Open Access Publications

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Open access publications by faculty, postdocs, and graduate students in the Department of Chemistry and Biochemistry.

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Anion Binding as a Strategy for the Synthesis of Porous Salts
(Chemistry of Materials, 2022-12-27) Antonio, Alexandra M.; Dworzak, Michael R.; Korman, Kyle J.; Yap, Glenn P. A.; Bloch, Eric D.
Porous salts have recently emerged as a promising new class of ultratunable permanently microporous solids. These adsorbents, which were first reported as ionic solids based on porous cations and anions, can be isolated from a wide variety of charged, permanently porous coordination cages. A challenge in realizing the full tunability of such systems, however, lies in the fact that the majority of coordination cages for which surface areas have been reported are comprised of charge-balanced inorganic and organic building blocks that result in neutral cages. As such, most reported permanently porous coordination cages cannot be used as reagents in the synthesis of porous salts. Here, we show that the facile reaction of TBAX (TBA+ = tetra-n-butylammonium; X = F– and Cl–) with molybdenum paddlewheel-based coordination cages of the M4L4 and M24L24 lantern and cuboctahedra structure types, respectively, affords charged cages by virtue of coordination of halide anions to the internal and/or external metal sites on these structures, as confirmed by single-crystal X-ray diffraction, X-ray photoelectron spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy. At a practical level, the TBAX/cage reactions, which are fully reversible upon isolation of the cage with the appropriate solvent, solubilize otherwise rigorously insoluble cages. This method significantly increases the solution processability of these highly porous solids. Toward the formation of new porous salts, halide binding also serves to incorporate charge on neutral cages and make them amenable to simple salt metathesis reactions to afford new porous salts based on anions and cations with intrinsic porosity. A combination of diffraction methods and a suite of spectroscopic tools confirms speciation of the isolated solids, which represent a new class of highly tunable porous salts. Ultimately, this work represents a roadmap for the preparation of new porous solids and showcases the utility and broad applicability of anion binding as a strategy for the synthesis of porous salts.
Attachment Chemistry of 4-Fluorophenylboronic Acid on TiO2 and Al2O3 Nanoparticles
(Chemistry of Materials, 2022-12-13) Byron, Carly; Silva-Quinones, Dhamelyz; Sarkar, Sucharita; Brown, Scott C.; Bai, Shi; Quinn, Caitlin M.; Grzenda, Zachary; Chinn, Mitchell S.; Teplyakov, Andrew V.
Surface modification of nanoparticulate TiO2 and Al2O3 materials with 4-fluorophenylboronic acid is investigated in order to both evaluate the novel surface modification schemes and develop spectroscopic labels for surface characterization. The chemistry of the modification is followed on all these surfaces using X-ray photoelectron spectroscopy, multinuclear (11B, 19F, and 13C) solid-state and solution NMR, and infrared spectroscopy to determine the binding modes of this compound using boron and fluorine as probe atoms. Density functional theory model calculations are utilized to visualize predicted surface species and to interpret the results of spectroscopic measurements. A comparison is made among TiO2 rutile, TiO2 anatase, and γ-Al2O3. On all three materials, the modification proceeds via the boronic functional groups, with metal oxide-controlled surface chemistry. The bonding configuration depends on the material and is dominated by a monodentate species for titania and by bidentate species for alumina. The surface structures determined to form on all the oxide semiconductors investigated suggest that sensitization or monolayer doping approaches with a well-defined chemical interaction via a boronic functionality can be developed.
Authentication of edible oils using an infrared spectral library and digital sample sets: A feasibility study
(Journal of Chemometrics, 2023-03-19) Sota-Uba, Isio; White, Collin G.; Booksh, Karl; Lavine, Barry K.
A potential method to determine whether two varieties of edible oils can be differentiated by Fourier transform infrared (FTIR) spectroscopy is proposed using digitally generated data of adulterated edible oils from an infrared (IR) spectral library. The first step is the evaluation of digitally blended data sets. Specifically, IR spectra of adulterated edible oils are computed from digitally blending experimental data of the IR spectra of an edible oil and the corresponding adulterant using the appropriate mixing coefficients to achieve the desired level of adulteration. To determine whether two edible oils can be differentiated by FTIR spectroscopy, pure IR spectra of the two edible oils are compared with IR spectra of two edible oils digitally mixed using a genetic algorithm for pattern recognition to solve a ternary classification problem. If the IR spectra of the two edible oils and their binary mixtures are differentiable from principal component plots of the spectral data, then differences between the IR spectra of these two edible oils are of sufficient magnitude to ensure that a reliable classification by FTIR spectroscopy can be obtained. Using this approach, the feasibility of authenticating edible oils such as extra virgin olive oil (EVOO) directly from library spectra is demonstrated. For this study, both digital and experimental data are combined to generate training and validation data sets to assess detection limits in FTIR spectroscopy for the adulterants.
Calcium Substitution in Rare-earth Metal Germanides with the Gd5Si4 Type Structure
(Journal of Inorganic and General Chemistry, 2022-02-23) Suen, Nian-Tzu; Bobev, Svilen
An extended series of rare-earth metal calcium germanides have been synthesized and structurally characterized. The compounds have the general formula RE5−xCaxGe4 (1.5
Chemometric Software supporting NSF Project Variable Selection for Remedying the Effects of Uncontrolled Variation in Data Driven Predictions
(Steven D. Brown, 2019-09-15) Poerio, Dominic V.; Kneale, Casey; Brown, Steven D.
Coupling Novel Probes with Molecular Localization Microscopy Reveals Cell Wall Homeostatic Mechanisms in Staphylococcus aureus
(ACS Chemical Biology, 2022-11-22) Lund, Victoria A.; Gangotra, Haneesh; Zhao, Zhen; Sutton, Joshua A. F.; Wacnik, Katarzyna; DeMeester, Kristen; Liang, Hai; Santiago, Cintia; Grimes, Catherine Leimkuhler; Jones, Simon; Foster, Simon J.
Bacterial cell wall peptidoglycan is essential for viability, and its synthesis is targeted by antibiotics, including penicillin. To determine how peptidoglycan homeostasis controls cell architecture, growth, and division, we have developed novel labeling approaches. These are compatible with super-resolution fluorescence microscopy to examine peptidoglycan synthesis, hydrolysis, and the localization of the enzymes required for its biosynthesis (penicillin binding proteins (PBPs)). Synthesis of a cephalosporin-based fluorescent probe revealed a pattern of PBPs at the septum during division, supporting a model of dispersed peptidoglycan synthesis. Metabolic and hydroxylamine-based probes respectively enabled the synthesis of glycan strands and associated reducing termini of the peptidoglycan to be mapped. Foci and arcs of reducing termini appear as a result of both synthesis of glycan strands and glucosaminidase activity of the major peptidoglycan hydrolase, SagB. Our studies provide molecular level details of how essential peptidoglycan dynamics are controlled during growth and division.
Dynamic regulation of Zn(II) sequestration by calgranulin C
(Protein Science, 2022-08-13) Wang, Qian; Kuci, Deniz; Bhattacharya, Shibani; Hadden-Perilla, Jodi A.; Gupta, Rupal
Calgranulin C performs antimicrobial activity in the human immune response by sequestering Zn(II). This biological function is afforded with the aid of two structurally distinct Ca(II)-binding EF hand motifs, wherein one of which bears an unusual amino acid sequence. Here, we utilize solution state NMR relaxation measurements to investigate the mechanism of Ca(II)-modulated enhancement of Zn(II) sequestration by calgranulin C. Using C13/N15 CPMG dispersion experiments we have measured pH-dependent major and minor state populations exchanging on micro-to-millisecond timescale. This conformational exchange takes place exclusively in the Ca(II)-bound state and can be mapped to residues located in the EF-I loop and the linker between the tandem EF hands. Molecular dynamics (MD) simulations spanning nano-to-microsecond timescale offer insights into the role of pH-dependent electrostatic interactions in EF-hand dynamics. Our results suggest a pH-regulated dynamic equilibrium of conformations that explore a range of “closed” and partially “open” sidechain configurations within the Zn(II) binding site. We propose a novel mechanism by which Ca(II) binding to a non-canonical EF loop regulates its flexibility and tunes the antimicrobial activity of calgranulin C.
Electronically Conductive Hydrogels by in Situ Polymerization of a Water-Soluble EDOT-Derived Monomer
(Advanced Engineering Materials, 2022-05-13) Nguyen, Dan My; Wu, Yuhang; Nolin, Abigail; Lo, Chun-Yuan; Guo, Tianzheng; Dhong, Charles; Martin, David C.; Kayser, Laure V.
Electronically conductive hydrogels have gained popularity in bioelectronic interfaces because their mechanical properties are similar to biological tissues, potentially preventing scaring in implanted electronics. Hydrogels have low elastic moduli, due to their high water content, which facilitates their integration with biological tissues. To achieve electronically conductive hydrogels, however, requires the integration of conducting polymers or nanoparticles. These “hard” components increase the elastic modulus of the hydrogel, removing their desirable compatibility with biological tissues, or lead to the heterogeneous distribution of the conductive material in the hydrogel scaffold. A general strategy to transform hydrogels into electronically conductive hydrogels without affecting the mechanical properties of the parent hydrogel is still lacking. Herein, a two-step method is reported for imparting conductivity to a range of different hydrogels by in-situ polymerization of a water-soluble and neutral conducting polymer precursor: 3,4–ethylenedioxythiophene diethylene glycol (EDOT-DEG). The resulting conductive hydrogels are homogenous, have conductivities around 0.3 S m−1, low impedance, and maintain an elastic modulus of 5–15 kPa, which is similar to the preformed hydrogel. The simple preparation and desirable properties of the conductive hydrogels are likely to lead to new materials and applications in tissue engineering, neural interfaces, biosensors, and electrostimulation.
Enhanced thermoelectric performance in the zintl antimonides (Ca,RE)9Cd4Sb9 (RE = rare-earth metal). Synergy between increased structural complexity and drive towards optimized chemical bonding
(Materials Today Advances, 2022-10-27) Ogunbunmi, M. O.; Baranets, S.; Bobev, S.
The interplay of structural complexity, high carrier mobility, and high density of states effective mass can play a pivotal role in achieving enhanced thermoelectric (TE) performance in candidate materials. In this regard, the Zintl phases represent a class of compounds that are susceptible to harboring these key ingredients. This, in addition to their amenability to various forms of chemical substitution mechanisms makes them a good choice of systems to explore systematically. Here we demonstrate the role-play of these ingredients in achieving excellent TE properties on single-crystals of Ca9–xREyCd4+δSb9 (RE = Ce, Pr, Nd, Sm, Gd, Tb; x ≈ 0.5–0.8, y ≈ 0.5–0.7, δ ≈ 0.25). These phases represent a new addition to the “9–4–9” family with intricate chemical bonding arising from both a purposely introduced disorder on Ca sites and the inherent presence of interstitial Cd positions. Many of the newly synthesized and characterized phases show moderate values of the Seebeck coefficient, lying in the range of 71–116 μV/K at 600 K and evolving as degenerate semiconductors. Simultaneously, the electrical resistivity ρ(T) of the measured samples can be as low as 0.18 mΩ cm at this temperature. As a result, the observed TE power factors in the Ce-, Nd-, and Sm-samples are in the range 6–46 μW/cm.K2. For Ca9–xCexCd4+δSb9, the estimated thermoelectric quality factor B > 0.4 at 300 K, which corresponds to a figure of merit zT ≥ 1. Calculations based on the single parabolic band (SPB) model show that the optimum region for thermoelectric performance requires carrier concentration n = 2–6 × 1019 cm−3 thus providing for an open window to further tune the TE properties.
Evolution of multiple spillover hydrogen species on anatase titanium dioxide
(Cell Reports Physical Science, 2022-12-21) Liu, Kairui; Hou, Guangjin; Gao, Pan; Nie, Xuezhong; Bai, Shi; Janik, Michael J.; Zhang, Z. Conrad
Hydrogen spillover is a widespread phenomenon on reducible metal oxide surfaces, with numerous observations confirming its occurrence. However, direct physical characterization of the spillover hydrogen species on an oxide catalyst support remains challenging. Differentiating the binding sites of specific spillover hydrogen species has been elusive. Herein, we vary temperature and reductive conditions, then quench and detect the physicochemical character of three distinct spillover hydrogen species on anatase titanium dioxide (TiO2-A) by deuterium magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy, aided by density functional theory. Fast cooling during the sample preparation is crucial in quenching the spillover deuterium species to enable MAS NMR detection. Energetically favorable spillover deuterium species evolve from deuteron to deuteride states with increasing reduction temperature. Prevailing deuteron species reside on the 2-fold-coordinated O2c site of the TiO2-A (101) surface at low reduction temperature. At high reduction temperature, deuterides residing at oxygen vacancies (Ti6c–D–Ti5c) are formed.
Experimental and Theoretical Study on the Substitution Patterns in Lithium Germanides: The Case of Li15Ge4 vs Li14ZnGe4
(European Journal of Inorganic Chemistry, 2021-12-23) Osman, Hussien H.; Bobev, Svilen
A new ternary lithium zinc germanide, Li13.83Zn1.17(2)Ge4, was synthesized by a high-temperature solid state reaction of the respective elements. The crystal structure was determined by single-crystal X-ray diffraction methods. The new phase crystallizes in the body-centered cubic space group /43d 3d (no. 220) with unit cell parameter of 10.695(1) Å. The crystal structure refinements show that the parent Li15Ge4 structure is stabilized as Li15−xZnxGe4 (x≈1) via random substitution of Li atoms by the one-electron-richer atoms of the element Zn, by virtue of which the number of valence electrons increases, leading to a more electronically stable system. The substitution effects in the parent Li15Ge4 structure were investigated through both theory and experiment, which confirm that the Zn atoms in this structure prefer to occupy only one of the two available crystallographic sites for Li. The preferred substitution pattern established from experimental results is supported by DFT electronic structure calculations, which also explore the subtleties of the chemical bonding and the electronic properties of the title compounds.
Graphene Absorption Enhanced by Quasi-Bound-State-in-Continuum in Long-Wavelength Plasmonic–Photonic System
(Advanced Optical Materials, 2022-09-07) Kananen, Thomas; Wiggins, Marcie; Wang, Zi; Wang, Feifan; Soman, Anishkumar; Booksh, Karl; Alù, Andrea; Gu, Tingyi
Graphene plasmonic structures can support enhanced and localized light–mater interactions within extremely small mode volumes. However, the external quantum efficiency of the resulting devices is fundamentally limited by material scattering and radiation loss. Here, such radiation loss channels are suppressed by tailoring the structure to support a symmetry-protected bound-state-in-the-continuum (BIC) system. With practical loss rates and doping level in graphene, over 90% absorption near critical coupling is expected from numerical simulation. Experimentally measured peak absorption of 68% is achieved in such a tailored graphene photonic–plasmonic system, with maximum 50% contrast to the control sample without graphene. Significant reduction of the plasmon absorption for a different spacer thickness verifies the sensitivity of the system to the quasi-BIC condition.
Iron Oxidation by a Fused Cytochrome-Porin Common to Diverse Iron-Oxidizing Bacteria
(mBio, 2021-07-27) Keffer, Jessica L.; McAllister, Sean M.; Garber, Arkadiy I.; Hallahan, Beverly J.; Sutherland, Molly C.; Rozovsky, Sharon; Chan, Clara S.
Iron (Fe) oxidation is one of Earth’s major biogeochemical processes, key to weathering, soil formation, water quality, and corrosion. However, our understanding of microbial contribution is limited by incomplete knowledge of microbial iron oxidation mechanisms, particularly in neutrophilic iron oxidizers. The genomes of many diverse iron oxidizers encode a homolog to an outer membrane cytochrome (Cyc2) shown to oxidize iron in two acidophiles. Phylogenetic analyses show Cyc2 sequences from neutrophiles cluster together, suggesting a common function, though this function has not been verified in these organisms. Therefore, we investigated the iron oxidase function of heterologously expressed Cyc2 from a neutrophilic iron oxidizer Mariprofundus ferrooxydans PV-1. Cyc2PV-1 is capable of oxidizing iron, and its redox potential is 208 ± 20 mV, consistent with the ability to accept electrons from Fe2+ at neutral pH. These results support the hypothesis that Cyc2 functions as an iron oxidase in neutrophilic iron-oxidizing organisms. The results of sequence analysis and modeling reveal that the entire Cyc2 family shares a unique fused cytochrome-porin structure, with a defining consensus motif in the cytochrome region. On the basis of results from structural analyses, we predict that the monoheme cytochrome Cyc2 specifically oxidizes dissolved Fe2+, in contrast to multiheme iron oxidases, which may oxidize solid Fe(II). With our results, there is now functional validation for diverse representatives of Cyc2 sequences. We present a comprehensive Cyc2 phylogenetic tree and offer a roadmap for identifying cyc2/Cyc2 homologs and interpreting their function. The occurrence of cyc2 in many genomes beyond known iron oxidizers presents the possibility that microbial iron oxidation may be a widespread metabolism.
Light and microwave driven spin pumping across FeGaB–BiSb interface
(Physical Review Materials, 2021-12-16) Sharma, Vinay; Wu, Weipeng; Bajracharya, Prabesh; To, Duy Quang; Johnson, Anthony; Janotti, Anderson; Bryant, Garnett W.; Gundlach, Lars; Jungfleisch, M. Benjamin; Budhani, Ramesh C.
Three-dimensional (3D) topological insulators (TIs) with large spin Hall conductivity have emerged as potential candidates for spintronic applications. Here, we report spin to charge conversion in bilayers of amorphous ferromagnet (FM) Fe78Ga13B9 (FeGaB) and 3D TI Bi85Sb15 (BiSb) activated by two complementary techniques: spin pumping and ultrafast spin-current injection. DC magnetization measurements establish the soft magnetic character of FeGaB films, which remains unaltered in the heterostructures of FeGaB-BiSb. Broadband ferromagnetic resonance (FMR) studies reveal enhanced damping of precessing magnetization and large value of spin mixing conductance (5.03×1019m–2) as the spin angular momentum leaks into the TI layer. Magnetic field controlled bipolar DC voltage generated across the TI layer by inverse spin Hall effect is analyzed to extract the values of spin Hall angle and spin diffusion length of BiSb. The spin pumping parameters derived from the measurements of the femtosecond light-pulse-induced terahertz emission are consistent with the result of FMR. The Kubo-Bastin formula and tight-binding model calculations shed light on the thickness-dependent spin-Hall conductivity of the TI films, with predictions that are in remarkable agreement with the experimental data. Our results suggest that room temperature deposited amorphous and polycrystalline heterostructures provide a promising platform for creating novel spin orbit torque devices.
Mirror-plane disorder in a nickel chloride Schiff base complex: a suitable case study for crystallographic instruction
(Acta Crystallographica Section C: Structural Chemistry, 2022-01-27) Hsuan, Chang; Chen, Wen-Ching; Shen, Jiun-Shian; Ong, Tiow-Gan; Wang, Vincent C.-C.; Yap, Glenn P. A.
The nickel chloride complex of the Schiff base N2,N20-propanediylbis(2,3-butanedione-2-imine-3-oxime), namely, chlorido(3,9-dimethylundeca-3,8-diene-2,10-dione 10-oxime 2-oximato- 4N,N0,N00,N000)nickel(II), [NiCl(C 11 H19 N4 O2 )], at 100 K crystallizes in the orthorhombic space group Cmce. The structure exhibits mirror disorder of the main molecule that is not present in the bromide analogue. The relatively small number of unique reflections in the data set and the disorder imposed by the crystallographic mirror plane present a challenging educational case study.
A molecular substitutional disorder case study suitable for instruction: L2CrII(THF)/L2[(trimethylsilyl)methyl]CrIII (L is 2,5-bis{[(2,6-diisopropylphenyl)imino]methyl}pyrrol-1-ide)
(Acta Crystallographica Section C: Structural Chemistry, 2022-04-06) Salisbury, Brian A.; Young, John F.; Theopold, Klaus H.; Yap, Glenn P. A.
A solution of CrII and CrIII complexes, bis(2,5-bis{[(2,6-diisopropylphenyl)imino]methyl}pyrrol-1-ido)(tetrahydrofuran)chromium(II)–bis(2,5-bis{[(2,6-diisopropylphenyl)imino]methyl}pyrrol-1-ido)[(trimethylsilyl)methyl]chromium(III) (0.88/0.12), [Cr(C30H38N3)2(C4H8O)]0.88[Cr(C30H38N3)2(C4H11Si)]0.12 or L2CrII(THF)/L2[(trimethylsilyl)methyl]CrIII (L = 2,5-bis{[(2,6-diisopropylphenyl)imino]methyl}pyrrol-1-ide and THF is tetrahydrofuran), in pentane crystallizes in the monoclinic space group P21/c. The structure obtained shows most of the atoms coincident but with THF disordered with the (trimethylsilyl)methyl ligand. Structures with this disorder, involving more than two or three atoms, seem to appear rarely in the literature; however, in this case, the data set is ideal for the crystallographic instruction of molecular substitution disorder.
Natural Product Synthesis by Intramolecular Alkylidene Carbene C−H Insertion
(European Journal of Organic Chemistry, 2022-03-15) Taber, Douglass F.
Direct functionalization of unactivated C−H bonds is of increasing importance in organic synthesis. Of the many methods that have been developed, intramolecular alkylidene C−H insertion, proceeding with retention of absolute configuration, is among the most versatile. This Review explores all examples of the use of intramolecular alkylidene C−H insertion in natural product synthesis.
A one-shot double-slice selection NMR method for biphasic systems
(Physical Chemistry Chemical Physics, 2022-07-18) Doolittle Catlin, Kaitlyn; Simmons, Julia; Bai, Shi
We propose a new and robust one-shot double-slice selection experiment to detect 1H NMR signals of biphasic systems simultaneously. The resultant spectrum contains opposite-phased peaks representing the chemical species from the two phases, respectively.
Photo-activatable Ub-PCNA probes reveal new structural features of the Saccharomyces cerevisiae Polη/PCNA complex
(Nucleic Acids Research, 2021-08-14) Shen, Siqi; Davidson, Gregory A; Yang, Kun; Zhuang, Zhihao
The Y-family DNA polymerase η (Polη) is critical for the synthesis past damaged DNA nucleotides in yeast through translesion DNA synthesis (TLS). TLS is initiated by monoubiquitination of proliferating cell nuclear antigen (PCNA) and the subsequent recruitment of TLS polymerases. Although individual structures of the Polη catalytic core and PCNA have been solved, a high-resolution structure of the complex of Polη/PCNA or Polη/monoubiquitinated PCNA (Ub-PCNA) still remains elusive, partly due to the disordered Polη C-terminal region and the flexibility of ubiquitin on PCNA. To circumvent these obstacles and obtain structural insights into this important TLS polymerase complex, we developed photo-activatable PCNA and Ub-PCNA probes containing a p-benzoyl-L-phenylalanine (pBpa) crosslinker at selected positions on PCNA. By photo-crosslinking the probes with full-length Polη, specific crosslinking sites were identified following tryptic digestion and tandem mass spectrometry analysis. We discovered direct interactions of the Polη catalytic core and its C-terminal region with both sides of the PCNA ring. Model building using the crosslinking site information as a restraint revealed multiple conformations of Polη in the polymerase complex. Availability of the photo-activatable PCNA and Ub-PCNA probes will also facilitate investigations into other PCNA-containing complexes important for DNA replication, repair and damage tolerance.
Proline C–H bonds as loci for proline assembly via C–H/O interactions
(ChemBioChem, 2022-09-21) Daniecki, Noah J.; Bhatt, Megh R.; Yap, Glenn P. A.; Zondlo, Neal Joseph
Proline residues within proteins lack a traditional hydrogen bond donor. However, the hydrogens of proline are all sterically accessible, with polarized C–H bonds that can be sites for molecular recognition. C–H/O interactions, between proline C–H bonds and oxygen lone pairs, have been previously identified as modes of recognition in proteins. A series of proline derivatives was synthesized, including 4R-hydroxyproline nitrobenzoate methyl ester, acylated on the proline nitrogen with bromoacetyl and glycolyl groups, and Boc-4S-(4-iodophenyl)hydroxyproline methyl amide. All three derivatives exhibited multiple close intermolecular C–H/O interactions in the crystallographic state, with H•••O distances as close as 2.3 Å. These observed distances are well below the 2.72 Å sum of the van der Waals radii of H and O. We further analyzed the role of C–H/O interactions in all previously crystallized derivatives of these amino acids, and found that all 26 structures exhibited close intermolecular C–H/O interactions. Finally, we analyzed all proline residues in the Cambridge Structural Database. The majority of these structures exhibited intermolecular C–H/O interactions at proline C–H bonds, suggesting that C–H/O interactions are an inherent and important mode for recognition of and higher-order assembly at proline residues. Due to steric accessibility and multiple polarized C–H bonds, proline residues are uniquely positioned as sites for binding and recognition via C–H/O interactions.

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