Extensive Hydrogen-bonding Network Research Articles

Hydrogen Bond Flexibility and Water Dynamics in the Far Red Fluorescent Protein TagRFP675

Next-generation far-red emitting fluorescent proteins (FPs) with enhanced fluorescence quantum yields and high photostability are highly desirable. They would enable deeper tissue penetration and lengthened imaging times given a lower autofluorescence background, lower light scattering, and higher transmission beyond 650 nm. The farthest red emitting FP engineered to date is TagRFP675, which has a 77 nm Stokes shift. The red emission of TagRFP675 is attributed to several H-bonding contacts involving Q41 and S28 at the N-acylimine position with additional H-bonds at the phenoxy end of the chromophore with N143, N158, and R197. We used molecular dynamics (MD) simulations to explore the relationship between flexibility of the chromophore environment and the large Stokes shift in TagRFP675 and related variants mKate and its mutant mKate-M41Q. Both TagRFP675 and mKate-M41Q have large Stokes shift as compared to mKate. Analysis of the hydrogen bonds around the chromophore reveal that in both TagRFP675 and mKate-M41Q have an extended hydrogen bond network connecting Q106-water-F65-Q41-S28 whereas in mKate, this network does not extend beyond F65. The water molecule involved in the hydrogen bond network shows significantly larger flexibility and mobility in TagRFP675 as compared to mKate-M41Q, highlighting the role of the chromophore environment for large bathochromic shifts.

Crystal structure of cis-bis-(μ-β-alanine-κ(2) O:O')bis[tri-chlorido-rhenium(III)](Re-Re) sesquihydrate.

The structure of the title compound, [Re2Cl6(C3H7NO2)2]·1.5H2O, comprises a dinuclear complex cation [Re-Re = 2.2494 (3) Å] involving cis-oriented double carboxyl-ate bridges, four equatorial chloride ions and two weakly bonded chloride ligands in the axial positions at the two rhenium(III) atoms. In the crystal, two complex mol-ecules and two water mol-ecules constitute hydrogen-bonded dimers, while an extensive hydrogen-bonding network involving the groups of the zwitterionic ligand is important for generation of the framework. An additional partially occupied water molecule is disordered over two sets of sites about a symmetry centre with a site-occupancy ratio of 0.3:0.2.

Kinetic and structural requirements for carbapenemase activity in GES-type β-lactamases.

Carbapenems are the last resort antibiotics for treatment of life-threatening infections. The GES β-lactamases are important contributors to carbapenem resistance in clinical bacterial pathogens. A single amino acid difference at position 170 of the GES-1, GES-2, and GES-5 enzymes is responsible for the expansion of their substrate profile to include carbapenem antibiotics. This highlights the increasing need to understand the mechanisms by which the GES β-lactamases function to aid in development of novel therapeutics. We demonstrate that the catalytic efficiency of the enzymes with carbapenems meropenem, ertapenem, and doripenem progressively increases (100-fold) from GES-1 to -5, mainly due to an increase in the rate of acylation. The data reveal that while acylation is rate limiting for GES-1 and GES-2 for all three carbapenems, acylation and deacylation are indistinguishable for GES-5. The ertapenem–GES-2 crystal structure shows that only the core structure of the antibiotic interacts with the active site of the GES-2 β-lactamase. The identical core structures of ertapenem, doripenem, and meropenem are likely responsible for the observed similarities in the kinetics with these carbapenems. The lack of a methyl group in the core structure of imipenem may provide a structural rationale for the increase in turnover of this carbapenem by the GES β-lactamases. Our data also show that in GES-2 an extensive hydrogen-bonding network between the acyl-enzyme complex and the active site water attenuates activation of this water molecule, which results in poor deacylation by this enzyme.

Crystal structure of penta-kis-(ethyl-enedi-amine-κ(2) N,N')lanthanum(III) trichloride-ethylene-diamine-dichloromethane (1/1/1).

We report here the crystal structure of a ten-coordinate lanthanum(III) metal coordinated by five bidentate ethyl-enedi-amine ligands, [La(C2H8N2)5]Cl3·C2H8N2·CH2Cl2. One free ethyl-enedi-amine mol-ecule and three Cl(-) anions are also located in the asymmetric unit. The overall structure is held together by an extensive hydrogen-bonding network between the Cl(-) anions and the NH groups on the metal-bound ethyl-enedi-amine ligands. The free ethyl-enedi-amine mol-ecule is held in an ordered position by additional hydrogen bonds involving both the chlorides and -NH groups on the metal-bound ligands. One highly disordered mol-ecule of di-chloro-methane is located on an inversion center; however, all attempts to model this disorder were unsuccessful. The electron density in this space was removed using the BYPASS procedure [van der Sluis & Spek (1990 ▶). Acta Cryst. A46, 194-201].

Effect of pressure on a mixed valence FeiiFeiii2complex

Intramolecular electron transfer (ET) in mixed valence (MV) oxo-centered [FeiiFeiii2O(carboxylate)6(ligand)3]·solvent complexes is highly dependent on temperature, on the nature of the ligands, and on the presence of crystal solvent molecules [1]. Whereas the effects of temperature, crystal solvent, and ligand variation on the details of the ET have been explored thoroughly, the effect of pressure is less well described [2]. The effect of pressure on the ET in MV Fe3O(cyanoacetate)6(water)3has been investigated with single crystal X-ray diffraction and Mössbauer spectroscopy. Previous multi-temperature studies have shown that at room temperature the ET between the three Fe sites is fast and the observed structure of the Fe3core is a perfectly equilateral triangle [3]. Cooling the complex below 130 K induces a phase transition as the ET slows down. Below 120 K the Fe3core is distorted due to the localization of the itinerant electron on one of the three Fe sites in the triangle (the complex is then in the valence trapped state). The valence trapping is complete within a temperature interval of just 10 K. The abruptness of the transition has been attributed to the extended hydrogen bond network involving water ligands and cyano groups, promoting intermolecular cooperative effects. The high-pressure X-ray diffraction data show that there is a 900flip of half the cyano groups at 3.5 GPa, which dramatically changes the hydrogen bond network. At a slightly higher pressure, a phase transition is found to occur. The five single crystals investigated all broke into minor fragments at the transition; however triclinic unit cells, similar to the low temperature unit cell, could be indexed from selected spots. Additional evidence that the complex is valence trapped comes from high pressure Mössbauer spectra measured above the phase transition (4 GPa). The relationship between valence trapping and the structural changes will in this work be highlighted using void space and Hirshfeld surface analysis.

Structural Insights into the Low pH Adaptation of a Unique Carboxylesterase from Ferroplasma: Altering the pH Optima of Two Carboxylesterases

To investigate the mechanism for low pH adaptation by a carboxylesterase, structural and biochemical analyses of EstFa_R (a recombinant, slightly acidophilic carboxylesterase from Ferroplasma acidiphilum) and SshEstI (an alkaliphilic carboxylesterase from Sulfolobus shibatae DSM5389) were performed. Although a previous proteomics study by another group showed that the enzyme purified from F. acidiphilum contained an iron atom, EstFa_R did not bind to iron as analyzed by inductively coupled plasma MS and isothermal titration calorimetry. The crystal structures of EstFa_R and SshEstI were determined at 1.6- and 1.5-Å resolutions, respectively. EstFa_R had a catalytic triad with an extended hydrogen bond network that was not observed in SshEstI. Quadruple mutants of both proteins were created to remove or introduce the extended hydrogen bond network. The mutation on EstFa_R enhanced its catalytic efficiency and gave it an alkaline pH optimum, whereas the mutation on SshEstI resulted in opposite effects (i.e. a decrease in the catalytic efficiency and a downward shift in the optimum pH). Our experimental results suggest that the low pH optimum of EstFa_R activity was a result of the unique extended hydrogen bond network in the catalytic triad and the highly negatively charged surface around the active site. The change in the pH optimum of EstFa_R happened simultaneously with a change in the catalytic efficiency, suggesting that the local flexibility of the active site in EstFa_R could be modified by quadruple mutation. These observations may provide a novel strategy to elucidate the low pH adaptation of serine hydrolases.

Detection of conformation types of cyclosporin retaining intramolecular hydrogen bonds by mass spectrometry.

Cyclosporin is a family of neutral cyclic undecapeptides widely used for the prevention of organ transplant rejection and controlling viral infection. The equilibrium of conformations assumed by cyclosporin A in response to the solvent environment is thought to play a critical role in enabling good membrane penetration, which improves upon shielding the polarity of the molecule through forming intramolecular hydrogen bonds. However, the distribution of structures and their internal hydrogen bond geometries have not been elucidated thus far across the series of cyclosporins. Herein, we elucidate the conformational heterogeneity of cyclosporins using a set of analytical approaches including ion mobility mass spectrometry, hydrogen-deuterium exchange, and molecular dynamics simulation. Ion mobility measurements reveal a specific conformational distribution for each cyclosporin derivative in a structure-dependent manner. In general, we observe that the more compact conformer is associated with a greater frequency of intramolecular hydrogen bonds. Cyclosporin A is populated by structures with an extensive hydrogen bond network that is lacking in cyclosporin H, which is composed predominantly of a single compact conformation. The slower dynamics of cyclosporin H backbone is also consistent with the lack of hydrogen bonds. Furthermore, we find a strong correlation between the steric bulk of the side chain at position 2 of cyclosporin and the distribution of conformers due to differential accommodation of side chains within the macrocycle, and also report a wide range of conformational dynamics in solution.

Comparison of x-ray absorption spectra between water and ice: new ice data with low pre-edge absorption cross-section.

The effect of crystal growth conditions on the O K-edge x-ray absorption spectra of ice is investigated through detailed analysis of the spectral features. The amount of ice defects is found to be minimized on hydrophobic surfaces, such as BaF2(111), with low concentration of nucleation centers. This is manifested through a reduction of the absorption cross-section at 535 eV, which is associated with distorted hydrogen bonds. Furthermore, a connection is made between the observed increase in spectral intensity between 544 and 548 eV and high-symmetry points in the electronic band structure, suggesting a more extended hydrogen-bond network as compared to ices prepared differently. The spectral differences for various ice preparations are compared to the temperature dependence of spectra of liquid water upon supercooling. A double-peak feature in the absorption cross-section between 540 and 543 eV is identified as a characteristic of the crystalline phase. The connection to the interpretation of the liquid phase O K-edge x-ray absorption spectrum is extensively discussed.

The influence of hydrogen bonding on the planar arrangement of melamine in crystal structures of its solvates, cocrystals and salts

The hydrogen bonding patterns of melamine as well as mono- and diprotonated melamine have been analysed in five crystal structures of a solvate, a cocrystal and three organic and inorganic salts, namely, melamine DMSO solvate ([mel]·DMSO (1)), melamine theobromine cocrystal ([mel]·[TBR]3 (2)), dimelaminium ethylenediaminetetraacetate ([mel-H]2[EDTA-H2]·2H2O (3)), anhydrous dimelaminium sulfate ([mel-H]2[SO4] (4)), and anhydrous melaminium dinitrate ([mel-H2][NO3]2 (5)). Melamine is a versatile molecular building block (tecton) in cocrystals, solvates and salts. Depending on the degree of protonation and/or other molecules or ions present in the structure, parallels could be drawn to determine whether melamine is arranged in a cross-linked manner, in undulating sheets or in the form of perfectly planar sheets in the structure. Graph set analysis was used to compare the geometry of hydrogen bond interactions of the new structures with those of other structures published in the literature. Solvent drop-assisted solid state reactions (kneading) were performed for “green” synthesis of the compounds. The organic salt 3 has high thermal stability as shown by variable-temperature X-ray powder diffraction, which is presumably related to its extensive hydrogen bond network. Rietveld refinements were carried out on laboratory powder diffraction data to confirm the structures of the compounds obtained from single-crystal data.

CO2 Absorption in the Protic Ionic Liquid Ethylammonium Nitrate

We present a first principles molecular dynamics study of carbon dioxide solvation by protic ionic liquids using ethylammonium nitrate as an example solvent. Microheterogeneity of the alkyl chains and the extended hydrogen bond network could be observed. Thus, the entire structure of the investigated protic ionic liquid mixed with CO2 closely resembles the one of the pure liquid. Our data indicates that CO2 most likely creates an energy loss due to entering the liquid via the too-small voids. But this is fully compensated by specific attractive interactions of CO2 with the cation and anions of ethylammonium nitrate. This result might serve as an explanation for the question of why the volume of the ionic liquid is not increasing through CO2 uptake. The CO2 cluster formation, which shows a structure similar to supercritical CO2, is guided by the dominance of the nonpolar groups in the CO2 solvation shell.

Structural Basis for Reversible Phosphorolysis and Hydrolysis Reactions of 2-O-α-Glucosylglycerol Phosphorylase

2-O-α-Glucosylglycerol phosphorylase (GGP) from Bacillus selenitireducens catalyzes both the reversible phosphorolysis of 2-O-α-glucosylglycerol (GG) and the hydrolysis of β-d-glucose 1-phosphate (βGlc1P). GGP belongs to the glycoside hydrolase (GH) family 65 and can efficiently and specifically produce GG. However, its structural basis has remained unclear. In this study, the crystal structures of GGP complexed with glucose and the glucose analog isofagomine and glycerol were determined. Subsite -1 of GGP is similar to those of other GH65 enzymes, maltose phosphorylase and kojibiose phosphorylase, whereas subsite +1 is largely different and is well designed for GG recognition. An automated docking analysis was performed to complement these crystal structures, βGlc1P being docked at an appropriate position. To investigate the importance of residues at subsite +1 in the bifunctionality of GGP, we constructed mutants at these residues. Y327F and K587A did not show detectable activities for either reverse phosphorolysis or βGlc1P hydrolysis. Y572F also showed significantly reduced activities for both of these reactions. In contrast, W381F showed significantly reduced reverse phosphorolytic activity but retained βGlc1P hydrolysis. The mode of substrate recognition and the reaction mechanisms of GGP were proposed based on these analyses. Specifically, an extensive hydrogen bond network formed by Tyr-327, Tyr-572, Lys-587, and water molecules contributes to fixing the acceptor molecule in both reverse phosphorolysis (glycerol) and βGlc1P hydrolysis (water) for a glycosyl transfer reaction. This study will contribute to the development of a large scale production system of GG by facilitating the rational engineering of GGP.

Interaction of a long alkyl chain protic ionic liquid and water.

A combined experimental/theoretical approach has been used to investigate the role of water in modifying the microscopic interactions characterizing the optical response of 1-butyl-ammonium nitrate (BAN) water solutions. Raman spectra, dominated by the signal from the protic ionic liquid, were collected as a function of the water content, and the corresponding spatial organization of the ionic couples, as well as their local arrangement with water molecules, was studied exploiting classical molecular dynamics calculations. High quality spectroscopic data, combined with a careful analysis, revealed that water affects the vibrational spectrum BAN in solution: as the water concentration is increased, peaks assigned to stretching modes show a frequency hardening together with a shape narrowing, whereas the opposite behavior is observed for peaks assigned to bending modes. Calculation results clearly show a nanometric spatial organization of the ionic couples that is not destroyed on increasing the water content at least within an intermediate range. Our combined results show indeed that small water concentrations even increase the local order. Water molecules are located among ionic couples and are closer to the anion than the cation, as confirmed by the computation of the number of H-bonds which is greater for water-anion than for water-cation. The whole results set thus clarifies the microscopic scenario of the BAN-water interaction and underlines the main role of the extended hydrogen bond network among water molecules and nitrate anions.

Characterization of molecular associations involving l-ornithine and α-ketoglutaric acid: Crystal structure of l-ornithinium α-ketoglutarate

The crystal structure of L-ornithinium α-ketoglutarate (C5H13N2O2, C5H5O5) has been solved by direct methods using single crystal X-ray diffraction data. It crystallizes in the monoclinic system, space group P21, unit cell parameters a=15.4326(3), b=5.2015(1), c=16.2067(3) Å and β=91.986(1)°, containing two independent pairs of molecular ions in the asymmetric unit. An extensive hydrogen-bond network and electrostatic charges due to proton transfer provide an important part of the cohesive energy of the crystal. The conformational versatility of L-ornithine and α-ketoglutaric acid is illustrated by the present results and crystal structures available from the Cambridge Structural Database.

Evidence from FTIR difference spectroscopy that D1-Asp61 influences the water reactions of the oxygen-evolving Mn4CaO5 cluster of photosystem II.

Understanding the mechanism of photosynthetic water oxidation requires characterizing the reactions of the water molecules that serve as substrate or that otherwise interact with the oxygen-evolving Mn4CaO5 cluster. FTIR difference spectroscopy is a powerful tool for studying the structural changes of hydrogen bonded water molecules. For example, the O-H stretching mode of water molecules having relatively weak hydrogen bonds can be monitored near 3600 cm(-1), the D-O-D bending mode can be monitored near 1210 cm(-1), and highly polarizable networks of hydrogen bonds can be monitored as broad features between 3000 and 2000 cm(-1). The two former regions are practically devoid of overlapping vibrational modes from the protein. In Photosystem II, water oxidation requires a precisely choreographed sequence of proton and electron transfer steps in which proton release is required to prevent the redox potential of the Mn4CaO5 cluster from rising to levels that would prevent its subsequent oxidation. Proton release takes place via one or more proton egress pathways leading from the Mn4CaO5 cluster to the thylakoid lumen. There is growing evidence that D1-D61 is the initial residue of one dominant proton egress pathway. This residue interacts directly with water molecules in the first and second coordination spheres of the Mn4CaO5 cluster. In this study, we explore the influence of D1-D61 on the water reactions accompanying oxygen production by characterizing the FTIR properties of the D1-D61A mutant of the cyanobacterium, Synechocystis sp. PCC 6803. On the basis of mutation-induced changes to the carbonyl stretching region near 1747 cm(-1), we conclude that D1-D61 participates in the same extensive networks of hydrogen bonds that have been identified previously by FTIR studies. On the basis of mutation-induced changes to the weakly hydrogen-bonded O-H stretching region, we conclude that D1-D61 interacts with water molecules that are located near the Cl(-)(1) ion and that deprotonate or participate in stronger hydrogen bonds as a result of the S1 to S2 and S2 to S3 transitions. On the basis of the elimination of a broad feature between 3100 and 2600 cm(-1), we conclude that the highly polarizable network of hydrogen bonds whose polarizability or protonation state increases during the S1 to S2 transition involves D1-D61. On the basis of the elimination of features in the D-O-D bending region, we conclude that D1-D61 forms a hydrogen bond to one of the H2O molecules whose H-O-H bending mode changes in response to the S1 to S2 transition. The elimination of this H2O molecule in the D1-D61A mutant provides one rationale for the decreased efficiency of water oxidation in this mutant. Finally, we discuss reasons why the recent conclusion that a substrate-containing cluster of five water molecules accepts a proton from the Mn4CaO5 cluster during the S1 to S2 transition and deprotonates during subsequent S state transitions should be reassessed.

Design and synthesis of highly potent HIV-1 protease inhibitors with novel isosorbide-derived P2 ligands

The design, synthesis, and biological evaluation of a series of six HIV-1 protease inhibitors incorporating isosorbide moiety as novel P2 ligands are described. All the compounds are very potent HIV-1 protease inhibitors with IC50 values in the nanomolar or picomolar ranges (0.05–0.43nM). Molecular docking studies revealed the formation of an extensive hydrogen-bonding network between the inhibitor and the active site. Particularly, the isosorbide-derived P2 ligand is involved in strong hydrogen bonding interactions with the backbone atoms.

Vanadium complexes with side chain functionalized N-salicylidene hydrazides: Hydrogen-bonding relays as structural directive for supramolecular interactions

The synthesis and spectroscopic characterization of dioxidovanadium(V) complexes with hydrazone Schiff-base ligands derived from salicylaldehyde and ω-hydroxy functionalized carbonic acid hydrazides with three different chain lengths are reported. This includes three series, the ammonium and potassium salts of the complex anions with the twofold deprotonated ligand systems as well as the corresponding neutral complexes [VO2(HL)]. For four complexes within these series crystal structures are reported, namely the full series of complexes with the ligand system containing the shortest chain length (i.e. ammonium salt, potassium salt, and neutral complex) and the ammonium salt with the ligand based on the longest side chain. All complexes posses a dioxidovanadium group with a five-coordinated vanadium atom in a distorted square pyramidal geometry. Within the series of ammonium salts an extensive hydrogen-bonding network including the side chain hydroxyl end group is observed which is virtually independent of the side chain length. Whereas the structure of the potassium salt is governed by ionic contacts resulting in a bilayered packing arrangement with intercalated potassium ions. In the case of the neutral complex a two-dimensional hydrogen-bonding network is observed. Magic angle spinning solid-state 51V NMR is used to characterize all three series of complexes. The variation of the chemical shift anisotropy parameters is found to be strongly dependent on differences in the supramolecular structure of the compounds, such as hydrogen bonding or crystal packing, which are caused by variation of the protonation state, counterion, and side chain length.

Synthesis, Characterization, and Crystal Structure of Sodium (Methyl α-d-Mannopyranosid)uronate Monohydrate

TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl)-mediated oxidation of methyl α-<sc>d</sc>-mannopyranoside with sodium hypochlorite gave sodium (methyl α-<sc>d</sc>-mannopyranosid)uronate, which was obtained as a crystalline monohydrate in ~70% yield without chromatography. Its purity was proved by NMR spectroscopy, a newly developed HPLC method, combustion analysis, and X-ray crystallography. The crystal structure, solved from synchrotron diffraction data in space group <i>P</i>2₁2₁2₁, revealed that the packing of the uronate molecules is connected by an extensive network of hydrogen bonds, and that the Na⁺ ion is coordinated by six oxygen ligands from one water and three surrounding sugar molecules.

Synthesis, supramolecular structure, and energetic properties of the first metal–organic nitrotetrazolate, [Me3Sn(μ-OH)SnMe3(μ-OH)SnMe3(H2O)][NT] (NT = 5-nitrotetrazolate)

The first metal–organic 5-nitrotetrazolate, [Me3Sn(μ-OH)SnMe3(μ-OH)SnMe3(H2O)][NT] (2), has been prepared by a simple metathetical reaction between Me3SnCl and sodium 5-nitrotetrazolate dihydrate (=NaNT, 1). A single-crystal X-ray diffraction study revealed the presence of the trinuclear [Me3Sn(μ-OH)SnMe3(μ-OH)SnMe3(H2O)]+ cation to which the 5-nitrotetrazolate anion is coordinated via a ring-N atom. The NT anion is further engaged in four different OH⋯N and OH⋯N hydrogen bonds involving the remaining three ring-nitrogen atoms and one oxygen of the nitro group, leading to an extensive supramolecular hydrogen-bonded network in the solid state. Despite its very low N content of only 10.65%, compound 2 is highly impact-sensitive (<2.5J) and can be classified as a primary explosive.

Network of Hydrogen Bonds near the Oxygen-Evolving Mn4CaO5 Cluster of Photosystem II Probed with FTIR Difference Spectroscopy

We previously provided experimental evidence that an extensive network of hydrogen bonds exists near the oxygen-evolving Mn4CaO5 cluster in photosystem II and that elements of this network form part of a dominant proton-egress pathway leading from the Mn4CaO5 cluster to the thylakoid lumen. The evidence was based on (i) the elimination of the same ν(C═O) mode of a protonated carboxylate group in the S2-minus-S1 FTIR difference spectrum of wild-type PSII core complexes from the cyanobacterium Synechocystis sp. PCC 6803 by the mutations D1-E65A, D2-E312A, and D1-E329Q and (ii) the substantial decrease in the efficiency of the S3 to S0 transition caused by the mutations D1-D61A, D1-E65A, and D2-E312A. The eliminated ν(C═O) mode corresponds to an unidentified carboxylate group whose pKa value decreases in response to the increased charge that develops on the Mn4CaO5 cluster during the S1 to S2 transition. In the current study, we have extended our work to include the ν(C═O) regions of other Sn+1-minus-Sn FTIR difference spectra and to additional mutations of residues inferred to participate in networks of hydrogen bonds near the Mn4CaO5 cluster or leading from the Mn4CaO5 cluster to the thylakoid lumen. Our data suggest that a different carboxylate group has its pKa value increased during the S2 to S3 transition and that a third carboxylate group experiences a change in its environment during the S0 to S1 transition. The pKa values that shift during the S1 to S2 and S2 to S3 transitions appear to be restored during the S3 to S0 transition. The D1-R334A mutation decreases or eliminates the same ν(C═O) modes from the S2-minus-S1 and S3-minus-S2 spectra as mutations D1-E65A, D2-E312A, and D1-E329Q and substantially decreases the efficiency of the S3 to S0 transition. We conclude that D1-R334 participates in the same dominant proton-egress pathway that was identified in our previous study. The D1-Q165E mutation leaves the ν(C═O) region of the S2-minus-S1 FTIR difference spectrum intact, but it eliminates a mode from this region of the S3-minus-S2 spectrum. We conclude that D1-Q165 participates in an extensive network of hydrogen bonds that that extends across the Mn4CaO5 cluster to the D1-E65/D2-E312 dyad and that includes D1-E329 and several water molecules including the W2 and W3 water ligands of the Mn4CaO5 cluster's dangling MnA4 and Ca ions, respectively. The D2-E307Q, D2-D308N, D2-E310Q, and D2-E323Q mutations alter the ν(C═O) regions of none of the FTIR difference spectra. We conclude that these four residues are located far from the three unidentified carboxylate groups that give rise to the ν(C═O) features observed in the FTIR difference spectra.

Exfoliation of Graphite with Triazine Derivatives under Ball-Milling Conditions: Preparation of Few-Layer Graphene via Selective Noncovalent Interactions

A ball-milling treatment can be employed to exfoliate graphite through interactions with commercially available melamine under solid conditions. This procedure allows the fast production of relatively large quantities of material with a low presence of defects. The milling treatment can be modulated in order to achieve graphene flakes with different sizes. Once prepared, the graphene samples can be redispersed in organic solvents, water, or culture media, forming stable dispersions that can be used for multiple purposes. In the present work, we have screened electron-rich benzene derivatives along with triazine derivatives in their respective ability to exfoliate graphite. The results suggest that the formation of a hydrogen-bonding network is important for the formation of multipoint interactions with the surfaces of graphene, which can be used for the exfoliation of graphite and the stabilization of graphene in different solvents. Aminotriazine systems were found to be the best partners in the preparation and stabilization of graphene layers in different solvents, while the equivalent benzene derivatives did not show comparable exfoliation ability. Computational studies have also been performed to rationalize the experimental results. The results provide also the basis for further work in the preparation of noncovalently modified graphene, where derivatives of aminotriazines can be designed to form extensive hydrogen-bond 2D networks on the graphene surface with the aim of manipulating their electronic and chemical properties.