ramachandran plot distributions or residue side-chain rotameric states. However, using Ramachandran plots or rotameric states as refinement targets diminishes the validating power of these tools. Therefore, finding additional model-validation criteria that are not used or are difficult to use as refinement goals is desirable. Hydrogen bonds are one of the important noncovalent interactions that shape and maintain protein structure. These interactions can be characterized by a specific geometry of hydrogen donor and acceptor atoms. Systematic analysis of these geometries performed for quality-filtered high-resolution models of proteins from the Protein Data Bank shows that they have a distinct and a conserved distribution. Here, it is demonstrated how this information can be used for atomic model validation.