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1.
U Burget  G Zundel 《Biopolymers》1987,26(1):95-108
(L -His)n- dihydrogen phosphate systems are studied by ir spectroscopy in the presence of various cations and as a function of the degree of hydration. Ir continua indicate that (I) OH … N ? O?…H+N (IIR) hydrogen bonds are formed and that these bonds show high proton polarizability, which increases from the Li+ to the K+ system. In the K+?system, His-Pi-Pi chains are formed, showing particularly high proton polarizability due to collective proton motion within both hydrogen bonds. The OH N ? O?…H?N equilibria are determined from ir bands. With the Li+ system, 55% of the protons are present at the histidine residues; this percentage is smaller with the Na+ system (41%), and amounts to only 32% with the K+ system. With the increasing degree of hydration the removal of the degeneracy of νas?PO2?3 vanishes, indicating loosening of the cations from the phosphates. Nevertheless, the hydrogen bond acceptor O atom becomes more negative; a shift of the equilibrium to the right is observed in the OH… N ? O?…H+N bond. This is explained by the strong interaction of the dipole of the hydrogen bonds with the water molecules. All these results show that protons can be shifted easily in these hydrogen bonds due to their high proton polarizability. The transfer equilibria can be controlled easily by local electrical fields. In addition, these results may be of significance when phosphates interact with proteins.  相似文献   

2.
OH…N ? O?…H+N hydrogen bonds formed between N-all-transretinylidene butylamine (Schiff base) and phenols (1:1) are studied by IR spectroscopy. It is shown that both proton limiting structures of these hydrogen bonds have the same weight with Δ pKa (50%) = (pKa protonated Schiff base minus pKa phenol) = 5.5. With the largely symmetrical systems, continua demonstrate that these hydrogen bonds show great proton polarizability. In the Schiff base + tyrosine system in a non-polar solvent the residence time of the proton at the tyrosine residue is much larger than that at the Schiff base. In CH2CCl2 these hydrogen bonds show, however, still proton polarizability, i.e., the position of the proton transfer equilibrium OH…N ? O?…H+N is shifted to and fro as function of the nature of the environment of this hydrogen bond. Consequences regarding bacteriorhodopsin are discussed.  相似文献   

3.
Polyhistidine-carboxylic acid systems are studied by ir spectroscopy. It is shown that OH ?N ? O?…H+N bonds formed between carboxylic groups and histidine residues are easily polarizable proton-transfer hydrogen bonds when the pKa of the protonated histidine residues is about 2.8 units larger than that of the carboxylic groups. From these results it bis concluded that OH ?N ? O? ?H+N bonds between glutamic or aspartic acid histidine residues in proteins may be easily polarizable proton-transfer bonds. Furthermore, it is demonstrated that water molecules shift the proton-transfer equilibria in these hydrogen bonds in favor of the polar structure, i.e., due to water or polar environments OH ?N ? O? ?H+N bonds with smaller ΔpKa values become easily polarizable proton-transfer hydrogen bonds. A consideration of the amide bands of polyhistidine shows that it can be present in five different conformations. It is shown that these conformational changes are strongly related to the degree of proton transfer. Hence, the degree of proton transfer, the degree of hydration, and conformation are not independent of each other, but are strongly coupled. Further proof for the interdependence of proton transfer and conformational changes are hysteresis effects, which are observed with studies of polyhistidine dependent on carboxylic acid, adsorption and desorption. OH ?N ? O? ?H+N bonds between aspartic and glutamic acid and histidine residues are present in hemoglobin, in ribonucleases, and in proteases, whereby this type of bond is preferentially found in the active centers of these enzymes. It is pointed out that hydrogen bonds with such interaction properties should be of great significance for structure and especially functions of proteins in which they are present.  相似文献   

4.
(L -Cys)n, (L -Lys)n, and (L -Glu)n were studied by ir spectroscopy in terms of their degree of deprotonation or protonation. It is shown that structurally symmetrical, easily polarizable SH ?S? ? ?S ?HS, N+H ?N ? N ?H+N, and OH ?O? ? ?O ?HO hydrogen bonds are formed between the side chains. The different wave number distributions of the ir continua caused by these hydrogen bonds show that the barrier in the double-minimum proton potential decreases in the series of these hydrogen bonds. The stability of these hydrogen bonds against hydration increases in this series. The OH ?O? ? ?O ?HO bonds are not broken by small amounts of water. With (L -Cys)n the formation of easily polarizable hydrogen bonds and a β-structure–coil transition are strongly interdependent. As a result of this coupling effect, the β-structure–coil transition becomes cooperative. With (L -Glu)n, the formation of the polarizable hydrogen bonds and the observed conformational change are independent processes. The (L -Glu)n conformation changes from α-helix to coil only if more than 80% of the residues are deprotonated. Finally, on the basis of the various types of easily polarizable hydrogen bonds, charge shifts in active centers of enzymes and the proton-conducting mechanism through hydrophobic regions of biological membranes are discussed.  相似文献   

5.
We studied films of poly(L -tyrosine) with hydrogen phosphate (residue/phosphate, 1:1) by ir spectroscopy. The influences of the alkali cations (Li+, Na+, K+) and of the degree of hydration were clarified. If Li+ ions are present, the OH ??OP hydrogen bonds formed in the dried films between the tyrosine OH groups and hydrogen phosphate are asymmetrical. The formation of hydrogen phosphate–hydrogen phosphate hydrogen bonds is prevented by the presence of the Li+ ions. With an increase in the degree of hydration, the tyrosine–phosphate bonds are not broken but become slightly stronger. Completely different behaviour is found if K+ ions are present. In dry films, the OH ??OP ? O? ?HOP hydrogen bonds formed between tyrosine and hydrogen phosphate show large proton polarizability. The tyrosine proton has a noticeable residence time at the acceptor O atom of the phosphate. The difference in the behaviour of the system with K+ ions when compared to the system with Li+ ions can be explained, since the hydrogen acceptor O atom of phosphate ions is more negatively charged due to the weaker influence of the K+ ions. Furthermore, POH ??OP hydrogen bonds between hydrogen phosphate molecules are formed. With an increase in the degree of hydration, the tyrosine–hydrogen phosphate hydrogen bonds are broken, all tyrosine protons are found at the tyrosine residues, and the -PO3? groupings are in a symmetrical environment, indicating that the K+ ions are removed from these groupings. If the degree of hydration increases further, hydrogen-bonded systems such as hydrogen phosphate–water–hydrogen phosphate are formed that show large proton polarizability due to collective proton motion. When Na+ ions are present, the OH ??OP ? O? ?HOP hydrogen bonds formed in dry films still show proton polarizability, but the residence time of the tyrosine proton at the phosphate is very short.  相似文献   

6.
The nature of hydrogen bonds formed between carboxylic acid residues and histidine residues in proteins is studied by ir spectroscopy. Poly(glutamic acid) [(Glu)n] is investigated with various monomer N bases. The position of the proton transfer equilibrium OH…?N ? O?…?H+N is determined considering the bands of the carboxylic group. It is shown that largely symmetrical double minimum energy surfaces are present in the OH…?N ? O?…?H+N bonds when the pKa of the protonated N base is two values larger than that of the carboxylic groups of (Glu)n. Hence OH…?N ? O?…?H+N bonds between glutamic and aspartic acid residues and histidine residues in proteins may be easily polarizable proton transfer hydrogen bonds. The polarizability of these bonds is one to two orders of magnitude larger than usual electron polarizabilities; therefore, these bonds strongly interact with their environment. It is demonstrated that water molecules shift these proton transfer equilibria in favor of the polar proton boundary structure. The access of water molecules to such bonds in proteins and therefore the position of this proton transfer equilibrium is dependent on conformation. The amide bands show that (Glu)n is α-helical with all systems. The only exception is the (Glu)n-n-propylamine system. When this system is hydrated (Glu)n is α-helical, too. When it is dried, however, (Glu)n forms antiparallel β-structure. This conformational transition, dependent on degree of hydration, is reversible. An excess of n-propylamine has the same effect on conformation as hydration.  相似文献   

7.
Abstract

2′-Deoxycytidine hemidihydrogenphosphate has been crystallized in the hexagonal space group P62 with α=25.839(3), c = 12.529(1) Å. The structure has been solved using the Patterson search method. The asymmetric unit contains two protonated, base-paired 2′-deoxycytidine dimers and two H2PO4 ? anions. The C+·C base pairs are composed of a protonated and a neutral species each and are triple H-bonded, the central N(3)…N(3) bonds being 2.850(7) and 2.884(5) Å. The conformations of the four nucleosides fall in the same category (sugar puckers 2·-endo, glycosidic links anti) but in one of them the glycosidic torsion angle is quite low with consequences in other geometrical parameters. The H2PO4 ? anions are located on twofold axes and form two types of tight columns with P…P separations about 4.18 Å The neighboring units along a column are linked via two very short O…H…O hydrogen bonds (O…O about 2.49 Å) leading to effective equalization of the P-O bonds. The base pairs of the two dC+·dC cations are coplanar and form layers perpendicular to the phosphate columns repeating every c/3. Within the layers, the dimers form a network through 0(5′)…O(2) hydrogen bonds but their primary intermolecular interactions have the form of H-bond anchors [N(4)-H…O-P and 0(3′)-H…O-P] to the phosphate groups.  相似文献   

8.
(L -Cys)n + N-base systems and (L -Cys)n + (L -Lys)n systems were studied by ir spectroscopy. It is shown that in the water-free systems, SH ?N ? S? ?H+N hydrogen bonds are formed. With the (L -Cys)n + N-base systems, both proton-limiting structures in the SH ?N ? S? ?H+N bonds have equal weight when the pKa of the protonated N-base is 2 pKa units larger than that of (L -Cys)n. The same is true with the water-free (L -Cys)n + (L -Lys)n system. Thus, with regard to the type of proton potentials present, these hydrogen bonds are proton-transfer hydrogen bonds showing very large proton polarizabilities. This is confirmed by the occurrence of continua in the ir spectra. Small amounts of water open these hydrogen bonds and increase the transfer of the proton to (L -Lys)n. In the (L -Lys)n + N-base systems, with increasing proton transfer the backbone of (L -Cys)n changes from antiparallel β-structure to coil. In (L -Cys)n + (L -Lys)n, the conformation is determined by the (L -Lys)n conformation and changes depending on the chain length of (L -Lys)n. Finally, the reactivity increase in the active center of fatty acid synthetase, which should be caused by the shift of a proton, is discussed on the basis of the great proton polarizability of the cysteine–lysine hydrogen bonds.  相似文献   

9.
Abstract

The 1H NMR relaxation effects produced by paramagnetic Cr(III) complexes on nucleoside 5′-mono- and -triphosphates in D2O solution at Ph′=3 were measured. The paramagnetic probes were [Cr(III)(H2O) 6]3+, [Cr(III)(H2O)3 (HATP)], [Cr(III)(H2O)3(HCTP)] and [Cr(III) (H2O)3(UTP)?, while the matrix nucleotides (0.1 M) were H2AMP, HIMP?, and H2ATP2-. For the aromatic base protons, the ratios of the transverse to longitudinal paramagnetic relaxation rates (R2p/R1p) for the [Cr(III)(H2O)6]3+/H2ATP2-, [Cr(III)(H2O)3(HATP)]/H2ATP2-, [Cr(III)(H2O)3(HCTP)]/H2ATP2 and [Cr(III)(H2O)3(UTP)]?/H2ATP2 systems were below 2.33 so the dipolar term predominates. For a given nucleotide, R1p for the purine H(8) signal was larger than for the H(2) signal with the [Cr(III)(H2O)6]3+ probe, while R1p for the H(2) signal was larger with all the other Cr(III) probes. Molecular mechanics computations on the [Cr(III)(H2O)4(HPP)(α,β)], [Cr(III)(NH3)4(HPP)(α,β)], [Co(III)(NH3)3(H2PPP)(α,βγ)] and [Co(III)(NH3)4(HPP)(α,β)] complexes gave calculated energy-minimized geometries in good agreement with those reported in crystal structures. The molecular mechanics force constants found were then used to calculate the geometry of the inner sphere [Cr(III)(H2O)6]3+ and [Cr(III)(H2O)3(HATP)(α,βγ)] complexes as well as the structures of the outer sphere [Cr(III) (H2O)6]3+-(H2AMP) and [Cr(III)(H2O)6]-(HIMP)? species. The gas-phase structure of the [Cr(III)(H2O)3(HATP)(α,βγ)] complex shows the existence of a hydrogen bond interaction between a water ligand and the adenine N(7) (O…N = 2.82 Å). The structure is also stabilized by intramolecular hydrogen bonds involving the -O(2′)H group and the adenine N(3) (O…N = 2.80 Å) as well as phosphate oxygen atoms and a water molecule (O…O = 2.47 Å). The metal center has an almost regular octahedral coordination geometry.

The structures of the two outer-sphere species reveal that the phosphate group interacts strongly with the hexa-aquochromium probe. In both complexes, the nucleotides have a similar “anti” conformation around the N(9)-C(l′) glycosidic bond. However, a very important difference characterizes the two structures. For the (HIMP)? complex, strong hydrogen bond interactions exist between one and two water ligands and the inosine N(7) and O(6) atoms, respectively (O…O = 2.63 Å O…N = 2.72, 2.70 Å). For the H2AMP complex, the [Cr(III) (H2O)c]3+ cation does not interact with N(7) since it is far from the purine system. Hydrogen bonds occur between water ligands and phosphate oxygens. The Cr-H(8) and Cr-H(2) distances revealed by the energy-minimized geometries for the two outer sphere species were used to calculate the R1p values for the H(8) and H(2) signals for comparison with the observed R1p values: 0.92(c), 1.04(ob) (H(8)) and 0.06(c), 0.35(ob) (H(2)) for H2AMP; and 3.76(c), 4.53(ob) (H(8)) and 0.16(c), 0.77(ob) s?1 (H(2)) for HIMP?. These results suggest that the dynamic relaxation effects can be only partially understood with molecular mechanics computations, although the success of the geometry calculations suggests that future efforts in the development of computational methods are justified.  相似文献   

10.
The X-ray structure analysis of a crystalline sample of 2-azabicyclo-[2,2,2]-octanone-3 or 3-isoquinuclidone shows that the molecules of this compound are associated in centrosymmetrical dimers stabilized by two N? H? O?C hydrogen bonds in which the N,H,O atoms are nearly collinear. As a consequence of this interaction, the H atom is shifted from its usual position and the Cα? N? H angle is increased to 125°. Using infrared spectroscopy (νN–H frequency range), it is possible to demonstrate that 3-isoquinuclidone is mainly in a dimeric form when dissolved in an inert solvent such as CCl4 and to observe the dimer-monomer equilibrium on dilution from saturation to a low concentration (0.005 mole/l.). On the contrary, dimers are broken off when operating in a polar medium (acetonitrile, deuterochloroform). In the same experimental conditions, measurements of the J vicinal coupling constant, by nuclear magnetic resonance spectroscopy, afford a concentration-dependent result in the case of CCl4 solutions (increasing from 5.4 to 5.7 Hz when diluting from 0.5 to 0.005 mole/l.) and a constant one (5.8 Hz) in the case of CH3CN or CDCl3 solutions. Then the 0.4-Hz difference can be attributed to geometrical changes in the Hα? Cα? N? H system when dimers are broken off and the valence angle Cα? N? H consequently decreases from 125° to its standard value (about 115°). This experimental observation is consistent with the result of a theoretical analysis performed by the INDO method. Then it seems that the use of the formulas proposed by Karplus to account for the valence angle distorsions in ethane-like systems, in the case of the Hα? Cα? N? H sequence, could yield overstimated corrections.  相似文献   

11.
The character of the bridged hydrogen atom (Hb) of B2H6 has become a hot issue in recent years. In this work, the complexes B2H6?·?·?·?NH3, B2H2X4?·?·?·?nNH3 (n?=?1, 2) and 2HF?·?·?·?B2H2X4?·?·?·?2NH3 (X?=?Cl, Br, I) were constructed and studied based on the M06-2X calculations to investigate how to enhance the Hb?·?·?·?N hydrogen-bonded interaction. When the terminal hydrogen atoms (Ht) of B2H6 were replaced by X (X?=?Cl, Br, I) atoms, the Hb?·?·?·?N hydrogen bond were strengthened. According to the electrostatic potentials in B2H2X4, two HF molecules were added to the interspace of the B-H-B-H four-membered ring of the B2H2X4?·?·?·?2NH3 complexes, and H?·?·?·?X hydrogen bond formed, resulting in further enhancing effect of Hb?·?·?·?N hydrogen bond. As a result, the positive cooperative effect of Hb?·?·?·?N hydrogen bond and H?·?·?·?X hydrogen bond do enhance the interactions of each other. The two measures not only enhance the strength of Hb?·?·?·?N hydrogen bond, but also achieve the goal to make the Hb?·?·?·?N hydrogen bond perpendicular to B?·?·?·?B direction.
Graphical Abstract Enhancing the hydrogen bond between the bridged hydrogen atom of diborane and ammonia?
  相似文献   

12.
Using density functional theory calculations, we investigated properties of a functionalized BC2N nanotube with NH3 and five other NH2-X molecules in which one of the hydrogen atoms of NH3 is substituted by X = ?CH3, ?CH2CH3, ?COOH, ?CH2COOH and ?CH2CN functional groups. It was found that NH3 can be preferentially adsorbed on top of the boron atom, with adsorption energy of ?12.0 kcal mol?1. The trend of adsorption-energy change can be correlated with the trend of relative electron-withdrawing or -donating capability of the functional groups. The adsorption energies are calculated to be in the range of ?1.8 to ?14.2 kcal mol?1, and their relative magnitude order is found as follows: H2N(CH2CH3) > H2N(CH3) > NH3 > H2N(CH2COOH) > H2N(CH2CN) > H2N(COOH). Overall, the functionalization of BC2N nanotube with the amino groups results in little change in its electronic properties. The preservation of electronic properties of BC2N coupled with the enhancement of solubility renders their chemical modification with either NH3 or amino functional groups to be a way for the purification of BC2N nanotubes.  相似文献   

13.
The binding of the fluorescent analog of adenosine diphosphate (ADP)1, 1,N6-ethenoadenosine diphosphate (εADP) to myosin and its subfragments, heavy meromyosin (HMM) and subfragment one (S1), has been studied under analagous conditions to those previously used in comparable studies on the binding of ADP to these molecules. The results indicate that there are two binding sites for εADP on myosin and HMM, and one site on S1. The dissociation constants for all had an identical value, within experimental error, of 2.0 (± .5) × 10?5 M?1. This is identical to the values found by Young (J. Biol. Chem., 242, 2790 (1967)) for ADP. In addition, the kinetics of hydrolysis of εATP versus ATP by S1 were studied. Values of Vmax and Km were 25 μM phosphate sec?1 (gm protein)?1 and 5 × 10?5 M?1 for ATP, and 80 μN phosphate sec?1 (gm protein)?1 and 45 × 10?5 M?1 for εATP. The results indicate that the increased Vmax that occurs when εATP is used as a substitute for ATP is not due to either an increased binding affinity of ATP for myosin and its subfragments, nor due to a decreased binding affinity of εATP versus ADP. This in turn suggests that the increase in Vmax may be due to an increased hydrolytic rate of εATP vs ATP in the enzyme substrate complex.  相似文献   

14.
A correlation between the rate of ATP synthesis by F0F1 ATP synthase and formate oxidation by formate hydrogen lyase (FHL) has been found in inside-out membrane vesicles of the Escherichia coli mutant JW 136 (Δhyahyb) with double deletions of hydrogenases 1 and 2, grown anaerobically on glucose in the absence of external electron acceptors at pH 6.5. ATP synthesis was suppressed by the H+-ATPase inhibitors N,N′-dicyclohexylcarbodiimide, sodium azide, and the uncoupler carbonyl cyanide m-chlorophenylhydrazone. Copper ions inhibited formate-dependent hydrogenase and ATP-synthase activities but did not affect the ATPase activity of the vesicles. The maximal rate of ATP synthesis (0.83 μmol/min per mg protein) was determined at simultaneous application of sodium formate, ADP, and inorganic phosphate, and was stimulated by K+ ions. The results confirm the assumption of a dual role of hydrogenase 3, the formate hydrogen lyase subunit that can couple the reduction of protons to H2 and their translocation through membrane with chemiosmotic synthesis of ATP.  相似文献   

15.
The interaction processes of trace amounts of N-methyl-2-pyrrolidinone (NMP), CS2/NMP (1:1 by volume) and pure NMP solvent with the hydrogen bond of OH?N in coal were constructed and simulated by density functional theory methods. The distances and bond orders between the main related atoms, and the hydrogen bond energy of OH?N were calculated. The calculated results show that pure NMP solvent does not weaken the hydrogen bond of OH?N in coal. However, trace amounts of NMP and CS2/NMP (1:1 by volume) have a strong capacity to weaken the hydrogen bond of OH?N in coal. The H2–N3 distances are elongated from 1.87 Å to 3.80 Å and 3.44 Å, the bond orders of H2–N3 all disappear, and the corresponding hydrogen bond energies of OH?N in coal decrease from 45.72 kJ mol?1 to 7.06 and 11.24 kJ mol?1, respectively. These results show that CS2 added to pure NMP solvent plays an important role in releasing the original capacity of NMP to weaken the hydrogen bond of OH?N in coal, in agreement with experimental observations.  相似文献   

16.
The oxidation enthalpy of reduced flavin mononucleotide at pH 7.0 in 0.2 m phosphate buffer has been studied by determining the heat associated with the reaction: FMNH2 + 2 Fe(CN)?36 ? FMN + 2 Fe(CN)?46 + 2 H+. (a) (The quinone, semiquinone, and hydroquinone forms of FMN are represented as FMN, FMNH, and FMNH2, respectively.) Calorimetric experiments were performed in a flow microcalorimeter which was modified to prevent sample contamination by oxygen. The enthalpy observed for reaction (a), after correction for dilution and buffer effects, was ?39.2 ± 0.4 kcal (mole FMNH2)?1 at 25 °C. The potential difference, ΔE′, developed by reaction (a) was determined potentiometrically and corresponded to a free energy change, ΔG′, of ?30.3 kcal (mole FMNH2)?1. The resulting entropy change, ΔS′, was thus calculated to be ?29.8 e.u. Reaction (a) was also studied at temperatures of 7 °C and 35.5 °C. ΔCp′ for the reaction was calculated as ?155 ± 18 cal deg?1 (mole FMNH2)?1 at 20 °C. ΔH′ for the reaction (b), FMNH2 ? FMN + H2, (b) was calculated as +14.2 ± 0.7 kcal mole?1 at 25 °C, relative to the enthalpy of the hydrogen electrode being identically equal to zero at all values of pH and temperature. The free energy at pH 7.0 for reaction (b), calculated from the potential was found to be ?9.7 kcal mole?1, which resulted in an entropy for reaction (b) of 80.2 e.u. A thermal titration of reaction (a) was used to calculate the thermodynamic parameters for the formation of semiquinone dimer according to the reaction FMNH2 + FMN ? (·FMNH)2. (c) The free energy, enthalpy, and entropy changes for reaction (c) were estimated to be ?6.1 kcal mole?1, ?7 kcal mole?1, and ?3 e.u., respectively.  相似文献   

17.
Calorimetric studies of the reduction of free oxygen in solution by sodium dithionite are in agreement with a stoichiometry of 2 moles Na2S2O4 per mole of oxygen. The reaction is biphasic with ΔHt - 118±7 kcal mol?1 (?494 ± 29 kJ mol?1). The initial phase of the reaction proceeds with an enthalpy change of ca ?20 kcal (?84 kJ) and occurs when 0.5 moles of dithionite have been added per mole dioxygen present. This could be interpreted as the enthalpy change for the addition of a single electron to form the superoxide anion. Further reduction of the oxygen to water by one or more additional steps is accompanied by an enthalpy change of ca ?100 kcal (?418. 5 kJ). Neither of these reductive phases is consistent with the formation of hydrogen peroxide as an intermediate. The reduction of hydrogen peroxide by dithionite in 0.1 M phosphate buffer, pH 7.15, is a much slower process and with an enthalpy change of ca ? 74 kcal mol?1 (?314 kJ mol?1). Dissociation of oxyhemoglobin induced by the reduction of free oxygen tension with dithionite also shows a stoichiometry of 2 moles dithionite per mole oxygen present and an enthalpy change of ca. ?101 ±9 kcal mol?1 (?423± 38 kJ mol?1). The difference in the observed enthalpies (reduction of dioxygen vs. oxyhemoglobin) has been attributed to the dissociation of oxyhemoglobin, which is 17 kcal mol?1 (71 kJ mol?1).  相似文献   

18.
Histidinium perchlorate having protecting groups at the α-amino and α-carboxylate group is studied by IR spectroscopy as function of the addition of protected histidine molecules. An intense continuous absorption arises, indicating that the N+H…N ? N…H+N formed are easily polarizable hydrogen bonds. From the integral absorbance of a band the concentration of the histidine-histidinium complex, i.e. the concentration of the easily polarizable hydrogen bonds is determined. It is shown that the absorbance of the continuum increases in proportion to the concentration of the easily polarizable N+H…N ? N…H+N bonds. Finally, it is discussed that via such an easily polarizable histidine-histidinium hydrogen bond a proton translocation in the active center of ribonuclease A may occur.  相似文献   

19.
Nonprecious metal catalysts (NPMCs) Fe? N? C are promising alternatives to noble metal Pt as the oxygen reduction reaction (ORR) catalysts for proton‐exchange‐membrane fuel cells. Herein, a new modulation strategy is reported to the active moiety Fe? N4 via a precise “single‐atom to single‐atom” grafting of a Pt atom onto the Fe center through a bridging oxygen molecule, creating a new active moiety of Pt1? O2? Fe1? N4. The modulated Fe? N? C exhibits remarkably improved ORR stabilities in acidic media. Moreover, it shows unexpectedly high catalytic activities toward oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), with overpotentials of 310 mV for OER in alkaline solution and 60 mV for HER in acidic media at a current density of 10 mA cm?2, outperforming the benchmark RuO2 and comparable with Pt/C(20%), respectively. The enhanced multifunctional electrocatalytic properties are associated with the newly constructed active moiety Pt1? O2? Fe1? N4, which protects Fe sites from harmful species. Density functional theory calculations reveal the synergy in the new active moiety, which promotes the proton adsorption and reduction kinetics. In addition, the grafted Pt1? O2? dangling bonds may boost the OER activity. This study paves a new way to improve and extend NPMCs electrocatalytic properties through a precisely single‐atom to single‐atom grafting strategy.  相似文献   

20.
An apolar synthetic octapeptide, Boc-(Ala-Aib)4-OMe, was crystallized in the triclinic space group P1 with cell dimensions a = 11.558 Å, b = 11.643 Å, c = 9.650 Å, α = 120.220°, β = 107.000°, γ = 90.430°, V = 1055.889 Å3, Z = 1, C34H60O11N8·H2O. The calculated crystal density was 1.217 g/cm3 and the absorption coefficient ? was 6.1. All the intrahelical hydrogen bonds are of the 310 type, but the torsion angles, ? and ψ, of Ala(5) and Ala(7) deviate from the standard values. The distortion of the 310-helix at the C-terminal half is due to accommodation of the bulky Boc group of an adjacent peptide in the nacking. A water molecule is held between the N-terminal of one peptide and the C-terminal of the other. The oxygen atom of water forms hydrogen bonds with N (1) -H and N (2) -H, which are not involved in the intrahelical hydrogen bonds. The hydrogen atoms of water also formed hydrogen bonds with carbonyl oxygens of the adjacent peptide molecule. On the other hand, 1H-nmr analysis revealed that the octapeptide took an α-helical structure in a CD3CN solution. The longer peptides, Boc-(Ala-Aib)6-OMe and Boc-(Ala-Aib)8-OMe, were also shown to take an α-helical structure in a CD3CN solution. An α-helical conformation of the hexadecapeptide in the solid state was suggested by x-ray analysis of the crystalline structure. Thus, the critical length for transition from the 310- to α-helix of Boc-(Ala-Aib)n-OMe is 8. © 1993 John Wiley & Sons, Inc.  相似文献   

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